[Codegen] Alter the default promotion for saturating adds and subs

The default promotion for the add_sat/sub_sat nodes currently does:
   1. ANY_EXTEND iN to iM
   2. SHL by M-N
   3. [US][ADD|SUB]SAT
   4. L/ASHR by M-N
If the promoted add_sat or sub_sat node is not legal, this can produce code
that effectively does a lot of shifting (and requiring large constants to be
materialised) just to use the overflow flag. It is simpler to just do the
saturation manually, using the higher bitwidth addition and a min/max against
the saturating bounds. That is what this patch attempts to do.

Differential Revision: https://reviews.llvm.org/D68643

llvm-svn: 374373

1 files changed, 61 insertions, 31 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp b/llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp
index e094981a191..3c8f63e46bc 100644
--- a/llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp
@@ -642,48 +642,78 @@ SDValue DAGTypeLegalizer::PromoteIntRes_Overflow(SDNode *N) {
 }
 
 SDValue DAGTypeLegalizer::PromoteIntRes_ADDSUBSAT(SDNode *N) {
-  // For promoting iN -> iM, this can be expanded by
-  // 1. ANY_EXTEND iN to iM
-  // 2. SHL by M-N
-  // 3. [US][ADD|SUB]SAT
-  // 4. L/ASHR by M-N
+  // If the promoted type is legal, we can convert this to:
+  //   1. ANY_EXTEND iN to iM
+  //   2. SHL by M-N
+  //   3. [US][ADD|SUB]SAT
+  //   4. L/ASHR by M-N
+  // Else it is more efficient to convert this to a min and a max
+  // operation in the higher precision arithmetic.
   SDLoc dl(N);
   SDValue Op1 = N->getOperand(0);
   SDValue Op2 = N->getOperand(1);
   unsigned OldBits = Op1.getScalarValueSizeInBits();
 
   unsigned Opcode = N->getOpcode();
-  unsigned ShiftOp;
-  switch (Opcode) {
-  case ISD::SADDSAT:
-  case ISD::SSUBSAT:
-    ShiftOp = ISD::SRA;
-    break;
-  case ISD::UADDSAT:
-  case ISD::USUBSAT:
-    ShiftOp = ISD::SRL;
-    break;
-  default:
-    llvm_unreachable("Expected opcode to be signed or unsigned saturation "
-                     "addition or subtraction");
-  }
 
   SDValue Op1Promoted = GetPromotedInteger(Op1);
   SDValue Op2Promoted = GetPromotedInteger(Op2);
-
   EVT PromotedType = Op1Promoted.getValueType();
   unsigned NewBits = PromotedType.getScalarSizeInBits();
-  unsigned SHLAmount = NewBits - OldBits;
-  EVT SHVT = TLI.getShiftAmountTy(PromotedType, DAG.getDataLayout());
-  SDValue ShiftAmount = DAG.getConstant(SHLAmount, dl, SHVT);
-  Op1Promoted =
-      DAG.getNode(ISD::SHL, dl, PromotedType, Op1Promoted, ShiftAmount);
-  Op2Promoted =
-      DAG.getNode(ISD::SHL, dl, PromotedType, Op2Promoted, ShiftAmount);
-
-  SDValue Result =
-      DAG.getNode(Opcode, dl, PromotedType, Op1Promoted, Op2Promoted);
-  return DAG.getNode(ShiftOp, dl, PromotedType, Result, ShiftAmount);
+
+  if (TLI.isOperationLegalOrCustom(Opcode, PromotedType)) {
+    unsigned ShiftOp;
+    switch (Opcode) {
+    case ISD::SADDSAT:
+    case ISD::SSUBSAT:
+      ShiftOp = ISD::SRA;
+      break;
+    case ISD::UADDSAT:
+    case ISD::USUBSAT:
+      ShiftOp = ISD::SRL;
+      break;
+    default:
+      llvm_unreachable("Expected opcode to be signed or unsigned saturation "
+                       "addition or subtraction");
+    }
+
+    unsigned SHLAmount = NewBits - OldBits;
+    EVT SHVT = TLI.getShiftAmountTy(PromotedType, DAG.getDataLayout());
+    SDValue ShiftAmount = DAG.getConstant(SHLAmount, dl, SHVT);
+    Op1Promoted =
+        DAG.getNode(ISD::SHL, dl, PromotedType, Op1Promoted, ShiftAmount);
+    Op2Promoted =
+        DAG.getNode(ISD::SHL, dl, PromotedType, Op2Promoted, ShiftAmount);
+
+    SDValue Result =
+        DAG.getNode(Opcode, dl, PromotedType, Op1Promoted, Op2Promoted);
+    return DAG.getNode(ShiftOp, dl, PromotedType, Result, ShiftAmount);
+  } else {
+    if (Opcode == ISD::USUBSAT) {
+      SDValue Max =
+          DAG.getNode(ISD::UMAX, dl, PromotedType, Op1Promoted, Op2Promoted);
+      return DAG.getNode(ISD::SUB, dl, PromotedType, Max, Op2Promoted);
+    }
+
+    if (Opcode == ISD::UADDSAT) {
+      APInt MaxVal = APInt::getAllOnesValue(OldBits).zext(NewBits);
+      SDValue SatMax = DAG.getConstant(MaxVal, dl, PromotedType);
+      SDValue Add =
+          DAG.getNode(ISD::ADD, dl, PromotedType, Op1Promoted, Op2Promoted);
+      return DAG.getNode(ISD::UMIN, dl, PromotedType, Add, SatMax);
+    }
+
+    unsigned AddOp = Opcode == ISD::SADDSAT ? ISD::ADD : ISD::SUB;
+    APInt MinVal = APInt::getSignedMinValue(OldBits).sext(NewBits);
+    APInt MaxVal = APInt::getSignedMaxValue(OldBits).sext(NewBits);
+    SDValue SatMin = DAG.getConstant(MinVal, dl, PromotedType);
+    SDValue SatMax = DAG.getConstant(MaxVal, dl, PromotedType);
+    SDValue Result =
+        DAG.getNode(AddOp, dl, PromotedType, Op1Promoted, Op2Promoted);
+    Result = DAG.getNode(ISD::SMIN, dl, PromotedType, Result, SatMax);
+    Result = DAG.getNode(ISD::SMAX, dl, PromotedType, Result, SatMin);
+    return Result;
+  }
 }
 
 SDValue DAGTypeLegalizer::PromoteIntRes_MULFIX(SDNode *N) {