[PATCH] D65474: [SCEV] Return zero from computeConstantDifference(X, X)

[Nikolai Bozhenov via Phabricator via llvm-commits]

n.bozhenov created this revision.
n.bozhenov added reviewers: sanjoy, llvm-commits.
Herald added a subscriber: hiraditya.
Herald added a project: LLVM.

Without this patch computeConstantDifference returns None for cases like
these:

  computeConstantDifference(%x, %x)
  computeConstantDifference({%x,+,16}, {%x,+,16})


Repository:
  rL LLVM

https://reviews.llvm.org/D65474

Files:
  llvm/lib/Analysis/ScalarEvolution.cpp


Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -9833,6 +9833,10 @@
   // We avoid subtracting expressions here because this function is usually
   // fairly deep in the call stack (i.e. is called many times).
 
+  // X - X = 0.
+  if (More == Less)
+    return APInt(getTypeSizeInBits(More->getType()), 0);
+
   if (isa<SCEVAddRecExpr>(Less) && isa<SCEVAddRecExpr>(More)) {
     const auto *LAR = cast<SCEVAddRecExpr>(Less);
     const auto *MAR = cast<SCEVAddRecExpr>(More);


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