[llvm] [InstCombine] Fix for folding `select` into floating point binary operators. (PR #83200)

[Paul Osmialowski via llvm-commits]

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@@ -536,19 +536,29 @@ Instruction *InstCombinerImpl::foldSelectIntoOp(SelectInst &SI, Value *TrueVal,
     // between 0, 1 and -1.
     const APInt *OOpC;
     bool OOpIsAPInt = match(OOp, m_APInt(OOpC));
-    if (!isa<Constant>(OOp) ||
-        (OOpIsAPInt && isSelect01(C->getUniqueInteger(), *OOpC))) {
-      Value *NewSel = Builder.CreateSelect(SI.getCondition(), Swapped ? C : OOp,
-                                           Swapped ? OOp : C, "", &SI);
-      if (isa<FPMathOperator>(&SI))
-        cast<Instruction>(NewSel)->setFastMathFlags(FMF);
-      NewSel->takeName(TVI);
-      BinaryOperator *BO =
-          BinaryOperator::Create(TVI->getOpcode(), FalseVal, NewSel);
-      BO->copyIRFlags(TVI);
-      return BO;
-    }
-    return nullptr;
+    if (isa<Constant>(OOp) &&
+        (!OOpIsAPInt || !isSelect01(C->getUniqueInteger(), *OOpC)))
+      return nullptr;
+
+    // If the false value is a NaN then we have that the floating point math
+    // operation in the transformed code may not preserve the exact NaN
+    // bit-pattern -- e.g. `fadd sNaN, 0.0 -> qNaN`.
+    // This makes the transformation incorrect since the original program would
+    // have preserved the exact NaN bit-pattern.
+    // Avoid the folding if the false value might be a NaN.
+    if (isa<FPMathOperator>(&SI) &&
+        !computeKnownFPClass(FalseVal, FMF, fcNan, &SI).isKnownNeverNaN())
+      return nullptr;
+
+    Value *NewSel = Builder.CreateSelect(SI.getCondition(), Swapped ? C : OOp,
+                                         Swapped ? OOp : C, "", &SI);
+    if (isa<FPMathOperator>(&SI))
+      cast<Instruction>(NewSel)->setFastMathFlags(FMF);
+    NewSel->takeName(TVI);
+    BinaryOperator *BO =
+        BinaryOperator::Create(TVI->getOpcode(), FalseVal, NewSel);
+    BO->copyIRFlags(TVI);
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pawosm-arm wrote:

> > I think this can or in the value flags from the select
> 
> I don't even know what does it mean :(

Ok, I guess I've deciphered it. I guess what you propose is to 'or' the flags copied from TVI with value flags from the select. Trouble is, SI don't have any flags, they were not generated by simplifyCFG pass since they were lost in SROA pass.

https://github.com/llvm/llvm-project/pull/83200