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

[Nema, Ashutosh via llvm-dev]

<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.
http://reviews.llvm.org/rL256194

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

Regards,
Ashutosh

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

Hi,

One day I worked hard and refactored the cost calculation for all X86 targets.
http://reviews.llvm.org/D15604
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

Hi,

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.

Thanks,
Ashutosh



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