[clang] [llvm] [IR] Allow fast math flags on fptrunc and fpext (PR #115894)

[John Brawn via cfe-commits]

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@@ -1875,13 +1873,17 @@ Instruction *InstCombinerImpl::visitFPTrunc(FPTruncInst &FPT) {
       // fptrunc (select Cond, (fpext X), Y --> select Cond, X, (fptrunc Y)
       Value *NarrowY = Builder.CreateFPTrunc(Y, Ty);
       Value *Sel = Builder.CreateSelect(Cond, X, NarrowY, "narrow.sel", Op);
+      if (auto *I = dyn_cast<Instruction>(Sel))
+        I->setFastMathFlags(Op->getFastMathFlags());
       return replaceInstUsesWith(FPT, Sel);
     }
     if (match(Op, m_Select(m_Value(Cond), m_Value(Y), m_FPExt(m_Value(X)))) &&
         X->getType() == Ty) {
       // fptrunc (select Cond, Y, (fpext X) --> select Cond, (fptrunc Y), X
       Value *NarrowY = Builder.CreateFPTrunc(Y, Ty);
       Value *Sel = Builder.CreateSelect(Cond, NarrowY, X, "narrow.sel", Op);
+      if (auto *I = dyn_cast<Instruction>(Sel))
+        I->setFastMathFlags(Op->getFastMathFlags());
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john-brawn-arm wrote:

IRBuilder::CreateSelect uses the default fast math flags of the IRBuilder, which will be the flags of the fptrunc. So we need this to make sure the new select has the same flags as the old select.

https://github.com/llvm/llvm-project/pull/115894