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RE: Question on TARGET_MMX and X86_TUNE_GENERAL_REGS_SSE_SPILL


Hi ,

> -----Original Message-----
> From: Ilya Enkovich [mailto:enkovich.gnu@gmail.com]
> Sent: Tuesday, April 26, 2016 7:09 PM
> To: Kumar, Venkataramanan <Venkataramanan.Kumar@amd.com>
> Cc: vmakarov@redhat.com; gcc@gcc.gnu.org; Uros Bizjak
> (ubizjak@gmail.com) <ubizjak@gmail.com>
> Subject: Re: Question on TARGET_MMX and
> X86_TUNE_GENERAL_REGS_SSE_SPILL
> 
> 2016-04-14 8:39 GMT+03:00 Kumar, Venkataramanan
> <Venkataramanan.Kumar@amd.com>:
> > Hi,
> >
> > X86_TUNE_GENERAL_REGS_SSE_SPILL: Try to spill general regs to SSE regs
> instead of memory.
> >
> > I tried enabling the above tuning with -march=bdver4 -Ofast -mtune-
> ctrl=general_regs_sse_spill.
> > I did not find any code differences.
> >
> > Looking at the below code to enable this tune,  mmx ISA needs to be turned
> off.
> >
> > static reg_class_t
> > ix86_spill_class (reg_class_t rclass, machine_mode mode) {
> >   if (TARGET_SSE && TARGET_GENERAL_REGS_SSE_SPILL && !
> TARGET_MMX
> >       && (mode == SImode || (TARGET_64BIT && mode == DImode))
> >       && rclass != NO_REGS && INTEGER_CLASS_P (rclass))
> >     return ALL_SSE_REGS;
> >   return NO_REGS;
> > }
> >
> > All processor variants enable MMX by default  and why we need to switch
> off mmx?
> 
> That really looks weird to me.  I ran SPEC2006 on Ofast + LTO with and
> without -mno-mmx and -mno-mmx gives (Haswell machine):
> 
> SPEC2006INT     :    +0.30%
> SPEC2006FP      :    +0.60%
> SPEC2006ALL     :    +0.48%
> 
> Which is quite surprising for disabling a hardware feature hardly used
> anywhere now.

As I said without mmx (-mno-mmx), the tune X86_TUNE_GENERAL_REGS_SSE_SPILL may be active now.
Not sure if there are any other reason.

> 
> 
> Thanks,
> Ilya
> 
> >
> > Thanks and regards,
> > Venkat.

Regards,
Venkat.

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