[llvm] [AMDGPU] Fold redundant inf/nan checks into frexp instructions (PR #214936)

[Matt Arsenault via llvm-commits]

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@@ -18829,6 +18829,131 @@ SDValue SITargetLowering::performClampCombine(SDNode *N,
   return getCanonicalConstantFP(DCI.DAG, SDLoc(N), N->getValueType(0), F);
 }
 
+/// Check if a value is a positive or negative infinity constant.
+static bool isInfinityFPConstant(SDValue V) {
+  auto *CFP = dyn_cast<ConstantFPSDNode>(V);
+  return CFP && CFP->getValueAPF().isInfinity();
+}
+
+SDValue
+SITargetLowering::performFrexpSelectCombine(SDNode *N,
+                                            DAGCombinerInfo &DCI) const {
+  // This optimization only applies when the hardware handles inf/nan correctly.
+  if (Subtarget->hasFractBug())
+    return SDValue();
+
+  SDValue Cond = N->getOperand(0);
+  SDValue TrueVal = N->getOperand(1);
+  SDValue FalseVal = N->getOperand(2);
+
+  // Determine which value is 0 and which might be the frexp result.
+  // Pattern 1: select cond, 0, frexp_result (cond true -> return 0)
+  // Pattern 2: select cond, frexp_result, 0 (cond false -> return 0)
+  // Only check FP zero - frexp returns FP or integer, handle separately.
+  SDValue FrexpVal;
+  bool CondSelectsZero; // If true, condition=true selects zero
+
+  bool TrueIsFPZero = isNullFPConstant(TrueVal);
+  bool FalseIsFPZero = isNullFPConstant(FalseVal);
+  bool TrueIsIntZero = isNullConstant(TrueVal);
+  bool FalseIsIntZero = isNullConstant(FalseVal);
+
+  if (TrueIsFPZero || TrueIsIntZero) {
+    FrexpVal = FalseVal;
+    CondSelectsZero = true;
+  } else if (FalseIsFPZero || FalseIsIntZero) {
+    FrexpVal = TrueVal;
+    CondSelectsZero = false;
+  } else {
+    return SDValue();
+  }
+
+  // Check if FrexpVal comes from amdgcn_frexp_exp or amdgcn_frexp_mant.
+  SDValue FrexpInput;
+  if (!sd_match(FrexpVal, m_IntrinsicWOChain<Intrinsic::amdgcn_frexp_exp>(
+                              m_Value(FrexpInput))) &&
+      !sd_match(FrexpVal, m_IntrinsicWOChain<Intrinsic::amdgcn_frexp_mant>(
+                              m_Value(FrexpInput))))
+    return SDValue();
----------------
arsenm wrote:

Maybe try matching against ISD::FREXP?

https://github.com/llvm/llvm-project/pull/214936