[llvm] r274098 - [ValueTracking] Teach computeKnownBits for PHI nodes to compute sign bit for a recurrence with a NSW addition.

[Huang, Li1 via llvm-commits]

Hi,

Is anyone already working on a fix for this issue? If not then I can work on this.


Thanks,
Li Huang


-----Original Message-----
From: Sanjoy Das [mailto:sanjoy at playingwithpointers.com] 
Sent: Wednesday, June 29, 2016 12:35 AM
To: Craig Topper <craig.topper at gmail.com>
Cc: Adam Nemet <anemet at apple.com>; llvm-commits <llvm-commits at lists.llvm.org>; Huang, Li1 <li1.huang at intel.com>
Subject: Re: [llvm] r274098 - [ValueTracking] Teach computeKnownBits for PHI nodes to compute sign bit for a recurrence with a NSW addition.

I think I have to redact my "I suspect this isn't anything complicated" tone. :)

To do the right thing in cases like the one you pastebinned, we'll have to track `IsSigned` on individual `NarrowIVDefUse` instances.

Then for instance, in

   %i.050 = phi i32 [ %add15, %for.body ], [ 0, %for.body.preheader ]
   %idxprom45 = zext i32 %i.050 to i64
   %arrayidx = getelementptr inbounds i32, i32* %A, i64 %idxprom45
   %0 = load i32, i32* %arrayidx, align 4
   %sub = add nsw i32 %i.050, -1
   %idxprom1 = sext i32 %sub to i64
   %arrayidx2 = getelementptr inbounds i32, i32* %B, i64 %idxprom1
   %1 = load i32, i32* %arrayidx2, align 4

we'll first replace `%idxprom45` with the zero extended wide IV, and then when we get to `%sub`, we'll replace it with `add %wide.zexted.iv, -1` but the corresponding `NarrowIVDefUse` we push to the worklist will have to remember that this new wide add instruction is really a `sext` of `%sub` (and accordingly handle its users).

I'll be on vacation from 30th June to 10th of July, but I should be able to get to this once I'm back (unfortunately, I probably won't have time before I leave).  If you want to fix this and have someone else or me (once I'm back) review it that's fine too.

Thanks,
-- Sanjoy