[llvm-dev] Question about __builtin_object_size

[Strahinja Petrovic via llvm-dev]

Thank you for replies. I'm using trunk version. I have problem in some 
tests with this behaviour. I also have fix for this problem just need an 
opinion if this should be fixed in clang. I can post patch on 
phabricator if you want to take a look.

Thanks,
Strahinja

On 09.02.2016. 20:32, George Burgess IV wrote:
> Hi!
>
> This is an artifact of how LLVM works. Essentially, LLVM detects that 
> `var` is unused and deletes it before it tries to lower the 
> `llvm.objectsize` (which is what clang lowers `__builtin_object_size` 
> to) call to a constant. While this isn't ideal, I don't think it 
> realistically a problem, because `var` must be otherwise unused for 
> this behavior to occur, and the whole purpose of objectsize functions 
> (so far) is to insert bounds checks when a variable is used.
>
> If this quirk is causing real problems for you, please let me know and 
> I'll see what I can do about fixing it. :)
>
> Thanks for pointing this out!
> George
>
> On Tue, Feb 9, 2016 at 11:06 AM, Xinliang David Li 
> <xinliangli at gmail.com <mailto:xinliangli at gmail.com>> wrote:
>
>     What version of clang/LLVM are you using? George (cc'ed ) has
>     improved __builtin_object_size support in Clang recently.
>
>     David
>
>     On Tue, Feb 9, 2016 at 6:37 AM, Strahinja Petrovic via llvm-dev
>     <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>
>         Hi,
>
>         I have question about __builtin_object_size behaviour. LLVM
>         for __builtin_object_size function doesn't calculate correct
>         value when pointer is used only in __builtin_object_size
>         function call, and never after that. For this case LLVM as
>         result generates 0 or -1 depends of second argument of
>         __builtin_object_size function. Is this correct behaviour or
>         it should work as gcc (gcc calculates correct value for this
>         case) ?
>
>         Example for this issue:
>
>         include <assert.h>
>         int main() {
>         struct V { char buf1[10];
>                    int b;
>                    char buf2[10];
>                   } var;
>
>                   char *p = &var.buf1[1], *q = &var.b;
>                   assert (__builtin_object_size (p, 0) == sizeof (var)
>         - 1);
>                   return 0;
>         }
>
>         Thanks,
>         Strahinja
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>
>
>

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