[llvm] [InstCombine] Make fptrunc combine use intersection of fast math flags (PR #118808)

[John Brawn via llvm-commits]

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@@ -1847,15 +1847,16 @@ Instruction *InstCombinerImpl::visitFPTrunc(FPTruncInst &FPT) {
   Value *X;
   Instruction *Op = dyn_cast<Instruction>(FPT.getOperand(0));
   if (Op && Op->hasOneUse()) {
-    // FIXME: The FMF should propagate from the fptrunc, not the source op.
     IRBuilder<>::FastMathFlagGuard FMFG(Builder);
+    FastMathFlags FMF = FPT.getFastMathFlags();
     if (isa<FPMathOperator>(Op))
-      Builder.setFastMathFlags(Op->getFastMathFlags());
+      FMF &= Op->getFastMathFlags();
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john-brawn-arm wrote:

Yes, I meant intersection.

https://github.com/llvm/llvm-project/pull/118808