[llvm-bugs] [Bug 51076] New: -Watomic-alignment not using available alignment info

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https://bugs.llvm.org/show_bug.cgi?id=51076

            Bug ID: 51076
           Summary: -Watomic-alignment not using available alignment info
           Product: clang
           Version: 12.0
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C
          Assignee: unassignedclangbugs at nondot.org
          Reporter: rth at twiddle.net
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

For i686:

----------------------------------
#include <stdint.h>

uint64_t foo(uint64_t *p)
{
  p = __builtin_assume_aligned(p, 8);
  return __atomic_load_n(p, __ATOMIC_RELAXED);
}

uint64_t bar(uintptr_t p)
{
  if (p & 7) return 0;
  return __atomic_load_n((uint64_t *)p, __ATOMIC_RELAXED);
}
----------------------------------

$ clang-12 -Wall -O -c z.c
z.c:6:10: warning: misaligned atomic operation may incur significant
performance penalty; the expected alignment (8 bytes) exceeds the actual
alignment (4 bytes) [-Watomic-alignment]
  return __atomic_load_n(p, __ATOMIC_RELAXED);
         ^
z.c:12:10: warning: misaligned atomic operation may incur significant
performance penalty; the expected alignment (8 bytes) exceeds the actual
alignment (4 bytes) [-Watomic-alignment]
  return __atomic_load_n((uint64_t *)p, __ATOMIC_RELAXED);
         ^
2 warnings generated.

In both cases, additional alignment information is available
beyond that of the uint64_t type, and it is not being used.

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