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

[via llvm-commits]

================
@@ -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) {
+  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 Max;
+  if (SDValue UMin = matchMinMax(In, ISD::UMIN, Max, /*Signed*/ false)) {
+    // C2 should be equal to UINT32_MAX / UINT16_MAX / UINT8_MAX according
+    // the element size of the destination type.
+    if (Max.isMask(VT.getScalarSizeInBits()))
+      return UMin;
+  }
+
+  return SDValue();
+}
+
+/// Detect patterns of truncation with signed saturation:
+/// (truncate (smin ((smax (x, signed_min_of_dest_type)),
+///                  signed_max_of_dest_type)) to dest_type)
+/// or:
+/// (truncate (smax ((smin (x, signed_max_of_dest_type)),
+///                  signed_min_of_dest_type)) to dest_type).
+/// With MatchPackUS, the smax/smin range is [0, unsigned_max_of_dest_type].
+/// Return the source value to be truncated or SDValue() if the pattern was not
+/// matched.
+static SDValue detectSSatSPattern(SDValue In, EVT VT) {
+  unsigned NumDstBits = VT.getScalarSizeInBits();
+  unsigned NumSrcBits = In.getScalarValueSizeInBits();
+  assert(NumSrcBits > NumDstBits && "Unexpected types for truncate operation");
+
+  APInt SignedMax, SignedMin;
+  SignedMax = APInt::getSignedMaxValue(NumDstBits).sext(NumSrcBits);
+  SignedMin = APInt::getSignedMinValue(NumDstBits).sext(NumSrcBits);
+  if (SDValue SMin = matchMinMax(In, ISD::SMIN, SignedMax, /*Signed*/ true)) {
+    if (SDValue SMax =
+            matchMinMax(SMin, ISD::SMAX, SignedMin, /*Signed*/ true)) {
+      return SMax;
+    }
+  }
+  if (SDValue SMax = matchMinMax(In, ISD::SMAX, SignedMin, /*Signed*/ true)) {
+    if (SDValue SMin =
+            matchMinMax(SMax, ISD::SMIN, SignedMax, /*Signed*/ true)) {
+      return SMin;
+    }
+  }
+  return SDValue();
+}
+
+/// Detect patterns of truncation with unsigned saturation:
+static SDValue detectSSatUPattern(SDValue In, EVT VT, SelectionDAG &DAG,
+                                  const SDLoc &DL) {
+  EVT InVT = In.getValueType();
+
+  // Saturation with truncation. We truncate from InVT to VT.
+  assert(InVT.getScalarSizeInBits() > VT.getScalarSizeInBits() &&
+         "Unexpected types for truncate operation");
+
+  APInt Min, Max;
+  SDValue SMax, SMin, UMin, First;
+  // (truncate (smin (smax (x, Min), Max)) to dest_type),
+  // (truncate (smax (smin (x, Max), Min)) to dest_type)
+  // (truncate (umin (smax (x, Min), Max)) to dest_type)
+  // where Min >= 0, Max is unsigned max of destination type and Min <= Max.
+  if (First = SMax = matchMinMax(In, ISD::SMAX, Min, /*Signed*/ false))
+    SMin = matchMinMax(First, ISD::SMIN, Max, /*Signed*/ false);
+  else if (First = SMin = matchMinMax(In, ISD::SMIN, Max, /*Signed*/ false))
+    SMax = matchMinMax(First, ISD::SMAX, Min, /*Signed*/ false);
+  else if (First = UMin = matchMinMax(In, ISD::UMIN, Max, /*Signed*/ false))
+    SMax = matchMinMax(UMin, ISD::SMAX, Min, /*Signed*/ false);
+
+  if (SMax && Min.isNonNegative() && Max.isMask(VT.getScalarSizeInBits())) {
----------------
ParkHanbum wrote:

@davemgreen how about this? 
```
static SDValue matchMin(SDValue V, unsigned Opcode, APInt &Limit) {
  if (V.getOpcode() == Opcode) {
    APInt C;
    if (ISD::isConstantSplatVector(V.getOperand(1).getNode(), C))
      if (C == Limit)
        return V.getOperand(0);
      if (C > Limit && Limit == 0)
        return V.getOperand(0);
  }
  return SDValue();
}
```



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