On Tue, 14 Jan 2014, Jay Foad wrote:
I'm using GCC 4.8.1 with eglibc 2.17 on an Intel CPU with SSE 4.1.
Given this source:
#include <math.h>
double f(double x){return floor x;}
gcc -msse4.1 -O -S generates:
f:
subq $8, %rsp
call floor
addq $8, %rsp
ret
It seems I need to add -fno-trapping-math to get inline code:
f:
roundsd $1, %xmm0, %xmm0
ret
But why? If I run the first version under a debugger, the call to
"floor" eventually resolves to eglibc's __floor_sse41, which just does
this:
roundsd $0x1,%xmm0,%xmm0
retq
So what is -ftrapping-math really buying me? Why can't the compiler
generate roundsd inline in all cases?
First, I think you want to file a PR on bugzilla, or your message will
just be forgotten.
The reason could be related to this note in the glibc manpage:
"SUSv2 and POSIX.1-2001 contain text about overflow (which might set
errno to ERANGE, or raise an FE_OVERFLOW exception). In practice, the
result cannot overflow on any current machine, so this error-handling
stuff is just nonsense."
--
Marc Glisse
