[llvm] [InstCombine] Fold fcmp ogt (x - y), 0 into fcmp ogt x, y #85245 (PR #85506)

[via llvm-commits]

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@@ -7973,11 +7973,36 @@ Instruction *InstCombinerImpl::visitFCmpInst(FCmpInst &I) {
   if (match(Op0, m_Instruction(LHSI)) && match(Op1, m_Constant(RHSC))) {
     switch (LHSI->getOpcode()) {
     case Instruction::FSub:
-      if ((Pred == FCmpInst::FCMP_OGT || Pred == FCmpInst::FCMP_OLT ||
-           Pred == FCmpInst::FCMP_ONE) &&
-          match(RHSC, m_AnyZeroFP()) &&
-          match(LHSI, m_FSub(m_Value(X), m_Value(Y))))
-        return new FCmpInst(Pred, X, Y);
+      switch (Pred) {
+      default:
+        break;
+      case FCmpInst::FCMP_UGT:
+      case FCmpInst::FCMP_ULT:
+      case FCmpInst::FCMP_UNE:
+      case FCmpInst::FCMP_OEQ:
+      case FCmpInst::FCMP_OGE:
+      case FCmpInst::FCMP_OLE: {
+        BinaryOperator *SubI = cast<BinaryOperator>(LHSI);
+        if (!computeKnownFPClass(SubI->getOperand(0), SubI->getFastMathFlags(),
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SahilPatidar wrote:

@jayfoad Perhaps we could handle this that way.
```cpp
        if (!computeKnownFPClass(SubI->getOperand(0), SubI->getFastMathFlags(),
                                 fcNan | fcInf, LHSI, 0)
                 .isKnownNever(fcNan | fcInf) &&
            !computeKnownFPClass(SubI->getOperand(1), SubI->getFastMathFlags(),
                                 fcNan | fcInf, LHSI, 0)
                 .isKnownNever(fcNan | fcInf))
```

https://github.com/llvm/llvm-project/pull/85506