[llvm] [DAGCombiner] Extend fp->int->fp optimizations to include clamping (PR #164502)

[Matt Arsenault via llvm-commits]

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@@ -19051,28 +19054,68 @@ static SDValue foldFPToIntToFP(SDNode *N, const SDLoc &DL, SelectionDAG &DAG,
   if (!TLI.isOperationLegal(ISD::FTRUNC, VT))
     return SDValue();
 
-  // fptosi/fptoui round towards zero, so converting from FP to integer and
-  // back is the same as an 'ftrunc': [us]itofp (fpto[us]i X) --> ftrunc X
-  SDValue N0 = N->getOperand(0);
-  if (N->getOpcode() == ISD::SINT_TO_FP && N0.getOpcode() == ISD::FP_TO_SINT &&
-      N0.getOperand(0).getValueType() == VT) {
-    if (DAG.getTarget().Options.NoSignedZerosFPMath)
-      return DAG.getNode(ISD::FTRUNC, DL, VT, N0.getOperand(0));
-  }
+  bool IsUnsigned = N->getOpcode() == ISD::UINT_TO_FP;
+  bool IsSigned = N->getOpcode() == ISD::SINT_TO_FP;
+  assert(IsSigned || IsUnsigned);
 
-  if (N->getOpcode() == ISD::UINT_TO_FP && N0.getOpcode() == ISD::FP_TO_UINT &&
-      N0.getOperand(0).getValueType() == VT) {
-    if (DAG.getTarget().Options.NoSignedZerosFPMath)
-      return DAG.getNode(ISD::FTRUNC, DL, VT, N0.getOperand(0));
+  bool IsSignedZeroSafe = DAG.getTarget().Options.NoSignedZerosFPMath;
+  // For signed conversions: The optimization changes signed zero behavior.
+  if (IsSigned && !IsSignedZeroSafe)
+    return SDValue();
+  // For unsigned conversions, we need FABS to canonicalize -0.0 to +0.0
+  // (unless NoSignedZerosFPMath is set).
+  if (IsUnsigned && !IsSignedZeroSafe && !TLI.isFAbsFree(VT))
+    return SDValue();
 
-    // Strict math: use FABS to handle negative inputs correctly.
-    if (TLI.isFAbsFree(VT)) {
-      SDValue Abs = DAG.getNode(ISD::FABS, DL, VT, N0.getOperand(0));
-      return DAG.getNode(ISD::FTRUNC, DL, VT, Abs);
-    }
+  // Collect potential clamp operations (innermost to outermost) and peel.
+  struct ClampOp {
+    unsigned Opcode;
+    SDValue Constant;
+  };
+  SmallVector<ClampOp, 2> Clamps;
+  unsigned MinOp = IsUnsigned ? ISD::UMIN : ISD::SMIN;
+  unsigned MaxOp = IsUnsigned ? ISD::UMAX : ISD::SMAX;
+  SDValue IntVal = N->getOperand(0);
+  constexpr unsigned MaxClampLevels = 2;
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arsenm wrote:

Canonically how could this ever be more than 2? 

https://github.com/llvm/llvm-project/pull/164502