[llvm] [DAG] Support saturated truncate (PR #99418)

[David Green via llvm-commits]

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@@ -14915,6 +14920,181 @@ SDValue DAGCombiner::visitEXTEND_VECTOR_INREG(SDNode *N) {
   return SDValue();
 }
 
+SDValue DAGCombiner::visitTRUNCATE_USAT(SDNode *N) {
+  EVT VT = N->getValueType(0);
+  SDValue N0 = N->getOperand(0);
+
+  auto MatchFPTOINT = [&](SDValue Val) -> SDValue {
+    if (Val.getOpcode() == ISD::FP_TO_SINT ||
+        Val.getOpcode() == ISD::FP_TO_UINT)
+      return Val;
+    return SDValue();
+  };
+
+  SDValue FPInstr;
+  if (N0.getOpcode() == ISD::SMAX) {
+    FPInstr = MatchFPTOINT(N0.getOperand(0));
+    if (!FPInstr)
+      FPInstr = MatchFPTOINT(N0.getOperand(1));
+  } else
+    FPInstr = MatchFPTOINT(N0);
+
+  if (FPInstr) {
+    EVT FPVT = FPInstr.getOperand(0).getValueType();
+    if (!DAG.getTargetLoweringInfo().shouldConvertFpToSat(ISD::FP_TO_UINT_SAT,
+                                                          FPVT, VT))
+      return SDValue();
+    return DAG.getNode(ISD::FP_TO_UINT_SAT, SDLoc(FPInstr), VT,
+                       FPInstr.getOperand(0),
+                       DAG.getValueType(VT.getScalarType()));
+  }
+
+  return SDValue();
+}
+
+// Match min/max and return limit value as a parameter.
+static SDValue matchMinMax(SDValue V, unsigned Opcode, APInt &Limit,
+                           bool Signed) {
----------------
davemgreen wrote:

I would be tempted to remove the Signed parameter and always pass in the APInt we expect. I think the masks can be created with something like `APInt::getLowBitsSet(NumSrcBits, NumDstBits)`

https://github.com/llvm/llvm-project/pull/99418