[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:

Do you mean these specific changes for setting the fast math flag on select instructions, or also the change at the top of visitFPTrunc to set the fast math flags in the builder based on the fptrunc? Either way it causes failures in the llvm/test/Transforms/InstCombine/fpcast.ll and llvm/test/Transforms/InstCombine/fptrunc.ll tests. Without setting the fast math flags in the select the failures are because the select gets the fast math flags from the fptrunc. Without setting the flags in the builder the opposite happens and the fptrunc gets the fast math flags from the select.

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