On 30 Jan 2009, at 05:11, Ross Smith wrote:
Zoltán Kócsi wrote:
On Thu, 29 Jan 2009 08:53:10 +
Andrew Haley a...@redhat.com wrote:
We're talking about gcc on ARM. gcc on ARM uses 0 for the null
pointer constant, therefore a linker cannot place an object at
address zero. All the rest is
This sounds like a genuine bug in gcc, then. As far as I can see,
Andrew is right -- if the ARM hardware requires a legitimate object
to be placed at address zero, then a standard C compiler has to use
some other value for the null pointer.
I think changing that would cause more trouble than
Erik Trulsson wrote:
On Wed, Jan 28, 2009 at 04:39:39PM +, Andrew Haley wrote:
6.3.2.3 Pointers
If a null pointer constant is converted to a pointer type, the
resulting pointer, called a null pointer, is guaranteed to compare
unequal to a pointer to any object or function.
This
On 2009-01-28 16:34:29 -0500, Robert Dewar wrote:
If 0 is a valid address, then it is improper for the compiler to use
0 to represent the null pointer.
I disagree. You can represent addresses with some bit set. If in
practice (at the processor level), this bit is ignored, then this
can really
On 2009-01-29 08:53:10 +, Andrew Haley wrote:
We're talking about gcc on ARM. gcc on ARM uses 0 for the null pointer
constant, therefore a linker cannot place an object at address zero.
All the rest is irrelevant.
How about using volatile, as suggested by Robert Dewar (if the gcc
On Thu, 29 Jan 2009 08:53:10 +
Andrew Haley a...@redhat.com wrote:
Erik Trulsson wrote:
On Wed, Jan 28, 2009 at 04:39:39PM +, Andrew Haley wrote:
6.3.2.3 Pointers
If a null pointer constant is converted to a pointer type, the
resulting pointer, called a null pointer, is
Zoltán Kócsi wrote:
On Thu, 29 Jan 2009 08:53:10 +
Andrew Haley a...@redhat.com wrote:
We're talking about gcc on ARM. gcc on ARM uses 0 for the null
pointer constant, therefore a linker cannot place an object at
address zero. All the rest is irrelevant.
Um, the linker *must* place the
On Tue, 27 Jan 2009 07:08:51 -0500
Robert Dewar de...@adacore.com wrote:
James Dennett wrote:
I don't know how much work it would be to disable this optimization
in gcc.
To me, it is always troublesome to talk of disable this optimization
in a context like this. The program in question
Zoltán Kócsi wrote:
On Tue, 27 Jan 2009 07:08:51 -0500
Robert Dewar de...@adacore.com wrote:
James Dennett wrote:
I don't know how much work it would be to disable this optimization
in gcc.
To me, it is always troublesome to talk of disable this optimization
in a context like this. The
Zoltán Kócsi wrote:
I don't mean to complain, but I happen to work with embedded systems.
I program them in C, or at least in a language that uses the syntactic
elements of C.
Yes, and that is an important distinction. If you program in C, you
should program in C (and know the language).
No, this is since C90; nothing has changed in this area. NULL
doesn't mean address 0, it means nothing. The C statement
if (ptr)
doesn't mean if ptr does not point to address zero, it means if ptr
points to something.
A question then:
How can I make a pointer to point to the integer
Zoltán Kócsi zol...@bendor.com.au writes:
How can I make a pointer to point to the integer located at address
0x0? It is a perfectly valid object, it exists, therefore I should be
able to get its address? In fact, if I have a CPU that starts its data
RAM at 0, then the first data object
Zoltán Kócsi wrote:
You say: You can't dereference a NULL pointer
I say: You shouldn't dereference a NULL pointer
Yes, but the standard says can't in effect, and gives
no idea of what it might mean to dereference a null
pointer.
I shouldn't but I most certainly can. I can generate a NULL
Paweł Sikora wrote:
do you say that we can't write a valid/defined program in C for
e.g. arm architercture? on some arms, the object (vector interrupt table)
starts at adress 0x0.
If 0 is a valid address, then it is improper for the compiler to use
0 to represent the null pointer. You may
On Wed, Jan 28, 2009 at 04:39:39PM +, Andrew Haley wrote:
Zoltán Kócsi wrote:
No, this is since C90; nothing has changed in this area. NULL
doesn't mean address 0, it means nothing. The C statement
if (ptr)
doesn't mean if ptr does not point to address zero, it means if ptr
On Mon, Jan 26, 2009 at 11:52 PM, zol...@bendor.com.au wrote:
I was debugging a function and by inserting the debug statement crashed
the system. Some investigation revealed that gcc 4.3.2 arm-eabi (compiled
from sources) with -O2 under some circumstances assumes that if a pointer
is
On Tue, 27 Jan 2009, James Dennett wrote:
On Mon, Jan 26, 2009 at 11:52 PM, zol...@bendor.com.au wrote:
I was debugging a function and by inserting the debug statement crashed
the system. Some investigation revealed that gcc 4.3.2 arm-eabi (compiled
from sources) with -O2 under some
James Dennett wrote:
On Mon, Jan 26, 2009 at 11:52 PM, zol...@bendor.com.au wrote:
I was debugging a function and by inserting the debug statement crashed
the system. Some investigation revealed that gcc 4.3.2 arm-eabi (compiled
from sources) with -O2 under some circumstances assumes that if
On Tue, Jan 27, 2009 at 9:33 AM, Paolo Bonzini bonz...@gnu.org wrote:
James Dennett wrote:
On Mon, Jan 26, 2009 at 11:52 PM, zol...@bendor.com.au wrote:
I was debugging a function and by inserting the debug statement crashed
the system. Some investigation revealed that gcc 4.3.2 arm-eabi
However, -fno-delete-null-pointer-checks will do.
Not for PTA though ;)
Care to expand?
Paolo
Not for PTA though ;)
Care to expand?
PTA tracks points-to-NULL as pointing to nothing.
This probably should be conditional on -fdelete-null-pointer-checks.
Otherwise *NULL and *anything won't alias.
Yes, you're right. I'll see if I can construct a testcase and a patch.
BTW, I was
On Tue, Jan 27, 2009 at 11:35 AM, Paolo Bonzini bonz...@gnu.org wrote:
Not for PTA though ;)
Care to expand?
PTA tracks points-to-NULL as pointing to nothing.
This probably should be conditional on -fdelete-null-pointer-checks.
Otherwise *NULL and *anything won't alias.
Yes, you're right.
Paolo Bonzini wrote:
Not for PTA though ;)
Care to expand?
PTA tracks points-to-NULL as pointing to nothing.
This probably should be conditional on -fdelete-null-pointer-checks.
Otherwise *NULL and *anything won't alias.
Yes, you're right. I'll see if I can construct a testcase and a
On Tue, Jan 27, 2009 at 11:29 AM, Paolo Bonzini bonz...@gnu.org wrote:
However, -fno-delete-null-pointer-checks will do.
Not for PTA though ;)
Care to expand?
PTA tracks points-to-NULL separately (as pointing to nothing). See
/* x = integer is all glommed to a single variable, which
Richard Guenther wrote:
On Tue, Jan 27, 2009 at 11:35 AM, Paolo Bonzini bonz...@gnu.org wrote:
Not for PTA though ;)
Care to expand?
PTA tracks points-to-NULL as pointing to nothing.
This probably should be conditional on -fdelete-null-pointer-checks.
Otherwise *NULL and *anything won't
James Dennett wrote:
I don't know how much work it would be to disable this optimization in gcc.
To me, it is always troublesome to talk of disable this optimization
in a context like this. The program in question is not C, and its
semantics cannot be determined from the C standard. If the
On Tue, 2009-01-27 at 00:11 -0800, James Dennett wrote:
On Mon, Jan 26, 2009 at 11:52 PM, zol...@bendor.com.au wrote:
I was debugging a function and by inserting the debug statement crashed
the system. Some investigation revealed that gcc 4.3.2 arm-eabi (compiled
from sources) with -O2
Laurent GUERBY wrote:
Just curious: is there a portable way to read from memory
address zero in C code? portable here means likely to work
on most compilers without exotic compile flags in 2009.
For C++, in *theory*, a reinterpret_castvoid*(0) yields a pointer with
the value 0 that is not
On 2009-01-27 07:08:51 -0500, Robert Dewar wrote:
[...]
Interestingly, my viewpoint on this from the compiler domain might
seem to be quite inconsistent. I think it is generally a bad idea
for compilers to aggressively optimize based on assumptions that
programs are free of these kinds of
Vincent Lefevre wrote:
On 2009-01-27 07:08:51 -0500, Robert Dewar wrote:
[...]
Interestingly, my viewpoint on this from the compiler domain might
seem to be quite inconsistent. I think it is generally a bad idea
for compilers to aggressively optimize based on assumptions that
programs are free
Sebastian Redl wrote:
Laurent GUERBY wrote:
Just curious: is there a portable way to read from memory
address zero in C code? portable here means likely to work
on most compilers without exotic compile flags in 2009.
For C++, in *theory*, a reinterpret_castvoid*(0) yields a pointer with
On Tue, Jan 27, 2009 at 05:42:04AM -0800, Daniel Jacobowitz wrote:
On Tue, Jan 27, 2009 at 06:52:18PM +1100, zol...@bendor.com.au wrote:
// Debug( tst-value );
if ( ! tst ) {
ptr-next = (void *) 0;
break;
}
This optimization comes up on the
* Laurent GUERBY:
Just curious: is there a portable way to read from memory
address zero in C code? portable here means likely to work
on most compilers without exotic compile flags in 2009.
No, C hasn't got a notion of addresses. The situation is likely to
get even more extreme, not less
On Tue, 2009-01-27 at 21:37 +0100, Florian Weimer wrote:
* Laurent GUERBY:
Just curious: is there a portable way to read from memory
address zero in C code? portable here means likely to work
on most compilers without exotic compile flags in 2009.
No, C hasn't got a notion of
Laurent GUERBY laur...@guerby.net writes:
Just curious: is there a portable way to read from memory
address zero in C code? portable here means likely to work
on most compilers without exotic compile flags in 2009.
char *my_null_pointer;
char fn() { return *my_null_pointer; }
It will be
Robert Dewar de...@adacore.com writes:
Interestingly, my viewpoint on this from the compiler domain might
seem to be quite inconsistent. I think it is generally a bad idea
for compilers to aggressively optimize based on assumptions that
programs are free of these kinds of mistakes. I would
On Jan 27, 2009, at 5:10 PM, Ian Lance Taylor wrote:
Chris Lattner clatt...@apple.com writes:
On Jan 27, 2009, at 1:10 PM, Ian Lance Taylor wrote:
Laurent GUERBY laur...@guerby.net writes:
Just curious: is there a portable way to read from memory
address zero in C code? portable here
Chris Lattner clatt...@apple.com writes:
On Jan 27, 2009, at 5:10 PM, Ian Lance Taylor wrote:
Chris Lattner clatt...@apple.com writes:
On Jan 27, 2009, at 1:10 PM, Ian Lance Taylor wrote:
Laurent GUERBY laur...@guerby.net writes:
Just curious: is there a portable way to read from memory
On Jan 27, 2009, at 10:47 PM, Ian Lance Taylor wrote:
Is LLVM smart enough to optimize that away, even when using shared
libraries?
Yes absolutely. Just build with -fvisibility-hidden or use an export
map to say that my_null_pointer is not exported. If it is static, it
will also do it at
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