Re: extern const initialized warns in C

[David Brown]

On 21/01/18 08:12, Jay K wrote:
> extern const int foo = 123;
> 
> 
> 
> Why does this warn?
> This is a valid portable form, with the same meaning
> across all compilers, and, importantly, portably
> to C and C++.
> 
> I explicitly do not want to say:
> 
>   const int foo = 123
> 
> because I want the code to be valid and have the same meaning
> in C and C++ (modulo name mangling).
> 
> I end up with:
> 
> // Workaround gcc warning.
> #ifdef __cplusplus
> #define EXTERN_CONST extern const
> #else
> #define EXTERN_CONST const
> #endif
> 
> 
> EXTERN_CONST int foo = 123;
> 
> and having to explain it to people.
> 



This suggests that gcc authors consider mixing "extern" and
initialization to be such bad style that the compiler warns by default.
 But the "bug" is that there is no flag to turn off this warning.
(Ideally every warning should have a matching flag, even if the warning
is enabled by default.)

Usually you do not want to have "extern" and initialisation in the same
line - it indicates a questionable organisation of your sources which is
more likely to be error-prone than the standard idioms.  (I say
"questionable", not necessarily wrong - but certainly I would question
it if I saw it in source code.)

Normally you want:

// file.h
// declaration, not definition
extern const int foo;

// file.c
#include 
// definition
const int foo = 123;

// otherfile.c
#include 
int usefoo(void) { return foo; }


The key advantages of this sort of setup are a cleaner separation
between declarations (which you need to /use/ things) and the
definitions (which should normally only exist once in the program -
certainly for C).  The declarations and definitions only exist in one
place, and they are checked for consistency - there are no "extern"
declarations lying around in C files that might get out of step from
changes in the headers or other files with definitions.

To be consistent with this, and to work consistently with C and C++, I
have a strict policy that a C (or C++) file never contains  declarations
without definitions (and initialisations as needed), with each
definition either also declared as "extern" in a matching header file,
or it is declared as "static".

This sort of arrangement is very common - though many people are lazy
about using "static".  (In C++, you can also use anonymous namespaces,
but "static" works for consistency between C and C++.)


Still, gcc should have a flag to disable this warning if you have reason
to use "extern const int foo = 123;" - it is, after all, correctly
defined C code.



> $ cat 1.c
> extern const int foo = 123;
> $ $HOME/gcc720/bin/gcc -c -S 1.c
> 1.c:1:18: warning: 'foo' initialized and declared 'extern'
>  extern const int foo = 123;
>   ^~~
> $ $HOME/gcc720/bin/gcc -c -S -xc++ -Wall -pedantic 1$ $HOME/gcc720/bin/gcc -v
> Using built-in specs.
> 
> COLLECT_GCC=/Users/jay/gcc720/bin/gcc
> COLLECT_LTO_WRAPPER=/Users/jay/gcc720/libexec/gcc/x86_64-apple-darwin16.7.0/7.2.0/lto-wrapper
> Target: x86_64-apple-darwin16.7.0
> Configured with: ../gcc-7.2.0/configure -prefix=/Users/jay/gcc720 
> -disable-nls -disable-bootstrap
> Thread model: posix
> gcc version 7.2.0 (GCC) $ 
> 
> 
> Thank you,
>  - Jay
> 
> 
> 
>  
> 

Second GCC 7.2 Release Candidate available from gcc.gnu.org

[Richard Biener]

A second release candidate for GCC 7.3 is available from

ftp://gcc.gnu.org/pub/gcc/snapshots/7.3.0-RC-20180122/

and shortly its mirrors.  It has been generated from SVN revision 256937.

I have so far bootstrapped and tested the release candidate on
x86_64-unknown-linux-gnu.  Please test it and report any issues to
bugzilla.

If all goes well I'd like to release 7.3 on Thursday, January 25th.

Re: extern const initialized warns in C

[Jay K]

By this argument there is a missing warning for the equivalent:

  const int foo = 123;

with no previous extern declaration.

As well, there is no warning in C++.
All three constructs are equivalent, yet only one gets a warning.

Interesting point, that I had not realized, and with an often acceptable
workaround, however also there exist coding conventions that prohibit use of 
static.
Instead they "hide" things by omitting them from headers only.

That can still be worked around, just put the declaration right before the 
definition,
in the same source file.

I realize there are many arguments for and against file level static.

 - Jay  


From: David Brown 
Sent: Monday, January 22, 2018 8:32 AM
To: Jay K; gcc
Subject: Re: extern const initialized warns in C
  

On 21/01/18 08:12, Jay K wrote:
> extern const int foo = 123;
> 
> 
> 
> Why does this warn?
> This is a valid portable form, with the same meaning
> across all compilers, and, importantly, portably
> to C and C++.
> 
> I explicitly do not want to say:
> 
>   const int foo = 123
> 
> because I want the code to be valid and have the same meaning
> in C and C++ (modulo name mangling).
> 
> I end up with:
> 
> // Workaround gcc warning.
> #ifdef __cplusplus
> #define EXTERN_CONST extern const
> #else
> #define EXTERN_CONST const
> #endif
> 
> 
> EXTERN_CONST int foo = 123;
> 
> and having to explain it to people.
> 




45977 – "warning: 'i' initialized and declared 'extern ...
gcc.gnu.org
GCC Bugzilla – Bug 45977 "warning: 'i' initialized and declared 'extern'" could 
use a separate warning flag controlling it Last modified: 2017-07-26 15:36:22 
UTC

This suggests that gcc authors consider mixing "extern" and
initialization to be such bad style that the compiler warns by default.
 But the "bug" is that there is no flag to turn off this warning.
(Ideally every warning should have a matching flag, even if the warning
is enabled by default.)

Usually you do not want to have "extern" and initialisation in the same
line - it indicates a questionable organisation of your sources which is
more likely to be error-prone than the standard idioms.  (I say
"questionable", not necessarily wrong - but certainly I would question
it if I saw it in source code.)

Normally you want:

// file.h
// declaration, not definition
extern const int foo;

// file.c
#include 
// definition
const int foo = 123;

// otherfile.c
#include 
int usefoo(void) { return foo; }


The key advantages of this sort of setup are a cleaner separation
between declarations (which you need to /use/ things) and the
definitions (which should normally only exist once in the program -
certainly for C).  The declarations and definitions only exist in one
place, and they are checked for consistency - there are no "extern"
declarations lying around in C files that might get out of step from
changes in the headers or other files with definitions.

To be consistent with this, and to work consistently with C and C++, I
have a strict policy that a C (or C++) file never contains  declarations
without definitions (and initialisations as needed), with each
definition either also declared as "extern" in a matching header file,
or it is declared as "static".

This sort of arrangement is very common - though many people are lazy
about using "static".  (In C++, you can also use anonymous namespaces,
but "static" works for consistency between C and C++.)


Still, gcc should have a flag to disable this warning if you have reason
to use "extern const int foo = 123;" - it is, after all, correctly
defined C code.



> $ cat 1.c
> extern const int foo = 123;
> $ $HOME/gcc720/bin/gcc -c -S 1.c
> 1.c:1:18: warning: 'foo' initialized and declared 'extern'
>  extern const int foo = 123;
>   ^~~
> $ $HOME/gcc720/bin/gcc -c -S -xc++ -Wall -pedantic 1$ $HOME/gcc720/bin/gcc -v
> Using built-in specs.
> 
> COLLECT_GCC=/Users/jay/gcc720/bin/gcc
> COLLECT_LTO_WRAPPER=/Users/jay/gcc720/libexec/gcc/x86_64-apple-darwin16.7.0/7.2.0/lto-wrapper
> Target: x86_64-apple-darwin16.7.0
> Configured with: ../gcc-7.2.0/configure -prefix=/Users/jay/gcc720 
> -disable-nls -disable-bootstrap
> Thread model: posix
> gcc version 7.2.0 (GCC) $ 
> 
> 
> Thank you,
>  - Jay
> 
> 
> 
>  
> 

Re: extern const initialized warns in C

[Jay K]

Also the warning did not include a link explaining the desired workaround.


Since you advocate for static...and I know it has big value..

There are the following reasons against static:

 - It is prohibited in some coding conventions.
    They instead hide symbols by omitting them from any headers.

 - It allows/encourages symbols duplicated from a human point of view,
   leading to harder to read code; but this is also the point and good,
   it offers scope to pick shorter names, or at least hide
   names (you can still strive for globally unique names, in
   case the symbols later have to be made extern)
   
 - it leads to accidental duplication, static int foo = 123 in a header

 - There are toolsets that don't resolve statics in disassembly

 - It only allows for sharing within a file and hiding from all others,
   it doesn't allow sharing for within a few files and hiding from others

 - It sort of doesn't work with "unity builds" old fashioned LTO/LTCG where one
   source file includes the rest
   

   I think a linker switch to report symbols that could be static
   might be useful.
   
   I find the "scoping" too hard to pass it, and if I need to make
   the symbol extern in future, I can afford a rename to do it.
   

    - Jay




From: Jay K
Sent: Monday, January 22, 2018 9:31 AM
To: David Brown; gcc
Subject: Re: extern const initialized warns in C
  


By this argument there is a missing warning for the equivalent:

  const int foo = 123;

with no previous extern declaration.

As well, there is no warning in C++.
All three constructs are equivalent, yet only one gets a warning.

Interesting point, that I had not realized, and with an often acceptable
workaround, however also there exist coding conventions that prohibit use of 
static.
Instead they "hide" things by omitting them from headers only.

That can still be worked around, just put the declaration right before the 
definition,
in the same source file.

I realize there are many arguments for and against file level static.

 - Jay  


From: David Brown 
Sent: Monday, January 22, 2018 8:32 AM
To: Jay K; gcc
Subject: Re: extern const initialized warns in C
  

On 21/01/18 08:12, Jay K wrote:
> extern const int foo = 123;
> 
> 
> 
> Why does this warn?
> This is a valid portable form, with the same meaning
> across all compilers, and, importantly, portably
> to C and C++.
> 
> I explicitly do not want to say:
> 
>   const int foo = 123
> 
> because I want the code to be valid and have the same meaning
> in C and C++ (modulo name mangling).
> 
> I end up with:
> 
> // Workaround gcc warning.
> #ifdef __cplusplus
> #define EXTERN_CONST extern const
> #else
> #define EXTERN_CONST const
> #endif
> 
> 
> EXTERN_CONST int foo = 123;
> 
> and having to explain it to people.
> 




45977 – "warning: 'i' initialized and declared 'extern ...
gcc.gnu.org
GCC Bugzilla – Bug 45977 "warning: 'i' initialized and declared 'extern'" could 
use a separate warning flag controlling it Last modified: 2017-07-26 15:36:22 
UTC


45977 – "warning: 'i' initialized and declared 'extern ...
gcc.gnu.org
GCC Bugzilla – Bug 45977 "warning: 'i' initialized and declared 'extern'" could 
use a separate warning flag controlling it Last modified: 2017-07-26 15:36:22 
UTC

This suggests that gcc authors consider mixing "extern" and
initialization to be such bad style that the compiler warns by default.
 But the "bug" is that there is no flag to turn off this warning.
(Ideally every warning should have a matching flag, even if the warning
is enabled by default.)

Usually you do not want to have "extern" and initialisation in the same
line - it indicates a questionable organisation of your sources which is
more likely to be error-prone than the standard idioms.  (I say
"questionable", not necessarily wrong - but certainly I would question
it if I saw it in source code.)

Normally you want:

// file.h
// declaration, not definition
extern const int foo;

// file.c
#include 
// definition
const int foo = 123;

// otherfile.c
#include 
int usefoo(void) { return foo; }


The key advantages of this sort of setup are a cleaner separation
between declarations (which you need to /use/ things) and the
definitions (which should normally only exist once in the program -
certainly for C).  The declarations and definitions only exist in one
place, and they are checked for consistency - there are no "extern"
declarations lying around in C files that might get out of step from
changes in the headers or other files with definitions.

To be consistent with this, and to work consistently with C and C++, I
have a strict policy that a C (or C++) file never contains  declarations
without definitions (and initialisations as needed), with each
definition either also declared as "extern" in a matching header file,
or it is declared as "static".

This sort of arrangement is very common - though many people are lazy
about using "static".  (In C++, you 

Re: extern const initialized warns in C

[Franz Sirl]

Am 2018-01-21 um 13:08 schrieb Georg-Johann Lay:

Jay K schrieb:

extern const int foo = 123;

Why does this warn?
This is a valid portable form, with the same meaning
across all compilers, and, importantly, portably
to C and C++.


I also wondered about this.

In C99 §6.9.2 "External object definitions" there's actually
the following example in clause 4:

extern int i3 = 3; // definition, external linkage


See https://gcc.gnu.org/ml/gcc-patches/2018-01/msg01848.html for a patch 
to make it controllable, though I wrote it for the opposite reason, 
namely to be able to turn it into an error separately (and to sync with 
the clang option).


Franz

Forms to contribute to GCC

[Daniel Celis Garza]

Hello,

I'll be contributing to GCC by working on OpenCoarrays as my PhD programme's 
broadening project. I'd like to request the necessary forms to do so. As far as 
I'm aware, those are the copyright assignment form of all future changes and 
the employer disclaimer form.

Best wishes,
Daniel

Re: extern const initialized warns in C

[David Brown]

On 22/01/2018 10:31, Jay K wrote:


By this argument there is a missing warning for the equivalent:

   const int foo = 123;

with no previous extern declaration.


I would like to see such a warning.  There is "-Wmissing-declarations", 
but that applies only to functions and not to objects.


(Note that in C++, "const" objects without an "extern" declaration are 
effectively "static" - in C, without the storage-class specifier const 
objects have external linkage.)




As well, there is no warning in C++.
All three constructs are equivalent, yet only one gets a warning.


No, they are not the same - in C++ the linkage of a const is static 
unless you explicitly say "extern", and in C it is external unless you 
explicitly say "static".


Thus in "extern const int foo = 123;", the "extern" has a significant 
effect in C++ but in C it does nothing (other than inform the reader).


I would like to see the warning having a controlling flag.  It could 
perhaps be on by default in C and off by default in C++ to get the same 
effect as today - and then users can fine-tune to fit their style.




Interesting point, that I had not realized, and with an often acceptable
workaround, however also there exist coding conventions that prohibit use of 
static.


I have never heard of such a thing in a coding standard.  C++ 
conventions might encourage the use of anonymous namespaces rather than 
C-style "static" declarations, but that would not apply to C.  I would 
consider a coding convention that discouraged static to be seriously broken.



Instead they "hide" things by omitting them from headers only.


That is madness.  The symbols still have global linkage across the 
program, and you will get all sorts of problems when one file uses the 
same "local" identifier as another.  If you are lucky, your linker will 
tell you of the crash - but if you have enabled "common" data (i.e., you 
don't have the "-fno-common" flag) and have used the same identifier for 
two different "local" objects without explicit initialisation, you are 
going to have some serious and very hard to find bugs.


If someone asks you to write to such a coding convention, do your best 
to refuse.




That can still be worked around, just put the declaration right before the 
definition,
in the same source file.

I realize there are many arguments for and against file level static.



There are no /good/ arguments against file-level static in C, except 
perhaps temporarily while debugging (it can be easier to view non-static 
data in a debugger).  Any time file-level static can be used, it 
/should/ be used.


IMHO, of course.

mvh.,

David



  - Jay


From: David Brown 
Sent: Monday, January 22, 2018 8:32 AM
To: Jay K; gcc
Subject: Re: extern const initialized warns in C
   


On 21/01/18 08:12, Jay K wrote:

extern const int foo = 123;



Why does this warn?
This is a valid portable form, with the same meaning
across all compilers, and, importantly, portably
to C and C++.

I explicitly do not want to say:

    const int foo = 123

because I want the code to be valid and have the same meaning
in C and C++ (modulo name mangling).

I end up with:

// Workaround gcc warning.
#ifdef __cplusplus
#define EXTERN_CONST extern const
#else
#define EXTERN_CONST const
#endif


EXTERN_CONST int foo = 123;

and having to explain it to people.






45977 – "warning: 'i' initialized and declared 'extern ...
gcc.gnu.org
GCC Bugzilla – Bug 45977 "warning: 'i' initialized and declared 'extern'" could 
use a separate warning flag controlling it Last modified: 2017-07-26 15:36:22 UTC

This suggests that gcc authors consider mixing "extern" and
initialization to be such bad style that the compiler warns by default.
  But the "bug" is that there is no flag to turn off this warning.
(Ideally every warning should have a matching flag, even if the warning
is enabled by default.)

Usually you do not want to have "extern" and initialisation in the same
line - it indicates a questionable organisation of your sources which is
more likely to be error-prone than the standard idioms.  (I say
"questionable", not necessarily wrong - but certainly I would question
it if I saw it in source code.)

Normally you want:

// file.h
// declaration, not definition
extern const int foo;

// file.c
#include 
// definition
const int foo = 123;

// otherfile.c
#include 
int usefoo(void) { return foo; }


The key advantages of this sort of setup are a cleaner separation
between declarations (which you need to /use/ things) and the
definitions (which should normally only exist once in the program -
certainly for C).  The declarations and definitions only exist in one
place, and they are checked for consistency - there are no "extern"
declarations lying around in C files that might get out of step from
changes in the headers or other files with definitions.

To be consistent with this, and to work consistently with C and C++, I
have a strict 

Re: Status of m32c target?

[Martin Jambor]

Hi,

On Fri, Jan 19 2018, Sandra Loosemore wrote:
> On 01/19/2018 10:14 AM, Jeff Law wrote:
>
>> cc0 needs to die.  That doesn't mean that any particular target needs to
>> be dropped -- it just  means that someone has to step forward to do the
>> conversion.
>
> Unifying two parallel threads:  might this be a good project for GSoC?

I have no idea how big the task would be but it seems it could be quite
difficult.  Therefore we would need an experienced and extraordinarily
willing to help mentor.

Martin

Re: extern const initialized warns in C

[David Brown]

Hi,

I made some points in my other reply.  But for completeness, I'll tackle 
these too.


On 22/01/2018 10:38, Jay K wrote:

Also the warning did not include a link explaining the desired workaround.


Since you advocate for static...and I know it has big value..

There are the following reasons against static:

  - It is prohibited in some coding conventions.
     They instead hide symbols by omitting them from any headers.


As noted before, that is insane.  It gives no benefits but makes it easy 
to cause mistakes that are hard to find.




  - It allows/encourages symbols duplicated from a human point of view,
    leading to harder to read code; but this is also the point and good,
    it offers scope to pick shorter names, or at least hide
    names (you can still strive for globally unique names, in
    case the symbols later have to be made extern)


Omitting "static" also allows symbol duplication.  It just means that 
such duplication is an error in the code - which may or may not be 
caught at link time.


You /can/ have a coding convention that discourages duplicate symbol 
names - even when using "static".  That might help a little in 
understanding, but will quickly mean bloated source code that is harder 
to read and follow (because you end up with long-winded symbol names 
everywhere).


Such conventions are not scalable, are hopeless for multi-programmer 
projects, terrible for code re-use, and can make code far harder to read 
and write.


The scoping and naming in C is limited enough without omitting half the 
features it has to deal with modularisation.



  - it leads to accidental duplication, static int foo = 123 in a header


It is quite simple - don't do that.  It is appropriate for constant data 
- "static const int foo = 123;" in a header will be fine, because "foo" 
has the same value everywhere and is likely to be "optimised away". 
That is the reason C++ makes "const int foo = 123;" effectively static.


Headers (in C, and mostly in C++) are for /declarations/, not 
definitions - at least if you want to write structured and modular code.




  - There are toolsets that don't resolve statics in disassembly


Statics are local to the file.  Disassemblies should show them when they 
are used.  For the tiny, tiny proportion of C programmers that ever use 
a disassembler, if their toolchains are not good enough then they should 
get better toolchains.  It should /never/ be a problem when using 
assembly listing files generated by the compiler, which are almost 
always more useful than disassembling object code.


Making a coding convention to suit this requirement is like making 
gloves with 6 fingers so that they fit people with an extra digit.




  - It only allows for sharing within a file and hiding from all others,
    it doesn't allow sharing for within a few files and hiding from others


C has no good way to allow sharing between a few files and hiding from 
others.  Such shared identifiers must be program-wide global.  But that 
does /not/ mean you should make /everything/ program-wide global!  It 
means you should minimise such sharing, prefix such shared names in a 
way likely to minimise conflicts, and organise your source code modules 
as best you can.




  - It sort of doesn't work with "unity builds" old fashioned LTO/LTCG where one
    source file includes the rest


It works /exactly/ as well.  Such "unity builds" need symbols to have 
different identifiers - but you can quite happily declare them "static".


You seem to be under the impression that using "static" forces you to 
use duplicate names for different objects in different files.  That is 
not true - it merely /allows/ it.  You can still have a convention 
requiring different identifiers in different modules.  It is a bad idea, 
IMHO, but omitting "static" makes it far worse.




    I think a linker switch to report symbols that could be static
    might be useful.

    I find the "scoping" too hard to pass it, and if I need to make

    the symbol extern in future, I can afford a rename to do it.


I am not sure what you mean by that.

mvh.,

David





     - Jay




From: Jay K
Sent: Monday, January 22, 2018 9:31 AM
To: David Brown; gcc
Subject: Re: extern const initialized warns in C
   



By this argument there is a missing warning for the equivalent:

   const int foo = 123;

with no previous extern declaration.

As well, there is no warning in C++.
All three constructs are equivalent, yet only one gets a warning.

Interesting point, that I had not realized, and with an often acceptable
workaround, however also there exist coding conventions that prohibit use of 
static.
Instead they "hide" things by omitting them from headers only.

That can still be worked around, just put the declaration right before the 
definition,
in the same source file.

I realize there are many arguments for and against file level static.

  - Jay


From: David Brown 
Sent: Monday, January 22, 2018 8:32 AM

Re: Status of m32c target?

[Trevor Saunders]

On Mon, Jan 22, 2018 at 10:57:35AM +0100, Martin Jambor wrote:
> Hi,
> 
> On Fri, Jan 19 2018, Sandra Loosemore wrote:
> > On 01/19/2018 10:14 AM, Jeff Law wrote:
> >
> >> cc0 needs to die.  That doesn't mean that any particular target needs to
> >> be dropped -- it just  means that someone has to step forward to do the
> >> conversion.
> >
> > Unifying two parallel threads:  might this be a good project for GSoC?
> 
> I have no idea how big the task would be but it seems it could be quite
> difficult.  Therefore we would need an experienced and extraordinarily
> willing to help mentor.

I'd be willing to help students where I can, but unfortunately I suspect
this isn't a great project.  My memory of looking at what was involved
is that you needed to learn a lot about how rtl worked to even
understand what needed to be done much less actually change things and
then test them.  That said maybe if we don't tell them its really hard
they'll go and do it and prove me wrong.

Trev

> 
> Martin

Re: extern const initialized warns in C

[Jay K]

> I find the "scoping" too hard to pass it, and if I need to make
> the symbol extern in future, I can afford a rename to do it.


I mean, I actually like like the ability to shorten file level static symbols.


As you point out, true, you can have it both ways, static does not force 
shortening, merely allows it, and protects you from inadvertant duplication.


Really, the prohibition against file level static is used in large code bases.

You already have to chose unique extern names, and depend on the linker to 
diagnose duplicate externals for them.

Extending this by barring statics isn't necessarily significant.

Such code bases prefix every extern identifier with some "local" prefix, and 
any missing prefix is easy to spot and a stylistic mistake.

(i.e. local to the subsystem or directory -- I realize it is the very 
definition of "local" that makes or break this)


I understand that hiding by omission from headers is not hiding at the linker 
level.


I agree there are scalability problems with naming in C, but it isn't clear 
static helps significantly.


There is an interesting side effect though that I think is not very much 
appreciated.

Large C code bases are more amenable to plain text search than large C++ code 
bases, due to the "more uniqueness" of symbols.


This plain text search aspect is one of extremely few advantages I see to C 
over C++, perhaps the only one.


 - Jay



From: David Brown 
Sent: Monday, January 22, 2018 10:14 AM
To: Jay K; gcc
Subject: Re: extern const initialized warns in C

Hi,

I made some points in my other reply.  But for completeness, I'll tackle
these too.

On 22/01/2018 10:38, Jay K wrote:
> Also the warning did not include a link explaining the desired workaround.
>
>
> Since you advocate for static...and I know it has big value..
>
> There are the following reasons against static:
>
>   - It is prohibited in some coding conventions.
>  They instead hide symbols by omitting them from any headers.

As noted before, that is insane.  It gives no benefits but makes it easy
to cause mistakes that are hard to find.

>
>   - It allows/encourages symbols duplicated from a human point of view,
> leading to harder to read code; but this is also the point and good,
> it offers scope to pick shorter names, or at least hide
> names (you can still strive for globally unique names, in
> case the symbols later have to be made extern)

Omitting "static" also allows symbol duplication.  It just means that
such duplication is an error in the code - which may or may not be
caught at link time.

You /can/ have a coding convention that discourages duplicate symbol
names - even when using "static".  That might help a little in
understanding, but will quickly mean bloated source code that is harder
to read and follow (because you end up with long-winded symbol names
everywhere).

Such conventions are not scalable, are hopeless for multi-programmer
projects, terrible for code re-use, and can make code far harder to read
and write.

The scoping and naming in C is limited enough without omitting half the
features it has to deal with modularisation.

>
>   - it leads to accidental duplication, static int foo = 123 in a header

It is quite simple - don't do that.  It is appropriate for constant data
- "static const int foo = 123;" in a header will be fine, because "foo"
has the same value everywhere and is likely to be "optimised away".
That is the reason C++ makes "const int foo = 123;" effectively static.

Headers (in C, and mostly in C++) are for /declarations/, not
definitions - at least if you want to write structured and modular code.

>
>   - There are toolsets that don't resolve statics in disassembly

Statics are local to the file.  Disassemblies should show them when they
are used.  For the tiny, tiny proportion of C programmers that ever use
a disassembler, if their toolchains are not good enough then they should
get better toolchains.  It should /never/ be a problem when using
assembly listing files generated by the compiler, which are almost
always more useful than disassembling object code.

Making a coding convention to suit this requirement is like making
gloves with 6 fingers so that they fit people with an extra digit.

>
>   - It only allows for sharing within a file and hiding from all others,
> it doesn't allow sharing for within a few files and hiding from others

C has no good way to allow sharing between a few files and hiding from
others.  Such shared identifiers must be program-wide global.  But that
does /not/ mean you should make /everything/ program-wide global!  It
means you should minimise such sharing, prefix such shared names in a
way likely to minimise conflicts, and organise your source code modules
as best you can.

>
>   - It sort of doesn't work with "unity builds" old fashioned LTO/LTCG where 
> one
> source file includes the rest

It works /exactly/ as well.  Such "unity builds" ne

Re: extern const initialized warns in C

[David Brown]

On 22/01/2018 11:14, Jay K wrote:

I  meant:


extern const foo = 123;


does not warn in C++, but by these arguments, should.



Yes, I think it should.  But I am a compiler user, not a compiler 
author, so my bias is strongly towards /my/ code rather than a wider 
audience.





I understand that const int foo = 123 is static in C++.

It is this difference between C and C++ and the desire to write code 
that means the same in C and C++ is why I write extern const int foo = 
123 in the first place. If I was just writing C forever and not planning 
to compile as C++ ever then I would omit the redundant extern -- and not 
get a warning -- the other inconsistency!


As I suggested, put the declaration in the header and the definition in 
the source file.  Then it is the same code for C and C++, works 
correctly, and gives no warnings no matter what flags you use.  And it 
is modular, structured, and lets programmers see exactly what is 
"exported" from that C file by looking in a short header rather than 
digging through a long source file.





To repeat for clarity:


  1 C: extern const int foo = 123; => warns, for reasons explained and 
understood even if not agreed


  2 C: const int foo = 123; => means the same thing, but no warning; 
inconsistent?


  3 C++: extern const int foo = 123; => also no warning, inconsistent?



The prohibition against file level static is actually quite widespread 
and adhered to.




Can you give references or links?  As I say, I think such a convention 
is /seriously/ wrong.


(There are plenty of other conventions that I think are wrong - even 
famous and "professional" standards like MISRA have some daft ideas.)




Along with it, of course, comes a mandate to pick globally unique names.


That mandate I can understand.  There are rational justifications for 
it, even though I don't agree with them and think it is unworkable 
except for very small and limited programs.  But there is nothing to 
stop you using "static" along with the globally unique names.




Given a large C code base with many external identifiers, the extra step 
of barring static and requiring incrementally more symbols to be unique 
is not clearly a big deal. I already have to ensure uniqueness of many 
symbols, what is a few more?




The majority of your file-level identifiers in a C file will normally 
/not/ be exported - they will only ever be used internally.  I have no 
statistics, but I'd guess that under 10% of most C file's file-level 
identifiers are used externally in my "average" C file.  That means only 
10% need to be globally unique.





But I understand, perhaps in other systems, statics vastly outnumber 
externs, and the condition would be a large increment not a small one.




Yes, exactly.  Of course this sort of thing will vary somewhat depending 
on the type of coding you are doing.





While I struggle not bite my tongue advocating switching from C to C++, 
this factor seems much less sigificant or harmful.


I don't follow.  If you want to switch from C to C++, that's fine by me 
:-)  C++ gives you namespaces, which gives you another way to group your 
identifiers and control their scope.




I have actually seen and heard of the accidental duplication of symbols 
due to static multiple times.


If you put your static variables in the C file, it is not "accidental" 
duplication - they are different objects.  If you put static (non-const) 
variables in your header files, you have misunderstood how to use header 
files in C programming.




I also do a "unity build" (one source file includes the rest) of my own 
code to eek out a little extra performance (even while already using 
LTO/LTCG) and had to work out the static duplication.




Using "static" does not hinder that.  And it can be a significant 
benefit in performance.  So far, we have been looking at this from the 
viewpoint of code correctness, easy of reading and writing the code, and 
avoiding subtle errors.  But since you are interested in performance or 
code size, then you /really/ should be using "static" at every possible 
opportunity.  It makes a large difference - both for code size and 
speed, when you have "static" on your variables and functions that don't 
have to be globally visible.





You mention the debugger ease of use.

I haven't seen debuggers struggle with static but I have seen file 
dumpers struggle (not objdump per se, but similar)


I use the debugger as a better disassembler instead for this very 
reason, debugger can just load up a file for examination without running 
anything.




And then, you should realize there are very large systems that output 
nightly builds with symbols, taking hours on fast machines, where 
individual developers only build their little part of the tree, but 
debug anything.




I realise that.  And "static" is your friend there too.  I could not 
imagine trying to use a large build system or multiple developers where 
everything is global and shared, and you

Re: extern const initialized warns in C

[Jay K]

 > If you put static (non-const)
 > variables in your header files, you have misunderstood how to use header
 > files in C programming.


 Not me, and usually const, but people do it, both.
 Even the consts can get duplicated.
 Even the simple C++
   const int one = 1;


 I can take the address of.
 It is also unusual and maybe dumb. There are too many programmers
 writing C and C++ with too little oversight to rule these out.


 The static prohibition might be too closed to identity.


 I understand about the local edit, but it behooves
 one to make the non-edited build debugable too.


 I know you can only go so far, can't litter it with printf
 or breakpoints or volatile, nor can compile it unoptimized
 and ship it, but uniquifying function/data names seems
 maybe affordable for debuggability of official builds.


  > If you want to switch from C to C++, that's fine by me


 But the rest of my team has to agree.


  > C++ gives you namespaces, which gives you
  > nother way to group your identifiers and control their scope.


I know *all* about C++ but I think for newbies it is best
to start out focusing on member functions.
Pretend there is a type global namespace to replace C's function
global namespace. That is a huge improvement on its own.


 > It makes a large difference - both for code size and speed


 I only recently learned of how static impacts ELF visibility
 and therefore performance. Windows does not work this way,
 and I'm sure MacOS does either. Non-static on Windows does not imply
 replacable at dynamic link time, not even if the function is exported.
 Symbols are always resolved directly within the dll/sharedobject by
 the static linker if they are present with no pointer or stub in the way.
 (Ok, if you incorrectly annotate as __declspec(dllexport) and you don't
 use LTO/LTCG, then you will call through a pointer, but no stub,
 no actual inter-positionableness, and it is a rare occurence.)


 There is also always a two level namespace -- imported functions are qualified
 by the dll name they are expected to be in. For multiple dlls to export
 the same function name creates no ambiguity and implies no replacement
 of one by the other, and no semantic difference depending on load order.
 Unless someone writes very wierd code calling dlopen/dlsym like crazy.
 There is no LD_PRELOAD, slight loss, and replacing e.g. operator new
 isn't really possible process-wide, only within the scope of the static link,
 and even that is so rare, that it is probably sufficient.


 There is no going out of the way to accurately simulate the static linker
 at dynamic link time. Functions are only exported if they are annotated
 in source or listed in a separate file. Not just by being non-static.


  - Jay



From: David Brown 
Sent: Monday, January 22, 2018 10:42 AM
To: Jay K; gcc
Subject: Re: extern const initialized warns in C



On 22/01/2018 11:14, Jay K wrote:
> I  meant:
>
>
> extern const foo = 123;
>
>
> does not warn in C++, but by these arguments, should.
>

Yes, I think it should.  But I am a compiler user, not a compiler
author, so my bias is strongly towards /my/ code rather than a wider
audience.

>
>
> I understand that const int foo = 123 is static in C++.
>
> It is this difference between C and C++ and the desire to write code
> that means the same in C and C++ is why I write extern const int foo =
> 123 in the first place. If I was just writing C forever and not planning
> to compile as C++ ever then I would omit the redundant extern -- and not
> get a warning -- the other inconsistency!

As I suggested, put the declaration in the header and the definition in
the source file.  Then it is the same code for C and C++, works
correctly, and gives no warnings no matter what flags you use.  And it
is modular, structured, and lets programmers see exactly what is
"exported" from that C file by looking in a short header rather than
digging through a long source file.

>
>
> To repeat for clarity:
>
>
>   1 C: extern const int foo = 123; => warns, for reasons explained and
> understood even if not agreed
>
>   2 C: const int foo = 123; => means the same thing, but no warning;
> inconsistent?
>
>   3 C++: extern const int foo = 123; => also no warning, inconsistent?
>
>
>
> The prohibition against file level static is actually quite widespread
> and adhered to.
>

Can you give references or links?  As I say, I think such a convention
is /seriously/ wrong.

(There are plenty of other conventions that I think are wrong - even
famous and "professional" standards like MISRA have some daft ideas.)

>
> Along with it, of course, comes a mandate to pick globally unique names.

That mandate I can understand.  There are rational justifications for
it, even though I don't agree with them and think it is unworkable
except for very small and limited programs.  But there is nothing to
stop you using "static" along with the globally unique names.

>
> Given a large C code base wi

Retpolines and CFI

[Florian Weimer]

I tried this:

struct C {
  virtual ~C();
  virtual void f();
};

void
f (C *p)
{
  p->f();
  p->f();
}

with r256939 and -mindirect-branch=thunk -O2 on x86-64 GNU/Linux, and 
got this:


_Z1fP1C:
.LFB0:
.cfi_startproc
pushq   %rbx
.cfi_def_cfa_offset 16
.cfi_offset 3, -16
movq(%rdi), %rax
movq%rdi, %rbx
jmp .LIND1
.LIND0:
pushq   16(%rax)
jmp __x86_indirect_thunk
.LIND1:
call.LIND0
movq(%rbx), %rax
movq%rbx, %rdi
popq%rbx
.cfi_def_cfa_offset 8
movq16(%rax), %rax
jmp __x86_indirect_thunk_rax
.cfi_endproc

This doesn't look quite right.  x86-64 is supposed to have asynchronous 
unwind tables by default, but there is nothing that reflects the change 
in the (relative) frame address after .LIND0.  I think that region 
really has to be moved outside of the .cfi_startproc/.cfi_endproc bracket.


There is a different issue with the think itself.

__x86_indirect_thunk_rax:
.LFB2:
.cfi_startproc
call.LIND5
.LIND4:
pause
lfence
jmp .LIND4
.LIND5:
mov %rax, (%rsp)
ret
.cfi_endproc

If a signal is delivered after the mov has executed, the unwinder will 
eventually unwind through the signal frame and hit 
__x86_indirect_thunk_rax.  It does not treat it as a signal frame, so 
the return address of the stack is decremented by one, in an attempt to 
obtain a program counter value which is within the call instruction. 
However, in this scenario, the return address is actually the start of 
the function, and subtracting one moves the program counter out of the 
unwind region for that function.


Both issues are visible in GDB if you set breakpoints in the proper 
places because the frame information used for debugging is incorrect as 
well.


This probably does not concern the kernel that much, but it is 
definitely a problem for userspace.


Thanks,
Florian

Re: extern const initialized warns in C

[David Brown]

Hi,

I think we are getting quite a bit off-topic for the gcc list here.  We 
should probably take the discussion to somewhere like comp.lang.c.  So I 
shall limit things to just a couple of points to round off.


On 22/01/2018 12:27, Jay K wrote:

  > If you put static (non-const)
  > variables in your header files, you have misunderstood how to use header
  > files in C programming.

  Not me, and usually const, but people do it, both.
  Even the consts can get duplicated.
  Even the simple C++
    const int one = 1;




If they get allocated memory, then this will be duplicated across all 
units that have the same const (or static const for C).  But usually 
small constants do not get put in memory - they get used directly, and 
the opportunities for constant propagation optimisations typically 
outweigh the occasional duplication of memory.  Even taking the address 
of a const does not force it to have a memory allocation (gcc is smart 
here), and if you want you can use "-fmerge-all-constants" to save space 
on duplicate constants, at the cost of possibly causing trouble for code 
that expects separate constants to have separate addresses.



  I can take the address of.
  It is also unusual and maybe dumb. There are too many programmers
  writing C and C++ with too little oversight to rule these out.


  The static prohibition might be too closed to identity.

  I understand about the local edit, but it behooves
  one to make the non-edited build debugable too.


We are taking about wildly different kinds of debugging here.



  I know you can only go so far, can't litter it with printf
  or breakpoints or volatile, nor can compile it unoptimized
  and ship it, but uniquifying function/data names seems
  maybe affordable for debuggability of official builds.

   > If you want to switch from C to C++, that's fine by me

  But the rest of my team has to agree.

   > C++ gives you namespaces, which gives you
   > nother way to group your identifiers and control their scope.


I know *all* about C++ but I think for newbies it is best
to start out focusing on member functions.
Pretend there is a type global namespace to replace C's function
global namespace. That is a huge improvement on its own.



There is no pretence needed - there /is/ the same global namespace. 
Named and anonymous namespaces are an addition, not a replacement.




  > It makes a large difference - both for code size and speed

  I only recently learned of how static impacts ELF visibility
  and therefore performance. 


Elf visibility is a different thing.  Symbols can have external linkage 
for C but hidden visibility in elf.  Elf symbol visibility affects the 
time taken for linking, and only affects runtime performance in terms of 
dynamic linking time (time to start the executable and to link dynamic 
libraries).  It is something you want to handle carefully if you have 
large C++ libraries where you can easily end up with vast numbers of 
symbols.



Windows does not work this way,


Windows uses COFF format, not ELF.


  and I'm sure MacOS does either.


MacOS is BSD underneath, and uses ELF.


 Non-static on Windows does not imply
  replacable at dynamic link time, not even if the function is exported.
  Symbols are always resolved directly within the dll/sharedobject by
  the static linker if they are present with no pointer or stub in the way.
  (Ok, if you incorrectly annotate as __declspec(dllexport) and you don't
  use LTO/LTCG, then you will call through a pointer, but no stub,
  no actual inter-positionableness, and it is a rare occurence.)


None of this has anything to do with "static" and the significantly 
improved optimisations it allows.


Basically, if an object or function is static and does not "escape" (by 
passing its address out of the translation unit, or similar things), 
then the compiler knows all the uses of that object.  It can inline the 
function without making extra copies, it can remove some reads or writes 
to the variable or move them across function calls, it can omit static 
objects and functions that are not used, it can pre-compute results 
based on static consts because it knows they can never change. 
Declaring things "static" opens up a wide range of optimisations, many 
of which have knock-on effects allowing more optimisations.


If you are interested in code efficiency, and you are not using "static" 
whenever possible, you are hobbling your compiler.


mvh.,

David




  There is also always a two level namespace -- imported functions are 
qualified

  by the dll name they are expected to be in. For multiple dlls to export
  the same function name creates no ambiguity and implies no replacement
  of one by the other, and no semantic difference depending on load order.
  Unless someone writes very wierd code calling dlopen/dlsym like crazy.
  There is no LD_PRELOAD, slight loss, and replacing e.g. operator new
  isn't really possible process-wide, only within the scope of the 
static 

Re: Google Summer of Code 2018: Call for mentors and ideas

[Martin Liška]

On 01/19/2018 03:13 PM, Martin Jambor wrote:
> On Fri, Jan 19 2018, Martin Jambor wrote:
>> Hi,
>>
>> On Thu, Jan 18 2018, Martin Liška wrote:
>>> On 01/17/2018 06:54 PM, Martin Jambor wrote:
>>>
>> ...

 2) Martin Liška is willing to mentor either:
2a) -fsanitize=type (He provided URL https://reviews.llvm.org/D32197
but it gives me a 404 error) or its prototype, or
2b) bash code completion like:

 http://blog.llvm.org/2017/09/clang-bash-better-auto-completion-is.html
but frankly I am afraid it is too small to be a GSoC project, or
2c) textual representation of LTO stream a.k.a. lto-dump tool
>>>
>>> If there's an interest, I can specify in more detail these topics.
>>
>> First, we'll need (just) enough info to make the idea attractive for
>> students.  Then they will need to write a project proposal with
>> milestones and stuff
>> (https://google.github.io/gsocguides/student/writing-a-proposal).  I am
>> not sure how involved we will be at that point, but perhaps it is a good
>> idea to think about milestones anyway.
>>
>>> Note that
>>> David Malcolm is also interested in 2b) and he's willing to be co-mentor.
>>> The topic 2b) can be enlarged to an overhaul of option handling, with 
>>> possible
>>> rewritten of current AWK scripts.
>>
>> I am still a bit sceptical about the b option.  IIRC there was an
>> objection on the IRC that rewriting AWK scripts would require extensive
>> testing in a wide variety of often obscure environments.  That may make
>> it ill-suited for GSoC.  But well... I am not strictly against it, but
>> we may need to set the expectations accordingly ...which may not make it
>> attractive neither for students not for Google.
>>
> 
> Ah no, I confused myself.  That was Joseph's issue with the idea of
> replacing libiberty with gnulib.  The problem with this one was getting
> a consensus to move away from AWK to python which would introduce a new
> dependency of the gcc project.  Someone experienced (and determined)
> from the community would have to drive that decision, we may not expect
> a newcomer, let a alone a student, to do it.

Here I can probably trigger the discussion about the transition from AWK
to another programming language. It's expected that it will take some time.
That said, do we still want to list the project as candidate for GSoC?

Martin

> 
> Sorry for the confusion,
> 
> Martin
> 

Re: extern const initialized warns in C

[Jonathan Wakely]

On 21 January 2018 at 07:12, Jay K wrote:
> extern const int foo = 123;
>
>
>
> Why does this warn?
> This is a valid portable form, with the same meaning
> across all compilers, and, importantly, portably
> to C and C++.
>
> I explicitly do not want to say:
>
>   const int foo = 123
>
> because I want the code to be valid and have the same meaning
> in C and C++ (modulo name mangling).
>
> I end up with:
>
> // Workaround gcc warning.
> #ifdef __cplusplus
> #define EXTERN_CONST extern const
> #else
> #define EXTERN_CONST const
> #endif
>
>
> EXTERN_CONST int foo = 123;
>
> and having to explain it to people.

Why not simply:

extern const int foo;
const int foo = 123;

without the ugly macro?

Or, according to taste:

// Without an extern declaration, file-scope const variables are static:
extern const int foo;
const int foo = 123;

Or:

#ifdef __cplusplus
// Without an extern declaration, file-scope const variables are static:
extern const int foo;
#endif
const int foo = 123;

But your macro version is just ugly. And instead of complaining that
you have to explain it to people, why not explain it with a comment in
the code? That's what comments are for.

Re: extern const initialized warns in C

[Jonathan Wakely]

On 21 January 2018 at 12:08, Georg-Johann Lay wrote:
> Jay K schrieb:
>>
>> extern const int foo = 123;
>>
>> Why does this warn?
>> This is a valid portable form, with the same meaning
>> across all compilers, and, importantly, portably
>> to C and C++.
>
>
> I also wondered about this.
>
> In C99 §6.9.2 "External object definitions" there's actually
> the following example in clause 4:
>
> extern int i3 = 3; // definition, external linkage

That's a different case. There's no advantage to the 'extern' here,
because the code means the same thing in C and C++ without the
'extern', so just leave it out.

Re: Google Summer of Code 2018: Call for mentors and ideas

[Martin Liška]

On 01/19/2018 03:09 PM, Martin Jambor wrote:
> Hi,
> 
> On Thu, Jan 18 2018, Martin Liška wrote:
>> On 01/17/2018 06:54 PM, Martin Jambor wrote:
>>
> ...
>>>
>>> 2) Martin Liška is willing to mentor either:
>>>2a) -fsanitize=type (He provided URL https://reviews.llvm.org/D32197
>>>but it gives me a 404 error) or its prototype, or
>>>2b) bash code completion like:
>>>
>>> http://blog.llvm.org/2017/09/clang-bash-better-auto-completion-is.html
>>>but frankly I am afraid it is too small to be a GSoC project, or
>>>2c) textual representation of LTO stream a.k.a. lto-dump tool
>>
>> If there's an interest, I can specify in more detail these topics.
> 
> First, we'll need (just) enough info to make the idea attractive for
> students.  Then they will need to write a project proposal with
> milestones and stuff
> (https://google.github.io/gsocguides/student/writing-a-proposal).  I am
> not sure how involved we will be at that point, but perhaps it is a good
> idea to think about milestones anyway.

Hi.

If I'm correct time spent for programming is May 14, 2018 - August 6, 2018. 

Let me briefly describe the topics:

2a) A Type Sanitizer

Both LLVM and GCC compilers do share a common sanitizer library called 
libsanitizer.
The library has recently received support of typed-based sanitization (TySan).
Goal of the task would be to investigate and prototype usage of type-based
aliasing rules information, provided by GCC. As a result the information can be 
leverage
to detect violations of strict aliasing rules. Student can start with analysis 
of differences
of aliasing analysis in between the GCC and LLVM compiler.

2c) Textual Representation of LTO Object Files

Link-Time Optimization (LTO) is a technique where an intermediate 
representation of translation
units is serialized and loaded during link time. That leads to massive 
optimization opportunities
as a compiler can see the whole program. Current format of LTO object file is 
binary and we
would welcome to have also an equivalent textual representation. The format 
should be human readable
and easily adjustable. Such improvement would help a lot to reproduce and debug 
complication
LTO bugs that require multiple object files to be compiled.

Martin

> 
>> Note that
>> David Malcolm is also interested in 2b) and he's willing to be co-mentor.
>> The topic 2b) can be enlarged to an overhaul of option handling, with 
>> possible
>> rewritten of current AWK scripts.
> 
> I am still a bit sceptical about the b option.  IIRC there was an
> objection on the IRC that rewriting AWK scripts would require extensive
> testing in a wide variety of often obscure environments.  That may make
> it ill-suited for GSoC.  But well... I am not strictly against it, but
> we may need to set the expectations accordingly ...which may not make it
> attractive neither for students not for Google.
> 
> Martin
> 

Unused GCC builtins

[Manuel Rigger]

Hi everyone,

As part of my research, we have been analyzing the usage of GCC builtins
in 5,000 C GitHub projects. One of our findings is that many of these
builtins are unused, even though they are described in the documentation
(see https://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html#C-Extensions)
and obviously took time to develop and maintain. I’ve uploaded a CSV
file with the unused builtins to
http://ssw.jku.at/General/Staff/ManuelRigger/unused-builtins.csv.

Details: We downloaded all C projects from GitHub that had more than 80
GitHub stars, which yielded almost 5,000 projects with a total of more
than one billion lines of C code. We filtered GCC, forks of GCC, and
other compilers as we did not want to incorporate internal usages of GCC
builtins or test cases. We extracted all builtin names from the GCC
docs, and also tried to find such names in the source code, which we
considered as builtin usages. We excluded subdirectories with GCC or
Clang, and removed other false positives. In total, we found 320k
builtin usages in these projects, and 3030 unused builtins out of a
total of 6039 builtins.

What is your take on this? Do you believe that some of these unused
builtins could be removed from the GCC docs or deprecated? Or are they
used in special "niche" domains that we did not consider? If yes, do you
think it is worth to maintain them? Are some of them only used in C++
projects? Might it be possible to remove their implementations (which
has already happened for the Cilk Plus builtins)?

We would be glad for any feedback.

- Manuel

Re: Unused GCC builtins

[David Brown]

On 22/01/18 16:46, Manuel Rigger wrote:
> Hi everyone,
> 
> As part of my research, we have been analyzing the usage of GCC builtins
> in 5,000 C GitHub projects. One of our findings is that many of these
> builtins are unused, even though they are described in the documentation
> (see https://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html#C-Extensions)
> and obviously took time to develop and maintain. I’ve uploaded a CSV
> file with the unused builtins to
> http://ssw.jku.at/General/Staff/ManuelRigger/unused-builtins.csv.
> 
> Details: We downloaded all C projects from GitHub that had more than 80
> GitHub stars, which yielded almost 5,000 projects with a total of more
> than one billion lines of C code. We filtered GCC, forks of GCC, and
> other compilers as we did not want to incorporate internal usages of GCC
> builtins or test cases. We extracted all builtin names from the GCC
> docs, and also tried to find such names in the source code, which we
> considered as builtin usages. We excluded subdirectories with GCC or
> Clang, and removed other false positives. In total, we found 320k
> builtin usages in these projects, and 3030 unused builtins out of a
> total of 6039 builtins.
> 
> What is your take on this? Do you believe that some of these unused
> builtins could be removed from the GCC docs or deprecated? Or are they
> used in special "niche" domains that we did not consider? If yes, do you
> think it is worth to maintain them? Are some of them only used in C++
> projects? Might it be possible to remove their implementations (which
> has already happened for the Cilk Plus builtins)?
> 
> We would be glad for any feedback.
> 
> - Manuel
> 

Many of these are going to be used automatically by the compiler.  You
write "strdup" in your code, and the compiler treats it as
"__builtin_strdup".  I don't know that such functions need to be
documented as extensions, but they are certainly in use.

You will also find that a large number of the builtins are for specific
target processors, and projects using them are not going to turn up on
GitHub.  They will be used in embedded software that is not open source.

I am sure there are builtins that are rarely or never used - but I doubt
if it is anything like as many as you have identified from this survey.

Re: Unused GCC builtins

[Joel Sherrill]

On 1/22/2018 9:55 AM, David Brown wrote:

On 22/01/18 16:46, Manuel Rigger wrote:

Hi everyone,

As part of my research, we have been analyzing the usage of GCC builtins
in 5,000 C GitHub projects. One of our findings is that many of these
builtins are unused, even though they are described in the documentation
(see https://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html#C-Extensions)
and obviously took time to develop and maintain. I’ve uploaded a CSV
file with the unused builtins to
http://ssw.jku.at/General/Staff/ManuelRigger/unused-builtins.csv.

Details: We downloaded all C projects from GitHub that had more than 80
GitHub stars, which yielded almost 5,000 projects with a total of more
than one billion lines of C code. We filtered GCC, forks of GCC, and
other compilers as we did not want to incorporate internal usages of GCC
builtins or test cases. We extracted all builtin names from the GCC
docs, and also tried to find such names in the source code, which we
considered as builtin usages. We excluded subdirectories with GCC or
Clang, and removed other false positives. In total, we found 320k
builtin usages in these projects, and 3030 unused builtins out of a
total of 6039 builtins.

What is your take on this? Do you believe that some of these unused
builtins could be removed from the GCC docs or deprecated? Or are they
used in special "niche" domains that we did not consider? If yes, do you
think it is worth to maintain them? Are some of them only used in C++
projects? Might it be possible to remove their implementations (which
has already happened for the Cilk Plus builtins)?

We would be glad for any feedback.

- Manuel



Many of these are going to be used automatically by the compiler.  You
write "strdup" in your code, and the compiler treats it as
"__builtin_strdup".  I don't know that such functions need to be
documented as extensions, but they are certainly in use.

You will also find that a large number of the builtins are for specific
target processors, and projects using them are not going to turn up on
GitHub.  They will be used in embedded software that is not open source.

I am sure there are builtins that are rarely or never used - but I doubt
if it is anything like as many as you have identified from this survey.



My first thought was that there is a lot of free and open source 
software that is not hosted at github. Larger projects are often 
self-hosted. Does this list cover all GNU, Savannah, sourceware.org, 
Apache, KDE, *BSD, Mozilla, etc projects?


You might get lucky and some like RTEMS and FreeBSD (I think) have
a github mirror. But github is not the entire universe of free and
open source software.

--joel sherrill
RTEMS

Re: Unused GCC builtins

[Jakub Jelinek]

On Mon, Jan 22, 2018 at 04:55:42PM +0100, David Brown wrote:
> Many of these are going to be used automatically by the compiler.  You
> write "strdup" in your code, and the compiler treats it as
> "__builtin_strdup".  I don't know that such functions need to be
> documented as extensions, but they are certainly in use.
> 
> You will also find that a large number of the builtins are for specific
> target processors, and projects using them are not going to turn up on
> GitHub.  They will be used in embedded software that is not open source.

Not just that.  If the statistics e.g. ignored GCC headers, then obviously
it will miss most of the target builtins, because the normal and only
supported way for the target builtins is to use them through the intrinsic
inline functions or macros provided by those headers.
So, take those out (usually a vendor ABI is something that says what
intrinsics are provided, so even if you made statistics on what intrinsic is
used in the 5000 most popular projects, we still couldn't remove them) and
taking out the above category, where the builtins are just an alternative
for a standard function and depending on prototype and chosen standard some
functions are treated like builtins, pretty much nothing remains in your
survey.

Jakub

Re: Unused GCC builtins

[Andrew Pinski]

On Mon, Jan 22, 2018 at 7:55 AM, David Brown  wrote:
> On 22/01/18 16:46, Manuel Rigger wrote:
>> Hi everyone,
>>
>> As part of my research, we have been analyzing the usage of GCC builtins
>> in 5,000 C GitHub projects. One of our findings is that many of these
>> builtins are unused, even though they are described in the documentation
>> (see https://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html#C-Extensions)
>> and obviously took time to develop and maintain. I’ve uploaded a CSV
>> file with the unused builtins to
>> http://ssw.jku.at/General/Staff/ManuelRigger/unused-builtins.csv.
>>
>> Details: We downloaded all C projects from GitHub that had more than 80
>> GitHub stars, which yielded almost 5,000 projects with a total of more
>> than one billion lines of C code. We filtered GCC, forks of GCC, and
>> other compilers as we did not want to incorporate internal usages of GCC
>> builtins or test cases. We extracted all builtin names from the GCC
>> docs, and also tried to find such names in the source code, which we
>> considered as builtin usages. We excluded subdirectories with GCC or
>> Clang, and removed other false positives. In total, we found 320k
>> builtin usages in these projects, and 3030 unused builtins out of a
>> total of 6039 builtins.
>>
>> What is your take on this? Do you believe that some of these unused
>> builtins could be removed from the GCC docs or deprecated? Or are they
>> used in special "niche" domains that we did not consider? If yes, do you
>> think it is worth to maintain them? Are some of them only used in C++
>> projects? Might it be possible to remove their implementations (which
>> has already happened for the Cilk Plus builtins)?
>>
>> We would be glad for any feedback.
>>
>> - Manuel
>>
>
> Many of these are going to be used automatically by the compiler.  You
> write "strdup" in your code, and the compiler treats it as
> "__builtin_strdup".  I don't know that such functions need to be
> documented as extensions, but they are certainly in use.
>
> You will also find that a large number of the builtins are for specific
> target processors, and projects using them are not going to turn up on
> GitHub.  They will be used in embedded software that is not open source.

And the many of the target ones are used indirectly via another
function/macro (e.g. __builtin_ia32_ptestc256).  The function/macro is
defined in a header that GCC  provides too.

Thanks,
Andrew

>
> I am sure there are builtins that are rarely or never used - but I doubt
> if it is anything like as many as you have identified from this survey.
>
>
>

Re: extern const initialized warns in C

[Vincent Lefevre]

On 2018-01-22 10:53:55 +0100, David Brown wrote:
> On 22/01/2018 10:31, Jay K wrote:
> > 
> > By this argument there is a missing warning for the equivalent:
> > 
> >    const int foo = 123;
> > 
> > with no previous extern declaration.
> 
> I would like to see such a warning.  There is "-Wmissing-declarations",
> but that applies only to functions and not to objects.

I would like such a warning too. FYI, a few days I fixed in bug in
MPFR, which was defining an exported symbol outside of its prefix
under some conditions and could yield problems with multiple defined
symbols once linked with GMP. This remained undetected since this
was internal to some file and was related to code obtained from GMP
(with the preprocessor involved too). Such a warning would have
detected a potential issue via automated tests since the symbol
isn't pre-declared in .h files.

-- 
Vincent Lefèvre  - Web: 
100% accessible validated (X)HTML - Blog: 
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)

Re: Status of m32c target?

[Paul.Koning]

> On Jan 22, 2018, at 5:17 AM, Trevor Saunders  wrote:
> 
> On Mon, Jan 22, 2018 at 10:57:35AM +0100, Martin Jambor wrote:
>> Hi,
>> 
>> On Fri, Jan 19 2018, Sandra Loosemore wrote:
>>> On 01/19/2018 10:14 AM, Jeff Law wrote:
>>> 
 cc0 needs to die.  That doesn't mean that any particular target needs to
 be dropped -- it just  means that someone has to step forward to do the
 conversion.
>>> 
>>> Unifying two parallel threads:  might this be a good project for GSoC?
>> 
>> I have no idea how big the task would be but it seems it could be quite
>> difficult.  Therefore we would need an experienced and extraordinarily
>> willing to help mentor.
> 
> I'd be willing to help students where I can, but unfortunately I suspect
> this isn't a great project.  My memory of looking at what was involved
> is that you needed to learn a lot about how rtl worked to even
> understand what needed to be done much less actually change things and
> then test them.  That said maybe if we don't tell them its really hard
> they'll go and do it and prove me wrong.

Not only that, but it obviously requires understanding in detail how the 
particular target architecture works, for each target you want to rewrite.  
With good instruction set architecture documentation that's doable though not 
necessarily easy; if good documentation does not exist then it becomes harder 
still.

paul

Re: Bugzilla timing out

[Frank Ch. Eigler]

Hi -

> Problems are still occurring for me; Bugzilla gives me 504 Gateway
> Time-outs when I try to access it tonight...

OK, we reworked some of the database routine maintenance workload,
e.g., a nightly cleanup pass that was quite likely excessive, and
will keep monitoring.


- FChE

Re: Unused GCC builtins

[Florian Weimer]

* Manuel Rigger:

> Details: We downloaded all C projects from GitHub that had more than 80
> GitHub stars, which yielded almost 5,000 projects with a total of more
> than one billion lines of C code. We filtered GCC, forks of GCC, and
> other compilers as we did not want to incorporate internal usages of GCC
> builtins or test cases. We extracted all builtin names from the GCC
> docs, and also tried to find such names in the source code, which we
> considered as builtin usages.

You actually need to compile the sources with an instrumented compiler
to discover uses of built-ins.  Not all references will have verbatim,
textual references in source code, but their names are constructed
using preprocessor macros.  This happens for the majority of the
floating-point-related built-ins you listed, I think.

Lvn vrtsion

[Embargador]

LVM 2.02.168 2016-11-30 libary 1.02.137 Driver Versuon 4.35.0 :/#

[Embargador]

Re: Unstable build/host qsorts causing differing generated target code

[Michael Matz]

Hi,

On Fri, 12 Jan 2018, Joseph Myers wrote:

> On Fri, 12 Jan 2018, Alexander Monakov wrote:
> 
> > No. The qsort_chk effort was limited to catching instances where comparators
> > are invalid, i.e. lack anti-commutativity (may indicate A < B < A) or
> > transitivity property (may indicate A < B < C < A). Fixing them doesn't
> > imply making corresponding qsort invocations stable.
> 
> Incidentally, does it detect being invalid because of comparing A != A?  
> (I don't know if qsort implementations ever do compare an element to 
> itself, but I did once notice in a patch review that a comparator could 
> have compared an element unequal to itself).

Many years ago I ran into a qsort implementation (some Sun one? too long 
ago) where indeed qsort was calling cmp(a,a).  I ran into this case only 
because one of the cmp() implementations didn't return 0 then, and qsort 
was Very Surprised by that :)


Ciao,
Michael.

Re: Status of m32c target?

[Segher Boessenkool]

Hi!

On Mon, Jan 22, 2018 at 10:57:35AM +0100, Martin Jambor wrote:
> On Fri, Jan 19 2018, Sandra Loosemore wrote:
> > On 01/19/2018 10:14 AM, Jeff Law wrote:
> >
> >> cc0 needs to die.  That doesn't mean that any particular target needs to
> >> be dropped -- it just  means that someone has to step forward to do the
> >> conversion.
> >
> > Unifying two parallel threads:  might this be a good project for GSoC?
> 
> I have no idea how big the task would be but it seems it could be quite
> difficult.  Therefore we would need an experienced and extraordinarily
> willing to help mentor.

Firstly, you need to know RTL quite well.

Secondly, you need to know the target architecture really well.

And then, you "simply" need to find all places in the backend that
generate insns affecting the condition flags, and change those as
needed.  (And that is *not* just patterns in the .md files, in the
unlucky cases; you need to go over it *all*).

For quite a long time you cannot actually run any code to test, either,
so you'll have to stare at assembler code instead.

It's a bit like writing a new backend, except you have all this existing
code to worry about as well.  Unless you start from scratch (which may
not be such a bad idea: you get to modernise it all, and it isn't _really_
from scratch, you can peek at the old code and copy stuff from it).

But writing a backend is too much for a GSoC, even a small one.


Segher

https://open.spotify.com/track/1mXVgsBdtIVeCLJnSnmtdV

[Tanya Carter]

Sent from my iPhone