[all-commits] [llvm/llvm-project] 49f751: [DivRemPairs] Freeze operands if they can be undef...

[Juneyoung Lee via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 49f75132bcdfcbc23010252e04e43fe0278ae1e7
      https://github.com/llvm/llvm-project/commit/49f75132bcdfcbc23010252e04e43fe0278ae1e7
  Author: Juneyoung Lee <aqjune at gmail.com>
  Date:   2020-03-25 (Wed, 25 Mar 2020)

  Changed paths:
    M llvm/lib/Transforms/Scalar/DivRemPairs.cpp
    M llvm/test/Transforms/DivRemPairs/PowerPC/div-expanded-rem-pair.ll
    M llvm/test/Transforms/DivRemPairs/PowerPC/div-rem-pairs.ll
    M llvm/test/Transforms/DivRemPairs/X86/div-rem-pairs.ll

  Log Message:
  -----------
  [DivRemPairs] Freeze operands if they can be undef values

Summary:
DivRemPairs is unsound with respect to undef values.

```
      // bb1:
      //   %rem = srem %x, %y
      // bb2:
      //   %div = sdiv %x, %y
      // -->
      // bb1:
      //   %div = sdiv %x, %y
      //   %mul = mul %div, %y
      //   %rem = sub %x, %mul
```

If X can be undef, X should be frozen first.
For example, let's assume that Y = 1 & X = undef:
```
   %div = sdiv undef, 1 // %div = undef
   %rem = srem undef, 1 // %rem = 0
 =>
   %div = sdiv undef, 1 // %div = undef
   %mul = mul %div, 1   // %mul = undef
   %rem = sub %x, %mul  // %rem = undef - undef = undef
```
http://volta.cs.utah.edu:8080/z/m7Xrx5

Same for Y. If X = 1 and Y = (undef | 1), %rem in src is either 1 or 0,
but %rem in tgt can be one of many integer values.

This resolves https://bugs.llvm.org/show_bug.cgi?id=42619 .

This miscompilation disappears if undef value is removed, but it may take a while.
DivRemPair happens pretty late during the optimization pipeline, so this optimization seemed as a good candidate to fix without major regression using freeze than other broken optimizations.

Reviewers: spatel, lebedev.ri, george.burgess.iv

Reviewed By: spatel

Subscribers: wuzish, regehr, nlopes, nemanjai, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76483