http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54231
--- Comment #2 from Thiago Macieira <thiago at kde dot org> 2012-08-11 22:33:31
UTC ---
When adding the following source file to the library build:
====
#include <stdlib.h>
void bzero_sse2(char *, size_t);
void bzero_avx(char *, size_t);
extern int avx_supported;
void my_bzero(char *ptr, size_t n)
{
if (avx_supported)
bzero_avx(ptr, n);
else
bzero_sse2(ptr, n);
}
====
and compiling everything with -O2 -flto, GCC produces the following function:
00000000000002e0 <my_bzero>:
2e0: mov 0x200171(%rip),%rax # 200458 <my_bzero+0x200178>
2e7: mov (%rax),%eax
2e9: test %eax,%eax
2eb: jne 310 <my_bzero+0x30>
2ed: test %rsi,%rsi
2f0: vpxor %xmm0,%xmm0,%xmm0
2f4: je 30e <my_bzero+0x2e>
2f6: nopw %cs:0x0(%rax,%rax,1)
300: vmovntdq %xmm0,(%rdi)
304: add $0x10,%rdi
308: sub $0x1,%rsi
30c: jne 300 <my_bzero+0x20>
30e: repz retq
310: test %rsi,%rsi
313: je 30e <my_bzero+0x2e>
315: vpxor %xmm0,%xmm0,%xmm0
319: nopl 0x0(%rax)
320: vmovntdq %xmm0,(%rdi)
324: add $0x10,%rdi
328: sub $0x1,%rsi
32c: jne 320 <my_bzero+0x40>
32e: repz retq
As can be seen, VEX-prefixed instructions were used in both cases.
