[X86] TZCNT/LZCNT/BSR/BSF have weird SchedRW

[topperc]

Bugzilla Link	36881
Resolution	FIXED
Resolved on	Mar 26, 2018 11:53
Version	trunk
OS	Windows NT
Blocks	#31672 #36256
CC	@RKSimon
Fixed by commit(s)	328566

Extended Description

BSR and BSF are using WriteShift. TZCNT/LZCNT are using WriteIMul.

I assume that generally WriteShift would be faster than WriteIMul. But TZCNT/LZCNT are know to be better than BSR/BSF on some CPUs.

[topperc]

assigned to @RKSimon

[RKSimon]

Ideally we need to create WriteBitScan, WriteTZCNT, WriteLZCNT and WritePOPCNT - that seems to cover most target requirements (AMD bitscans are a lot worse than *ZCNT, but TZCNT/LZCNT are quite different) - will get this done shortly.

[topperc]

This reminded me of llvm/llvm-bugzilla-archive#34843

[RKSimon]

https://reviews.llvm.org/D44879

[RKSimon]

rL328566

[RKSimon]

mentioned in issue #36256