https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66337
Bug ID: 66337
Summary: __alignof__(s.d) incorrect for a double member with
-malign-power
Product: gcc
Version: 5.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
As mentioned in http://gcc.gnu.org/ml/gcc-patches/2015-05/msg02145.html, the
result of __alignof__(s.d) where s.d is a struct member of type double doesn't
reflect the POWER ABI requirements on such members when the -malign-power
option is specified and differs from the result returned by IBM xlc on AIX with
the equivalent option.
I searched the GCC test suite for tests exercising the option to see if I was
missing something but couldn't find any. I also searched the sources to see I
could find a comment explaining the difference. All I was able to find was the
following in config/rs6000/{freebsd64,linux64}.h:
/* PowerPC64 Linux word-aligns FP doubles when -malign-power is given. */
which still doesn't explain the result.
Test case run with gcc 4.8 and IBM xlc 12.1 on AIX 7.1 (GCC's result is the
same on Linux/POWER).
If the result is expected, I'm happy to update the documentation provided
someone can point me to the ABI document that outlines the requirements.
$ cat t.c && gcc -malign-power t.c && ./a.out && xlc -qalign=power t.c &&
./a.out
#include <stdio.h>
static struct { double d; } sd;
int main (void) {
printf ("%zu %zu\n", __alignof (sd), __alignof__ (sd.d));
return 0;
}
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