diff mbox series

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

Message ID alpine.DEB.2.22.394.2110271656160.1681686@digraph.polyomino.org.uk
State Committed
Commit 7ca9377bab849bbdb023ce23fb8237fa4b7a996e
Headers
Series Disable -Waggressive-loop-optimizations warnings in tst-dynarray.c |

Checks

Context Check Description
dj/TryBot-apply_patch success Patch applied to master at the time it was sent
dj/TryBot-32bit success Build for i686

Commit Message

Joseph Myers Oct. 27, 2021, 4:56 p.m. UTC 
  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.

Tested with build-many-glibcs.py (GCC mainline) for arm-linux-gnueabi,
where it restores a clean testsuite build.

Comments

Florian Weimer Oct. 29, 2021, 9:04 a.m. UTC | #1 
* 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
diff mbox series

Patch

diff --git a/malloc/tst-dynarray.c b/malloc/tst-dynarray.c
index c33505711f..955fab696c 100644
--- a/malloc/tst-dynarray.c
+++ b/malloc/tst-dynarray.c
@@ -20,6 +20,7 @@ 
 
 #include <errno.h>
 #include <stdint.h>
+#include <libc-diag.h>
 
 #define DYNARRAY_STRUCT dynarray_long
 #define DYNARRAY_ELEMENT long
@@ -476,8 +477,15 @@  test_long_overflow (void)
     struct dynarray_long dyn;
     dynarray_long_init (&dyn);
     errno = EINVAL;
+    DIAG_PUSH_NEEDS_COMMENT;
+    /* GCC 12 (on 32-bit platforms) warns that after inlining, a loop
+       iteration would invoke undefined behavior.  That loop iteration
+       can never be executed because an allocation of this size must
+       fail.  */
+    DIAG_IGNORE_NEEDS_COMMENT (12, "-Waggressive-loop-optimizations");
     TEST_VERIFY (!dynarray_long_resize
                  (&dyn, (SIZE_MAX / sizeof (long)) + 1));
+    DIAG_POP_NEEDS_COMMENT;
     TEST_VERIFY (errno == ENOMEM);
     TEST_VERIFY (dynarray_long_has_failed (&dyn));
   }
@@ -486,8 +494,15 @@  test_long_overflow (void)
     struct dynarray_long_noscratch dyn;
     dynarray_long_noscratch_init (&dyn);
     errno = EINVAL;
+    DIAG_PUSH_NEEDS_COMMENT;
+    /* GCC 12 (on 32-bit platforms) warns that after inlining, a loop
+       iteration would invoke undefined behavior.  That loop iteration
+       can never be executed because an allocation of this size must
+       fail.  */
+    DIAG_IGNORE_NEEDS_COMMENT (12, "-Waggressive-loop-optimizations");
     TEST_VERIFY (!dynarray_long_noscratch_resize
                  (&dyn, (SIZE_MAX / sizeof (long)) + 1));
+    DIAG_POP_NEEDS_COMMENT;
     TEST_VERIFY (errno == ENOMEM);
     TEST_VERIFY (dynarray_long_noscratch_has_failed (&dyn));
   }