On Thu, Dec 18, 2014 at 02:24:06PM +0100, Uros Bizjak wrote:
> > It removed 14309 redundant "xor %eax,%eax" instructions and saved about
> > 27KB. I am currently running the new kernel without any problem. OK
> > for trunk?
>
> How about skipping RAX setup unconditionally for !TARGET_SSE? Please
> see ix86_conditional_register_usage, where SSE registers are squashed
> for !TARGET_SSE, so it is not possible to use them even in the inline
> asm.
I'd say a problem is if a -mno-sse TU calls a vararg function (obviously
it can't pass any float/double arguments) to a function in a TU compiled
with -msse2 or higher where the stdarg pass can't figure out anything, say
#include <stdarg.h>
extern void bar (int, va_list);
void
foo (int x, ...)
{
va_list ap;
va_start (ap, x);
bar (x, ap);
va_end (ap);
}
If foo is compiled with gcc 4.4 and earlier, it might crash when called from
-mno-sse caller that would not xor %eax,%eax. If foo is compiled with gcc
4.5? and higher, then it might just randomly save all the xmm registers to
stack (as the test is %al != 0, I think it will be more likely that it will
save it unnecessarily than not).
So I view H.J.'s new option as a user guarantee the callee will be also
-mno-sse.
Jakub
