[llvm] [InstCombine] Fold expression using basic properties of floor and ceiling function (PR #107107)

[via llvm-commits]

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@@ -8178,6 +8178,75 @@ static Instruction *foldFCmpFSubIntoFCmp(FCmpInst &I, Instruction *LHSI,
   return nullptr;
 }
 
+static Instruction *foldFCmpWithFloorAndCeil(FCmpInst &I,
+                                             InstCombinerImpl &IC) {
+  Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
+  Type *OpType = LHS->getType();
+  CmpInst::Predicate Pred = I.getPredicate();
+
+  bool FloorX = match(LHS, m_Intrinsic<Intrinsic::floor>(m_Specific(RHS)));
+  bool CeilX = match(LHS, m_Intrinsic<Intrinsic::ceil>(m_Specific(RHS)));
+
+  if (!FloorX && !CeilX) {
+    if ((FloorX = match(RHS, m_Intrinsic<Intrinsic::floor>(m_Specific(LHS)))) ||
+        (CeilX = match(RHS, m_Intrinsic<Intrinsic::ceil>(m_Specific(LHS))))) {
+      std::swap(LHS, RHS);
+      Pred = I.getSwappedPredicate();
+    }
+  }
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c8ef wrote:

I believe we still need to include the `fcmp` instruction in the function to preserve the `fmf` flags. The current implementation should be sufficient.

https://github.com/llvm/llvm-project/pull/107107