[llvm-dev] X86 TRUNCATE cost for AVX & AVX2 mode

Nema, Ashutosh via llvm-dev llvm-dev at lists.llvm.org
Tue Apr 12 02:48:03 PDT 2016

<Copied Cong>

Thanks Elena.

Mostly I was interested in why such a high cost 30 kept for TRUNCATE v16i32 to v16i8 in SSE41.
Looking at the code it appears like TRUNCATE v16i32 to v16i8 in SSE41 is very expensive
vs SSE2. I feel this number should be same/close to the cost mentioned for same
operation in SSE2ConversionTbl.

Below patch from Cong Hou reduce cost for same operation in SSE2 mode.

Looks like as the part of same patch we should reduce cost for TRUNCATE v16i32 to v16i8 in SSE4.1 as well.


From: Demikhovsky, Elena [mailto:elena.demikhovsky at intel.com]
Sent: Monday, April 11, 2016 9:05 PM
To: Nema, Ashutosh <Ashutosh.Nema at amd.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>; Zuckerman, Michael <michael.zuckerman at intel.com>
Subject: RE: X86 TRUNCATE cost for AVX & AVX2 mode


One day I worked hard and refactored the cost calculation for all X86 targets.
But this revision was not accepted.

I fixed conversions, but assume that truncation suffers from the same problem.
I used "SplitFactor" in order to process wide types.

I'll be happy if you'll try to reanimate this work or part of it, because the huge numbers causes a non-optimal vectorization factor to be chosen.

-           Elena

From: Nema, Ashutosh [mailto:Ashutosh.Nema at amd.com]
Sent: Monday, April 11, 2016 16:51
To: Demikhovsky, Elena <elena.demikhovsky at intel.com<mailto:elena.demikhovsky at intel.com>>; Zuckerman, Michael <michael.zuckerman at intel.com<mailto:michael.zuckerman at intel.com>>
Cc: llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Subject: X86 TRUNCATE cost for AVX & AVX2 mode


I was going through the X86TTIImpl::getCastInstrCost, and got a doubt on cost
calculation for TRUNCATE instruction in AVX mode.

In AVX2ConversionTbl & AVXConversionTbl table there is no cost defined for
TRUNCATE v16i32 to v16i8, as a fallback it goes to SSE41ConversionTbl table and there
it finds cost as 30 for this operation. 30 cost for this operation looks very high.

Wondering why such a high cost kept for this, any pointers to understand this will be helpful.
In few cases this restricts better vectorization opportunities.

Other observations:
Cost for TRUNCATE v16i32 to v16i8 in SSE2ConversionTbl as 7.
Cost for TRUNCATE v8i32 to v8i8 is 2 in AVX2 and 4 in AVX mode.


Intel Israel (74) Limited

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