* why does gcc need -fno-trapping-math to generate roundsd instructions?
@ 2014-01-14 14:53 Jay Foad
2014-01-15 16:13 ` Marc Glisse
0 siblings, 1 reply; 2+ messages in thread
From: Jay Foad @ 2014-01-14 14:53 UTC (permalink / raw)
To: gcc-help
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?
Thanks,
Jay.
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: why does gcc need -fno-trapping-math to generate roundsd instructions?
2014-01-14 14:53 why does gcc need -fno-trapping-math to generate roundsd instructions? Jay Foad
@ 2014-01-15 16:13 ` Marc Glisse
0 siblings, 0 replies; 2+ messages in thread
From: Marc Glisse @ 2014-01-15 16:13 UTC (permalink / raw)
To: Jay Foad; +Cc: gcc-help
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
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