[PATCH] D51216: Fix IRBuilder.CreateFCmp(X, X) misfolding

[Andrew Scheidecker via Phabricator via llvm-commits]

AndrewScheidecker added a comment.

> Hm, can we use fast-math flags here? Namely, `nnan`.

It's possible, but not a trivial change. I added a nullable const FastMathFlags* parameter to:

- evaluateFCmpRelation and ConstantFoldCompareInstruction
- ConstantExpr::getCompare and ConstantExpr::getFCmp
- TargetFolder::CreateFCmp, ConstantFolder::CreateFCmp, and NoFolder

Then, IRBuilder::CreateFCmp can pass its current FastMathFlags through to the constant folder. Most places in LLVM I saw that were calling IRBuilder::CreateFCmp seemed to be setting the FastMathFlags, but there were a few transforms that didn't seem to be propagating them through:

- foldLogicOfFCmps in InstCombineAndOrXor.cpp
- foldVectorCmp in InstCombineCompares.cpp
- foldOpIntoPhi in InstructionCombining.cpp (line 993)
- FCmpSplitter in Scalarizer.cpp
- vectorizeTree(TreeEntry*) in SLPVectorizer.cpp (line 3223)

I stopped trying to propagate the fast-math flags dependency out when I got to ConstantFolding.cpp (i.e. the constant folding pass rather than the IRBuilder constant folder). Since ConstantExpr doesn't have fast-math flags, there isn't a way to transmit the fast-math flags from whatever produced the ConstantExpr.

So I think it's doable, but beyond the scope of this change.


Repository:
  rL LLVM

https://reviews.llvm.org/D51216