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

[Matt Arsenault via llvm-commits]

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@@ -8037,6 +8037,46 @@ Instruction *InstCombinerImpl::visitFCmpInst(FCmpInst &I) {
       if (Instruction *NV = FoldOpIntoSelect(I, cast<SelectInst>(LHSI)))
         return NV;
       break;
+    case Instruction::FSub:
+      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:
+        // Skip optimization: fsub x, y unless guaranteed !isinf(x) ||
+        // !isinf(y).
+        if (!LHSI->hasOneUse() ||
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arsenm wrote:

Instead of duplicating the hasOneUse check, you could hoist the whole inner pattern match below before the condition type handling. It would also probably be nicer to read splitting the whole switch into a helper function 

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