Merge r324195 into the 6.0 branch: [LV] Use Demanded Bits and ValueTracking for reduction type-shrinking

[llvmbot]

Bugzilla Link	36321
Resolution	FIXED
Resolved on	Feb 19, 2018 07:25
Version	trunk
OS	Windows NT
Blocks	#35152
Reporter	LLVM Bugzilla Contributor
CC	@zmodem

Extended Description

This commit fixed a bug in the loop vectorizer (#35734 ) and should probably be considered for inclusion in the 6.0.0 release.

[llvmbot]

• Diego who approved the original patch and might be able to sign off on this as a LV code owner.

[llvmbot]

• Michael Kuperstein, the LV code owner. I think he should be the one signing off on this.

Thanks,
Diego

[llvmbot]

SGTM, Diego.

@​mkuper: Do you have any opinion on including this fix in the 6.0.0 release? The patch was originally authored by Matt Simpson. I'm assisting Matt while he's out of the office for a few weeks.

[llvmbot]

My $0.02:

IIUC, the bug this fixes is a nasty miscompile, but it's fairly old - it's not a regression from 5.0. Is this the case? If so, maybe worth letting this bake a bit, and getting it into 6.0.1? It's fairly late in the 6.0 release process, and the patch doesn't seem trivial.

If it is a regression from 5.0, then it's probably worth to merge.

[zmodem]

I'm open to merging this.. let's just have it bake in trunk a day or two more.

[llvmbot]

Following up on Michael's comments.. the bug was filed in Dec '17, but the code that introduced the bug was committed in Aug. '15 (at least based on the comments in #35082 ), so this doesn't look to be a 5.0 regression.

I agree this is a non-trivial change. I'll let Hans decide if this should be included in 6.0.0 or 6.0.1.

[zmodem]

Following up on Michael's comments.. the bug was filed in Dec '17, but the
code that introduced the bug was committed in Aug. '15 (at least based on
the comments in #35082 ), so this doesn't look to be a 5.0 regression.

I agree this is a non-trivial change. I'll let Hans decide if this should
be included in 6.0.0 or 6.0.1.

It's been very quiet since this landed on trunk, so I went ahead and merged it in
r325508.