[PATCH] D76931: [ValueLattice] Distinguish between constant ranges with/without undef.

[Florian Hahn via Phabricator via llvm-commits]

fhahn updated this revision to Diff 253634.
fhahn added a comment.

I've adjusted the defaults after rG05f0e598ab26 <https://reviews.llvm.org/rG05f0e598ab265a80fedb23225cde4176f11774ac>: per default, markConstantRange & co assume that the constant range does not contain undef (existing behavior) and isConstantRange/getConstantRange allow constant ranges including undef (existing behavior).

mergeIn automatically marks constant ranges as containing undef based on the merged values. Clients that manually construct ValueLatticeElements are responsible for marking them as containing undef accordingly and also update isConstantRange/getConstantRange calls to exclude ranges including undef where it is not safe. One example is processAnd in CorrelatedValuePropagation, but there will be future changes needed to update all places in LVI that construct lattice elements manually. Currently it uses a helper that returns an optional constant range in multiple places so there's no easy way to recover the information if it may be undef.

I'll also update the example in the description shortly to the code below. %p will be represented as constantrange_including_undef [0, 256) and it would not be allowed to replace %res with %p. Simplifying the condition %f.1 based on range information including undef is allowed, if we replace it with a constant (and remove the original instruction that contained the use of the potentially undef value).

  define i32 @f(i32 %a, i1 %c) {
    br i1 %c, label %true, label %false
  true:
    %a.255 = and i32 %a, 255
    br label %exit
  false:
    br label %exit
  exit:
    %p = phi i32 [ %a.255, %true ], [ undef, %false ]
    %f.1 = icmp eq i32 %p, 300
    call void @use(i1 %f.1)
    %res = and i32 %p, 255
    ret i32 %res
  }

The changed test cases should give a good idea of the kind of improvements this gives over the singlecr approach.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D76931/new/

https://reviews.llvm.org/D76931

Files:
  llvm/include/llvm/Analysis/LazyValueInfo.h
  llvm/include/llvm/Analysis/ValueLattice.h
  llvm/lib/Analysis/LazyValueInfo.cpp
  llvm/lib/Analysis/ValueLattice.cpp
  llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
  llvm/test/Transforms/Attributor/range.ll
  llvm/test/Transforms/CorrelatedValuePropagation/merge-range-and-undef.ll
  llvm/test/Transforms/SCCP/phi-cycle.ll
  llvm/test/Transforms/SCCP/range-and-ip.ll
  llvm/test/Transforms/SCCP/range-and.ll

-------------- next part --------------
A non-text attachment was scrubbed...
Name: D76931.253634.patch
Type: text/x-patch
Size: 21589 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20200330/818cac9c/attachment-0001.bin>