Re: can we un-deprecate .ptr on arrays in @safe code? cf issue 18529

[Cym13 via Digitalmars-d]

On Wednesday, 28 February 2018 at 22:34:07 UTC, Dukc wrote:
I don't think a just iterated array is automatically set to 
null, so taking it's pointer won't hit a memory-proteted area. 
So undeprectating arr.ptr in @safe would break @safety and be a 
step backward.


If it cannot be proven safe by the compiler but is safe anyway it 
doesn't break @safe, it should just be @trusted. That's what 
@trusted is about, not about abusing functions to trick unsafe 
things into compiling @safe-ly.


If this is an issue, one can define a @trusted function which 
takes a starting pointer from array and casts it to size_t 
before returning it so memory corruption cannot happen via it.


On Tuesday, 27 February 2018 at 09:47:51 UTC, Stefan Koch wrote:

Checking if an array is the slice of another.


For that there is also std.array.overlap().

Re: can we un-deprecate .ptr on arrays in @safe code? cf issue 18529

[Dukc via Digitalmars-d]

I don't think a just iterated array is automatically set to null, 
so taking it's pointer won't hit a memory-proteted area. So 
undeprectating arr.ptr in @safe would break @safety and be a step 
backward.


If this is an issue, one can define a @trusted function which 
takes a starting pointer from array and casts it to size_t before 
returning it so memory corruption cannot happen via it.


On Tuesday, 27 February 2018 at 09:47:51 UTC, Stefan Koch wrote:

Checking if an array is the slice of another.


For that there is also std.array.overlap().

Re: can we un-deprecate .ptr on arrays in @safe code? cf issue 18529

[Walter Bright via Digitalmars-d]

On 2/27/2018 4:39 AM, Jonathan M Davis wrote:

Except that that's really not how @trusted is supposed to be used. The
programmer needs to verify that the caller is using a.ptr in a manner that
is actually @safe, because the compiler is not smart enough to determine
that for you. Wrapping it in an @trusted function means that the caller
won't get an error and that the programmer won't necessarily know that they
need to verify the calling code. It's the code that's using ptr that needs
to be verified, not the actual accessing of ptr.

Hiding the access to ptr within an @trusted function goes against that
entire idea of @trusted and makes it easy to use ptr without realizing that
you need to be checking the code that's using it, since you just called a
wrapper function to silence the compiler instead of listening the compiler
and studying the code using ptr to verify its @safety.


Yes, this bears repeating.

Re: can we un-deprecate .ptr on arrays in @safe code? cf issue 18529

[Seb via Digitalmars-d]

On Tuesday, 27 February 2018 at 11:33:04 UTC, Simen Kjærås wrote:
That only checks if the first element is the same. For a full 
'is slice' check you'd need something like this:


assert(a.ptr <= b.ptr && b.ptr + b.length <= a.ptr + 
a.length);


// Or:

auto c = b.ptr - a.ptr;
assert(c >= 0 && c + b.length <= a.length);

And trust me, the compiler complains about both of these. 
Possibly rightfully in the first example, but the latter never 
does anything scary with the given pointers.


--
  Simen


aka isSliceOf -> https://github.com/dlang/phobos/pull/6147

Re: can we un-deprecate .ptr on arrays in @safe code? cf issue 18529

[Timothee Cour via Digitalmars-d]

> Hm... borrowing from Timothee's suggestion:
> This would be fine and @safe, but may not be useful for all purposes. 
> However, it would fix your issue.


how about this: https://github.com/dlang/phobos/pull/6231

On Tue, Feb 27, 2018 at 12:09 PM, Steven Schveighoffer via
Digitalmars-d  wrote:
> On 2/27/18 3:00 PM, Steven Schveighoffer wrote:
>>
>> On 2/27/18 12:32 PM, Atila Neves wrote:
>>
>>> There's a common case where it's not equivalent - when the pointer is
>>> null. Imagine I have a C function I want to call:
>>>
>>> extern(C) void fun(int* things);
>>>
>>> Imagine also that it's ok to call with null. Well, now I can't use a
>>> slice to call this and have it be 1) @safe and 2) not throw RangeError. I
>>> ran into this the other way.
>>
>>
>> fun(x.length ? [0] : null);
>
>
> Hm... borrowing from Timothee's suggestion:
>
> @trusted @nogc pure nothrow
> T* pointer(T)(T[] a){
>return a.length > 0 ? a.ptr : null;
> }
>
> This would be fine and @safe, but may not be useful for all purposes.
> However, it would fix your issue.
>
> -Steve

Re: can we un-deprecate .ptr on arrays in @safe code? cf issue 18529

[Steven Schveighoffer via Digitalmars-d]

On 2/27/18 3:00 PM, Steven Schveighoffer wrote:

On 2/27/18 12:32 PM, Atila Neves wrote:

There's a common case where it's not equivalent - when the pointer is 
null. Imagine I have a C function I want to call:


extern(C) void fun(int* things);

Imagine also that it's ok to call with null. Well, now I can't use a 
slice to call this and have it be 1) @safe and 2) not throw 
RangeError. I ran into this the other way.


fun(x.length ? [0] : null);


Hm... borrowing from Timothee's suggestion:

@trusted @nogc pure nothrow
T* pointer(T)(T[] a){
   return a.length > 0 ? a.ptr : null;
}

This would be fine and @safe, but may not be useful for all purposes. 
However, it would fix your issue.


-Steve

Re: can we un-deprecate .ptr on arrays in @safe code? cf issue 18529

[Steven Schveighoffer via Digitalmars-d]

On 2/27/18 12:32 PM, Atila Neves wrote:

There's a common case where it's not equivalent - when the pointer is 
null. Imagine I have a C function I want to call:


extern(C) void fun(int* things);

Imagine also that it's ok to call with null. Well, now I can't use a 
slice to call this and have it be 1) @safe and 2) not throw RangeError. 
I ran into this the other way.


fun(x.length ? [0] : null);

I think even the compiler could elide the bounds check since you already 
did it, but I'm not sure that actually happens.


-Steve

Re: can we un-deprecate .ptr on arrays in @safe code? cf issue 18529

[ag0aep6g via Digitalmars-d]

On 02/27/2018 06:32 PM, Atila Neves wrote:

On Tuesday, 27 February 2018 at 12:39:04 UTC, Jonathan M Davis wrote:

[...]
In almost all cases, [0] is equivalent to a.ptr except that it does 
bounds checking, so it's actually @safe and thus doesn't need to be 
manually verified by the programmer, unlike your pointer function 
suggestion.


There's a common case where it's not equivalent - when the pointer is 
null. Imagine I have a C function I want to call:


extern(C) void fun(int* things);

Imagine also that it's ok to call with null. Well, now I can't use a 
slice to call this and have it be 1) @safe and 2) not throw RangeError. 
I ran into this the other way.


As Jonathan says, you have to manually verify that it's safe, because it 
generally isn't. `arr.ptr` can be invalid but not null, even in @safe code.


Consider:


void fun(int* things) @safe
{
int x = things is null ? 0 : *things;
import std.stdio;
writeln(x);
}

void main()
{
int[] arr;
fun(arr.ptr); /* Ok, prints "0". */

arr = [1, 2, 3];
fun(arr.ptr); /* Ok, prints "1".*/

arr = arr[$ .. $]; /* This is @safe. */
fun(arr.ptr); /* Not ok, prints garbage. */
}


The first two calls are actually safe and can be @trusted. The third 
call is invalid and must not be possible in @safe code.


Maybe it would be possible require `arr.ptr` to be valid or null in 
@safe code. This would outlaw `arr[$ .. $]` and bounds checking would 
have to catch it. I don't know if that would be any practical than 
banning `arr.ptr`.

Re: can we un-deprecate .ptr on arrays in @safe code? cf issue 18529

[Atila Neves via Digitalmars-d]

On Tuesday, 27 February 2018 at 12:39:04 UTC, Jonathan M Davis 
wrote:
On Tuesday, February 27, 2018 04:20:38 Timothee Cour via 
Digitalmars-d wrote:

[...]


Except that that's really not how @trusted is supposed to be 
used. The programmer needs to verify that the caller is using 
a.ptr in a manner that is actually @safe, because the compiler 
is not smart enough to determine that for you. Wrapping it in 
an @trusted function means that the caller won't get an error 
and that the programmer won't necessarily know that they need 
to verify the calling code. It's the code that's using ptr that 
needs to be verified, not the actual accessing of ptr.


Hiding the access to ptr within an @trusted function goes 
against that entire idea of @trusted and makes it easy to use 
ptr without realizing that you need to be checking the code 
that's using it, since you just called a wrapper function to 
silence the compiler instead of listening the compiler and 
studying the code using ptr to verify its @safety.



[...]


In almost all cases, [0] is equivalent to a.ptr except that 
it does bounds checking, so it's actually @safe and thus 
doesn't need to be manually verified by the programmer, unlike 
your pointer function suggestion.


There's a common case where it's not equivalent - when the 
pointer is null. Imagine I have a C function I want to call:


extern(C) void fun(int* things);

Imagine also that it's ok to call with null. Well, now I can't 
use a slice to call this and have it be 1) @safe and 2) not throw 
RangeError. I ran into this the other way.


Atila

Re: can we un-deprecate .ptr on arrays in @safe code? cf issue 18529

[Steven Schveighoffer via Digitalmars-d]

On 2/27/18 7:20 AM, Timothee Cour wrote:

this would be more bearable if there was a standard @trusted method to
get array `.ptr`, eg:
in `object.d` (so that it's indeed standard)

```
@trusted @nogc pure nothrow
auto pointer(T)(T a){
   return a.ptr;
}
```


That completely defeats the purpose of the restriction! Of course we 
aren't going to do that :)


This is the standard way (and it's on you to make sure you don't mess it 
up):


auto x = (() @trusted => a.ptr)();


again, the deprecation message is misleading because `[0]` isn't
equivalent to `a.ptr`


It isn't equivalent, but it is a @safe mechanism to do it. If you want 
to do unsafe things, then you need to use @trusted escapes, 
druntime/phobos is not going to help you there.


-Steve

Re: can we un-deprecate .ptr on arrays in @safe code? cf issue 18529

[Jonathan M Davis via Digitalmars-d]

On Tuesday, February 27, 2018 04:20:38 Timothee Cour via Digitalmars-d 
wrote:
> this would be more bearable if there was a standard @trusted method to
> get array `.ptr`, eg:
> in `object.d` (so that it's indeed standard)
>
> ```
> @trusted @nogc pure nothrow
> auto pointer(T)(T a){
>   return a.ptr;
> }
> ```

Except that that's really not how @trusted is supposed to be used. The
programmer needs to verify that the caller is using a.ptr in a manner that
is actually @safe, because the compiler is not smart enough to determine
that for you. Wrapping it in an @trusted function means that the caller
won't get an error and that the programmer won't necessarily know that they
need to verify the calling code. It's the code that's using ptr that needs
to be verified, not the actual accessing of ptr.

Hiding the access to ptr within an @trusted function goes against that
entire idea of @trusted and makes it easy to use ptr without realizing that
you need to be checking the code that's using it, since you just called a
wrapper function to silence the compiler instead of listening the compiler
and studying the code using ptr to verify its @safety.

>
> again, the deprecation message is misleading because `[0]` isn't
> equivalent to `a.ptr`
> having something like `pointer` (and making deprecation msg use that)
> would be a better mitigation

In almost all cases, [0] is equivalent to a.ptr except that it does bounds
checking, so it's actually @safe and thus doesn't need to be manually
verified by the programmer, unlike your pointer function suggestion.

If you have a use case where you need a.ptr rather than [0], then that
just means that you're going to have to verify that your code is @safe in
spite of using an @system operation and mark it @trusted if it is. Simply
treating a.ptr as @safe when it isn't or wrapping it in an @trusted function
just defeats the purpose of the whole @safety system.

Now, maybe in some simple cases, the compiler can be improved to detect that
what you're doing with a.ptr is actually @safe and not give an error, but as
long as it can't, any use of a.ptr needs to be verified by the programmer.

- Jonathan M Davis

Re: can we un-deprecate .ptr on arrays in @safe code? cf issue 18529

[Timothee Cour via Digitalmars-d]

this would be more bearable if there was a standard @trusted method to
get array `.ptr`, eg:
in `object.d` (so that it's indeed standard)

```
@trusted @nogc pure nothrow
auto pointer(T)(T a){
  return a.ptr;
}
```

again, the deprecation message is misleading because `[0]` isn't
equivalent to `a.ptr`
having something like `pointer` (and making deprecation msg use that)
would be a better mitigation



On Tue, Feb 27, 2018 at 3:56 AM, Jonathan M Davis via Digitalmars-d
 wrote:
> On Tuesday, February 27, 2018 11:33:04 Simen Kjærås via Digitalmars-d wrote:
>> And trust me, the compiler complains about both of these.
>> Possibly rightfully in the first example, but the latter never
>> does anything scary with the given pointers.
>
> As I understand it, the way that @safety checks generally work is they check
> whether a particular operation is @safe or not. They don't usually care
> about what is then done with the result. So, if you do something like take
> the address of something, that's immediately @system regardless of what you
> do with the result. That changes on some level with DIP 1000 and scope,
> because then it uses scope to ensure that the lifetime of stuff like
> pointers doesn't exceed the lifetime of what they point to so that it can
> then know that taking the address is @safe, but without DIP 1000, it takes
> very little for something to become @system. e.g. this is compiles with
> -dip1000 but otherwise doesn't:
>
> void main() @safe
> {
> int i;
> assert( !is null);
> }
>
> Now, the compiler does seem to be a bit smarter with dynamic arrays and ptr
> given that this compiles without -dip1000
>
> void main() @safe
> {
> int[] i;
> assert(i.ptr !is null);
> }
>
> However, this doesn't compile with -dip1000:
>
> void main() @safe
> {
> int[] i;
> auto j = i.ptr;
> assert(j !is null);
> }
>
> and not even this compiles with -dip1000:
>
> void main() @safe
> {
> int[] i;
> scope j = i.ptr;
> assert(j !is null);
> }
>
> though I'm inclined to think that that's a bug from what I understand of
> -dip1000.
>
> In any case, @safety checks tend to be fairly primitive, so once you start
> mucking around with pointers, it's not hard to write code that gets treated
> as @system because of a single expression in the code that is clearly @safe
> within the context of the function, but the compiler can't see it.
>
> And for better or worse, accessing a dynamic array's ptr member is now
> @system, because it's not @safe in all circumstances. If the compiler were
> smarter, then a number of uses of ptr would probably be @safe, but its
> analysis for stuff like that is usually pretty primitive, in part because
> making it sophisticated requires stuff like code flow analysis, which the
> compiler doesn't do a lot of, precisely because it is complicated and easy
> to get wrong. Walter is particularly leery about making it so that stuff is
> an error or not based on code flow analysis, and @safe falls into that camp.
> Clearly, some of that is going on with DIP 1000, but that seems to be
> largely by using the type system to solve the problem rather than doing much
> in the way of code flow analysis.
>
> - Jonathan M Davis
>
>

Re: can we un-deprecate .ptr on arrays in @safe code? cf issue 18529

[Jonathan M Davis via Digitalmars-d]

On Tuesday, February 27, 2018 11:33:04 Simen Kjærås via Digitalmars-d wrote:
> And trust me, the compiler complains about both of these.
> Possibly rightfully in the first example, but the latter never
> does anything scary with the given pointers.

As I understand it, the way that @safety checks generally work is they check
whether a particular operation is @safe or not. They don't usually care
about what is then done with the result. So, if you do something like take
the address of something, that's immediately @system regardless of what you
do with the result. That changes on some level with DIP 1000 and scope,
because then it uses scope to ensure that the lifetime of stuff like
pointers doesn't exceed the lifetime of what they point to so that it can
then know that taking the address is @safe, but without DIP 1000, it takes
very little for something to become @system. e.g. this is compiles with
-dip1000 but otherwise doesn't:

void main() @safe
{
int i;
assert( !is null);
}

Now, the compiler does seem to be a bit smarter with dynamic arrays and ptr
given that this compiles without -dip1000

void main() @safe
{
int[] i;
assert(i.ptr !is null);
}

However, this doesn't compile with -dip1000:

void main() @safe
{
int[] i;
auto j = i.ptr;
assert(j !is null);
}

and not even this compiles with -dip1000:

void main() @safe
{
int[] i;
scope j = i.ptr;
assert(j !is null);
}

though I'm inclined to think that that's a bug from what I understand of
-dip1000.

In any case, @safety checks tend to be fairly primitive, so once you start
mucking around with pointers, it's not hard to write code that gets treated
as @system because of a single expression in the code that is clearly @safe
within the context of the function, but the compiler can't see it.

And for better or worse, accessing a dynamic array's ptr member is now
@system, because it's not @safe in all circumstances. If the compiler were
smarter, then a number of uses of ptr would probably be @safe, but its
analysis for stuff like that is usually pretty primitive, in part because
making it sophisticated requires stuff like code flow analysis, which the
compiler doesn't do a lot of, precisely because it is complicated and easy
to get wrong. Walter is particularly leery about making it so that stuff is
an error or not based on code flow analysis, and @safe falls into that camp.
Clearly, some of that is going on with DIP 1000, but that seems to be
largely by using the type system to solve the problem rather than doing much
in the way of code flow analysis.

- Jonathan M Davis

Re: can we un-deprecate .ptr on arrays in @safe code? cf issue 18529

[Simen Kjærås via Digitalmars-d]

On Tuesday, 27 February 2018 at 09:58:00 UTC, bauss wrote:

On Tuesday, 27 February 2018 at 09:47:51 UTC, Stefan Koch wrote:

On Tuesday, 27 February 2018 at 09:23:19 UTC, bauss wrote:
On Tuesday, 27 February 2018 at 08:43:32 UTC, Timothee Cour 
wrote:
see rationale in 
https://issues.dlang.org/show_bug.cgi?id=18529


All I see is a rationale for how it can't be replaced, but 
not a rationale for actually doing it.


I can't imagine a "safe" situation where you want to do it.

Tbh. just put your code in @trusted


Checking if an array is the slice of another.


Like?

assert(a.ptr != b.ptr);

Which already works in @safe.


That only checks if the first element is the same. For a full 'is 
slice' check you'd need something like this:


assert(a.ptr <= b.ptr && b.ptr + b.length <= a.ptr + 
a.length);


// Or:

auto c = b.ptr - a.ptr;
assert(c >= 0 && c + b.length <= a.length);

And trust me, the compiler complains about both of these. 
Possibly rightfully in the first example, but the latter never 
does anything scary with the given pointers.


--
  Simen

Re: can we un-deprecate .ptr on arrays in @safe code? cf issue 18529

[bauss via Digitalmars-d]

On Tuesday, 27 February 2018 at 09:47:51 UTC, Stefan Koch wrote:

On Tuesday, 27 February 2018 at 09:23:19 UTC, bauss wrote:
On Tuesday, 27 February 2018 at 08:43:32 UTC, Timothee Cour 
wrote:
see rationale in 
https://issues.dlang.org/show_bug.cgi?id=18529


All I see is a rationale for how it can't be replaced, but not 
a rationale for actually doing it.


I can't imagine a "safe" situation where you want to do it.

Tbh. just put your code in @trusted


Checking if an array is the slice of another.


Like?

assert(a.ptr != b.ptr);

Which already works in @safe.

Re: can we un-deprecate .ptr on arrays in @safe code? cf issue 18529

[Stefan Koch via Digitalmars-d]

On Tuesday, 27 February 2018 at 09:23:19 UTC, bauss wrote:
On Tuesday, 27 February 2018 at 08:43:32 UTC, Timothee Cour 
wrote:

see rationale in https://issues.dlang.org/show_bug.cgi?id=18529


All I see is a rationale for how it can't be replaced, but not 
a rationale for actually doing it.


I can't imagine a "safe" situation where you want to do it.

Tbh. just put your code in @trusted


Checking if an array is the slice of another.

Re: can we un-deprecate .ptr on arrays in @safe code? cf issue 18529

[bauss via Digitalmars-d]

On Tuesday, 27 February 2018 at 08:43:32 UTC, Timothee Cour wrote:

see rationale in https://issues.dlang.org/show_bug.cgi?id=18529


All I see is a rationale for how it can't be replaced, but not a 
rationale for actually doing it.


I can't imagine a "safe" situation where you want to do it.

Tbh. just put your code in @trusted

Re: can we un-deprecate .ptr on arrays in @safe code? cf issue 18529

[Mike Franklin via Digitalmars-d]

On Tuesday, 27 February 2018 at 08:43:32 UTC, Timothee Cour wrote:

see rationale in https://issues.dlang.org/show_bug.cgi?id=18529


It looks like the actual deprecation was made with this PR:  
https://github.com/dlang/dmd/pull/5860  Meaning it's been 
deprecated for more than a year and a half.


I think you're going to have to take it up with Walter.

Mike