On Fri, Nov 22, 2024 at 9:33 PM Richard Biener <rguent...@suse.de> wrote: > > Similar to the X86_TUNE_AVX512_TWO_EPILOGUES tuning which enables > an extra 128bit SSE vector epilouge when doing 512bit AVX512 > vectorization in the main loop the following allows a 64bit SSE > vector epilogue to be generated when the previous vector epilogue > still had a vectorization factor of 16 or larger (which usually > means we are operating on char data). > > This effectively applies to 256bit and 512bit AVX2/AVX512 main loops, > a 128bit SSE main loop would already get a 64bit SSE vector epilogue. > > Together with X86_TUNE_AVX512_TWO_EPILOGUES this means three > vector epilogues for 512bit and two vector epilogues when enabling > 256bit vectorization. I have not added another tunable for this > RFC - suggestions on how to avoid inflation there welcome. > > This speeds up 525.x264_r to within 5% of the -mprefer-vector-size=128 > speed with -mprefer-vector-size=256 or -mprefer-vector-size=512 > (the latter only when -mtune-crtl=avx512_two_epilogues is in effect). > > I have not done any further benchmarking, this merely shows the > possibility and looks for guidance on how to expose this to the > uarch tunings or to the user (at all?) if not gating on any uarch > specific tuning. > > Note 64bit SSE isn't a native vector size so we rely on emulation > being "complete" (if not epilogue vectorization will only fail, so > it's "safe" in this regard). With AVX512 ISA available an alternative > is a predicated epilog, but due to possible STLF issues user control > would be required here. > > Bootstrapped on x86_64-unknown-linux-gnu, testing in progress > (I expect some fallout in scans due to some extra epilogues, let's see) I'll do some benchmark, Guess it should be ok. > > * config/i386/i386.cc (ix86_vector_costs::finish_cost): For an > 128bit SSE epilogue request a 64bit SSE epilogue if the 128bit > SSE epilogue VF was 16 or higher. > --- > gcc/config/i386/i386.cc | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc > index c7e70c21999..f2e8de3aafc 100644 > --- a/gcc/config/i386/i386.cc > +++ b/gcc/config/i386/i386.cc > @@ -25495,6 +25495,13 @@ ix86_vector_costs::finish_cost (const vector_costs > *scalar_costs) > && GET_MODE_SIZE (loop_vinfo->vector_mode) == 32) > m_suggested_epilogue_mode = V16QImode; > } > + /* When a 128bit SSE vectorized epilogue still has a VF of 16 or larger > + enable a 64bit SSE epilogue. */ > + if (loop_vinfo > + && LOOP_VINFO_EPILOGUE_P (loop_vinfo) > + && GET_MODE_SIZE (loop_vinfo->vector_mode) == 16 > + && LOOP_VINFO_VECT_FACTOR (loop_vinfo).to_constant () >= 16) > + m_suggested_epilogue_mode = V8QImode; > > vector_costs::finish_cost (scalar_costs); > } > -- > 2.43.0
-- BR, Hongtao
