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

[Matt Arsenault via llvm-commits]

================
@@ -18829,6 +18829,145 @@ SDValue SITargetLowering::performClampCombine(SDNode *N,
   return getCanonicalConstantFP(DCI.DAG, SDLoc(N), N->getValueType(0), F);
 }
 
+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)
+  SDValue FrexpVal;
+  bool CondSelectsZero; // If true, condition=true selects zero
+
+  auto isZero = [](SDValue V) {
+    if (auto *C = dyn_cast<ConstantSDNode>(V))
+      return C->isZero();
+    if (auto *C = dyn_cast<ConstantFPSDNode>(V))
+      return C->isZero();
+    return false;
+  };
+
+  if (isZero(TrueVal)) {
+    FrexpVal = FalseVal;
+    CondSelectsZero = true;
+  } else if (isZero(FalseVal)) {
+    FrexpVal = TrueVal;
+    CondSelectsZero = false;
+  } else {
+    return SDValue();
+  }
+
+  // Check if FrexpVal comes from amdgcn_frexp_exp or amdgcn_frexp_mant.
+  if (FrexpVal.getOpcode() != ISD::INTRINSIC_WO_CHAIN)
+    return SDValue();
+
+  unsigned IID = FrexpVal.getConstantOperandVal(0);
+  if (IID != Intrinsic::amdgcn_frexp_exp && IID != Intrinsic::amdgcn_frexp_mant)
+    return SDValue();
+
+  SDValue FrexpInput = FrexpVal.getOperand(1);
+
+  // Helper to strip fabs/fneg/fcopysign from a value.
+  auto peekFPSignOps = [](SDValue Val) {
----------------
arsenm wrote:

Pretty sure this helper exists already 

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