Re: Disable -Waggressive-loop-optimizations warnings in tst-dynarray.c

[Florian Weimer <fweimer@redhat.com>]

* Joseph Myers:

> My build-many-glibcs.py bot shows -Waggressive-loop-optimizations
> errors building the glibc testsuite for 32-bit architectures with GCC
> mainline, which seem to have appeared between GCC commits
> 4abc0c196b10251dc80d0743ba9e8ab3e56c61ed and
> d8edfadfc7a9795b65177a50ce44fd348858e844:
>
> In function 'dynarray_long_noscratch_resize',
>     inlined from 'test_long_overflow' at tst-dynarray.c:489:5,
>     inlined from 'do_test' at tst-dynarray.c:571:3:
> ../malloc/dynarray-skeleton.c:391:36: error: iteration 1073741823 invokes undefined behavior [-Werror=aggressive-loop-optimizations]
>   391 |             DYNARRAY_ELEMENT_INIT (&list->u.dynarray_header.array[i]);
> tst-dynarray.c:39:37: note: in definition of macro 'DYNARRAY_ELEMENT_INIT'
>    39 | #define DYNARRAY_ELEMENT_INIT(e) (*(e) = 23)
>       |                                     ^
> In file included from tst-dynarray.c:42:
> ../malloc/dynarray-skeleton.c:389:37: note: within this loop
>   389 |         for (size_t i = old_size; i < size; ++i)
>       |                                   ~~^~~~~~
> In function 'dynarray_long_resize',
>     inlined from 'test_long_overflow' at tst-dynarray.c:479:5,
>     inlined from 'do_test' at tst-dynarray.c:571:3:
> ../malloc/dynarray-skeleton.c:391:36: error: iteration 1073741823 invokes undefined behavior [-Werror=aggressive-loop-optimizations]
>   391 |             DYNARRAY_ELEMENT_INIT (&list->u.dynarray_header.array[i]);
> tst-dynarray.c:27:37: note: in definition of macro 'DYNARRAY_ELEMENT_INIT'
>    27 | #define DYNARRAY_ELEMENT_INIT(e) (*(e) = 17)
>       |                                     ^
> In file included from tst-dynarray.c:28:
> ../malloc/dynarray-skeleton.c:389:37: note: within this loop
>   389 |         for (size_t i = old_size; i < size; ++i)
>       |                                   ~~^~~~~~
>
> I don't know what GCC change made these errors appear, or why they
> only appear for 32-bit architectures.  However, the warnings appear to
> be both true (that iteration would indeed involve undefined behavior
> if executed) and useless in this particular case (that iteration is
> never executed, because the allocation size overflows and so the
> allocation fails - but the check for allocation size overflow is in a
> separate source file and so can't be seen by the compiler when
> compiling this test).  So use the DIAG_* macros to disable
> -Waggressive-loop-optimizations around the calls in question to
> dynarray_long_resize and dynarray_long_noscratch_resize in this test.

The glibc patch is okay, but I think this case is fairly common: A
failed allocation ensuring that loops do not wrap around the address
space.  So this could hint to a defect in the GCC warning.

Thanks,
Florian