2016-04-27 14:35 GMT+03:00 Kumar, Venkataramanan <venkataramanan.ku...@amd.com>: > Hi , > >> -----Original Message----- >> From: Ilya Enkovich [mailto:enkovich....@gmail.com] >> Sent: Tuesday, April 26, 2016 7:09 PM >> To: Kumar, Venkataramanan <venkataramanan.ku...@amd.com> >> Cc: vmaka...@redhat.com; gcc@gcc.gnu.org; Uros Bizjak >> (ubiz...@gmail.com) <ubiz...@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.ku...@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.
Surely that should be the main reason I see performance gain. So I want to ask the same question as you did: why does this important performance feature requires disabled MMX. This restriction exists from the very start of X86_TUNE_GENERAL_REGS_SSE_SPILL existence (at least in trunk) and no comments on why we have this restriction. Did you try to remove !TARGET_MMX and see what happens? Thanks, Ilya > >> >> >> Thanks, >> Ilya >> >> > >> > Thanks and regards, >> > Venkat. > > Regards, > Venkat.
