On Mon, Dec 03, 2012 at 10:18:56PM +0400, Konstantin Serebryany wrote:
> The LLVM implementation always used 32-byte alignment for stack redzones.
> I never actually did any performance checking on x86 (32-byte aligned
> vs 8-byte aligned),
> although I suspect 32-byte aligned redzones should be ~2x faster.
If the ~2x faster comes from unaligned vs. aligned integer stores, I can't
spot anything like that on e.g.
__attribute__((noinline, noclone)) void
foo (int *p)
{
int i;
for (i = 0; i < 32; i++)
p[i] = 0x12345678;
}
int
main (int argc, const char **argv)
{
char buf[1024];
int *p = &buf[argc - 1];
int i;
__builtin_printf ("%p\n", p);
for (i = 0; i < 100000000; i++)
foo (p);
return 0;
}
Time with zero arguments (i.e. argc 1) is the same as time with 1, 2 or 3
arguments on SandyBridge CPU. I guess there could be penalties on page
boundaries, etc., but I think hot caches is the usual operation on the
stack.
Jakub
