Very simple source, but total mess in generated code !
Or, am I doing something wrong ?
====
Source a.c:
typedef int v4si __attribute__((__vector_size__(16), __may_alias__));
typedef long long int64_t;
int64_t xor128fold (v4si s) {
int64_t *p = (int64_t*)&s;
return p[0] ^ p[1];
}
====
-O1 gives ugly, but "kind of correct" code
gcc -O1 a.c -c -o a.o
objdump -xd a.o
0000000000000000 <xor128fold>:
0: 66 0f 7f 44 24 d8 movdqa %xmm0,0xffffffffffffffd8(%rsp)
6: 48 8b 44 24 d8 mov 0xffffffffffffffd8(%rsp),%rax
b: 66 0f 7f 44 24 e8 movdqa %xmm0,0xffffffffffffffe8(%rsp)
11: 48 33 44 24 f0 xor 0xfffffffffffffff0(%rsp),%rax
16: c3 retq
====
-O2 (and -O3) gives entirely incorrect code
0000000000000000 <xor128fold>:
0: 66 0f 7f 44 24 d8 movdqa %xmm0,0xffffffffffffffd8(%rsp)
6: 48 8b 44 24 d8 mov 0xffffffffffffffd8(%rsp),%rax
b: 48 33 44 24 f0 xor 0xfffffffffffffff0(%rsp),%rax
10: 66 0f 7f 44 24 e8 movdqa %xmm0,0xffffffffffffffe8(%rsp)
16: c3 retq
====
Compiler
gcc -v
Using built-in specs.
Target: x86_64-unknown-linux-gnu
Configured with: ./configure --enable-languages=c,c++ --prefix <path omitted>
Thread model: posix
gcc version 4.4.2 (GCC)
====
Older compiler (gcc version 4.1.2 20080704 (Red Hat 4.1.2-46))
was able to generate correct code with -O1 (with or w/o "may_alias" in
typedef).
It generates same correct code with -O2/-O3 with "may_alias" in typedef.
0000000000000000 <xor128fold>:
0: 66 0f 7f 44 24 e8 movdqa %xmm0,0xffffffffffffffe8(%rsp)
6: 48 8b 44 24 e8 mov 0xffffffffffffffe8(%rsp),%rax
b: 48 33 44 24 f0 xor 0xfffffffffffffff0(%rsp),%rax
10: c3 retq
It generates totally incorrect code with -O2/-O3 without "may_alias" in
typedef.
0000000000000000 <xor128fold>:
0: 48 8b 44 24 e8 mov 0xffffffffffffffe8(%rsp),%rax
5: 48 33 44 24 f0 xor 0xfffffffffffffff0(%rsp),%rax
a: c3 retq
--
Summary: incorrect code on taking address of vectored type
argument
Product: gcc
Version: 4.4.2
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: Shvaiger_Felix at emc dot com
GCC build triplet: x86_64-unknown-linux-gnu
GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42437
