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

[David Green via llvm-commits]

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@@ -14915,6 +14920,159 @@ 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);
+  SDValue FPInstr = N0.getOpcode() == ISD::SMAX ? N0.getOperand(0) : N0;
+  if (FPInstr.getOpcode() == ISD::FP_TO_SINT ||
+      FPInstr.getOpcode() == ISD::FP_TO_UINT) {
+    EVT FPVT = FPInstr.getOperand(0).getValueType();
+    if (!DAG.getTargetLoweringInfo().shouldConvertFpToSat(ISD::FP_TO_UINT_SAT,
+                                                          FPVT, VT))
+      return SDValue();
+    SDValue Sat = DAG.getNode(ISD::FP_TO_UINT_SAT, SDLoc(FPInstr), VT,
+                              FPInstr.getOperand(0),
+                              DAG.getValueType(VT.getScalarType()));
+    return Sat;
+  }
+
+  return SDValue();
+}
+
+/// Detect patterns of truncation with unsigned saturation:
+///
+/// (truncate (umin (x, unsigned_max_of_dest_type)) to dest_type).
+/// Return the source value x to be truncated or SDValue() if the pattern was
+/// not matched.
+///
+static SDValue detectUSatUPattern(SDValue In, EVT VT) {
+  EVT InVT = In.getValueType();
+
+  // Saturation with truncation. We truncate from InVT to VT.
+  assert(InVT.getScalarSizeInBits() > VT.getScalarSizeInBits() &&
+         "Unexpected types for truncate operation");
+
+  // Match min/max and return limit value as a parameter.
+  auto MatchMinMax = [](SDValue V, unsigned Opcode, APInt &Limit) -> SDValue {
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davemgreen wrote:

MatchMinMax can be made a separate function so that it is shared between all the three new routines, if they work the same way (either return the constant and check it later or check for the correct constant).

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