Re: Should -msse3 enable fisttp

[Evan Cheng <evan dot cheng at apple dot com>]

Well, both Intel and AMD calls fisttp a SSE3 instruction even though  
it operates on the x87 stack ST(0). My argument is users who specify - 
msse3 to turn on SSE3 instructions would expect fisttp be turned on  
as well.

But according to the manual -msse3 does not turn on generation of  
SSE3 instructions:

> -mmmx
> -mno-mmx
> -msse
> -mno-sse
> -msse2
> -mno-sse2
> -msse3
> -mno-sse3
> -m3dnow
> -mno-3dnow
> These switches enable or disable the use of built-in functions that  
> allow direct access to the MMX, SSE, SSE2, SSE3 and 3Dnow  
> extensions of the instruction set.
> See X86 Built-in Functions, for details of the functions enabled  
> and disabled by these switches.
> To have SSE/SSE2 instructions generated automatically from floating- 
> point code, see -mfpmath=sse.

Thus the confusion.

Evan

On Oct 3, 2005, at 3:25 PM, Andrew Pinski wrote:

> On Oct 3, 2005, at 5:56 PM, Evan Cheng wrote:
>
>
> > My mistake. I misunderstood the meaning of -msse3 (it only enables  
> > the sse3 builtins). Please ignore.
>
>
> Actually it enables more than the builtins.  It enables the use sse3
> instructions.  This is just like -maltivec on PowerPC and -msse and  
> -msse
> on x86, etc.
>
> Hmm, but from the original patch:
> http://gcc.gnu.org/ml/gcc-patches/2005-03/msg01119.html
> "BTW: Regarding TARGET_FISTTP macro: according to documentation,  
> fisttp insn indeed depends on (TARGET_80387 && TARGET_SSE3).  
> However, this insn is not a SSE3 instruction, so it should not be  
> disabled by -mno-sse3 flag."
>
> And then RTH agreed:
> http://gcc.gnu.org/ml/gcc-patches/2005-03/msg01432.html
>
> So from the sound of it fisttp is not a SSE3 instruction.
>
> Thanks,
> Andrew Pinski