https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70451
Bug ID: 70451
Summary: x86 over align struct
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: hjl.tools at gmail dot com
CC: ubizjak at gmail dot com
Target Milestone: ---
Target: x86
[hjl@gnu-6 gcc]$ cat /tmp/a.i
typedef struct xxx
{
unsigned long long a1;
unsigned long long a2;
unsigned long long a3;
unsigned long long a4;
} xxx;
xxx b;
[hjl@gnu-6 gcc]$ gcc -S /tmp/a.i
[hjl@gnu-6 gcc]$ cat a.s
.file "a.i"
.comm b,32,32
.ident "GCC: (GNU) 5.3.1 20160212 (Red Hat 5.3.1-4)"
.section .note.GNU-stack,"",@progbits
[hjl@gnu-6 gcc]$ ./xgcc -B./ -S /tmp/a.i
[hjl@gnu-6 gcc]$ cat a.s
.arch armv8-a+fp+simd
.file "a.i"
.comm b,32,8
.ident "GCC: (GNU) 6.0.0 20160304 (experimental)"
.section .note.GNU-stack,"",@progbits
[hjl@gnu-6 gcc]$ cat a.s
.text
.file "/tmp/a.i"
.type b,@object # @b
.comm b,32,8
.ident "clang version 3.7.0 (tags/RELEASE_370/final)"
.section ".note.GNU-stack","",@progbits
[hjl@gnu-6 gcc]$
According to x86 psABIs, b should be aligned at 8 bytes, not 32 bites.
There are 2 issues:
1. Waste of space to align b to 32 bytes.
2. Since malloc/new only returns 8-byte aligned memory (i386) or 16-byte
aligned memory (x86-64), pointer to xxx from malloc/new won't be 32-byte
aligned.
With SSE/AVX optimization for memcpy/memset, I get segfault on unaligned
memory with pointer from malloc/new since alignment is incorrect.
