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

[David Green via llvm-commits]

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@@ -14915,6 +14920,166 @@ 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) {
+  if (V.getOpcode() == Opcode) {
+    if (Signed) {
+      APInt C;
+      if (ISD::isConstantSplatVector(V.getOperand(1).getNode(), C) &&
+          C == Limit)
+        return V.getOperand(0);
+    } else if (ISD::isConstantSplatVector(V.getOperand(1).getNode(), Limit))
+      return V.getOperand(0);
+  }
+  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");
+
+  APInt C1, C2;
+  if (SDValue UMin = matchMinMax(In, ISD::UMIN, C2, /*Signed*/ false)) {
+    // C2 should be equal to UINT32_MAX / UINT16_MAX / UINT8_MAX according
+    // the element size of the destination type.
+    if (C2.isMask(VT.getScalarSizeInBits())) {
+      // (truncate (umin (smax (x, C1), C2)))
+      // where C1 == 0, C2 is unsigned max of destination type.
+      if (SDValue SMax = matchMinMax(UMin, ISD::SMAX, C1, /*Signed*/ false)) {
----------------
davemgreen wrote:

I think this logic should be in detectSSatUPattern.

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