Commit cbefc36f authored by Hans Wennborg's avatar Hans Wennborg
Browse files

Switch lowering: omit range check for bit tests when default is unreachable (PR43129) 

This is modeled after the same functionality for jump tables, which was
added in r357067.

Differential revision: https://reviews.llvm.org/D68131

llvm-svn: 373431
parent 70f70035

llvm/include/llvm/CodeGen/SwitchLoweringUtils.h

+2 −1
Original line number Diff line number Diff line
@@ -212,13 +212,14 @@ struct BitTestBlock { 
  BitTestInfo Cases;

  BranchProbability Prob;

  BranchProbability DefaultProb;

  bool OmitRangeCheck;



  BitTestBlock(APInt F, APInt R, const Value *SV, unsigned Rg, MVT RgVT, bool E,

               bool CR, MachineBasicBlock *P, MachineBasicBlock *D,

               BitTestInfo C, BranchProbability Pr)

      : First(std::move(F)), Range(std::move(R)), SValue(SV), Reg(Rg),

        RegVT(RgVT), Emitted(E), ContiguousRange(CR), Parent(P), Default(D),

        Cases(std::move(C)), Prob(Pr) {}

        Cases(std::move(C)), Prob(Pr), OmitRangeCheck(false) {}

};



/// Return the range of values within a range.

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

+23 −17
Original line number Diff line number Diff line
@@ -2622,17 +2622,11 @@ void SelectionDAGBuilder::visitBitTestHeader(BitTestBlock &B, 
  // Subtract the minimum value.

  SDValue SwitchOp = getValue(B.SValue);

  EVT VT = SwitchOp.getValueType();

  SDValue Sub = DAG.getNode(ISD::SUB, dl, VT, SwitchOp,

                            DAG.getConstant(B.First, dl, VT));



  // Check range.

  const TargetLowering &TLI = DAG.getTargetLoweringInfo();

  SDValue RangeCmp = DAG.getSetCC(

      dl, TLI.getSetCCResultType(DAG.getDataLayout(), *DAG.getContext(),

                                 Sub.getValueType()),

      Sub, DAG.getConstant(B.Range, dl, VT), ISD::SETUGT);

  SDValue RangeSub =

      DAG.getNode(ISD::SUB, dl, VT, SwitchOp, DAG.getConstant(B.First, dl, VT));



  // Determine the type of the test operands.

  const TargetLowering &TLI = DAG.getTargetLoweringInfo();

  bool UsePtrType = false;

  if (!TLI.isTypeLegal(VT)) {

    UsePtrType = true;
@@ -2645,6 +2639,7 @@ void SelectionDAGBuilder::visitBitTestHeader(BitTestBlock &B, 
        break;

      }

  }

  SDValue Sub = RangeSub;

  if (UsePtrType) {

    VT = TLI.getPointerTy(DAG.getDataLayout());

    Sub = DAG.getZExtOrTrunc(Sub, dl, VT);
@@ -2660,16 +2655,24 @@ void SelectionDAGBuilder::visitBitTestHeader(BitTestBlock &B, 
  addSuccessorWithProb(SwitchBB, MBB, B.Prob);

  SwitchBB->normalizeSuccProbs();



  SDValue BrRange = DAG.getNode(ISD::BRCOND, dl,

                                MVT::Other, CopyTo, RangeCmp,

  SDValue Root = CopyTo;

  if (!B.OmitRangeCheck) {

    // Conditional branch to the default block.

    SDValue RangeCmp = DAG.getSetCC(dl,

        TLI.getSetCCResultType(DAG.getDataLayout(), *DAG.getContext(),

                               RangeSub.getValueType()),

        RangeSub, DAG.getConstant(B.Range, dl, RangeSub.getValueType()),

        ISD::SETUGT);



    Root = DAG.getNode(ISD::BRCOND, dl, MVT::Other, Root, RangeCmp,

                       DAG.getBasicBlock(B.Default));

  }



  // Avoid emitting unnecessary branches to the next block.

  if (MBB != NextBlock(SwitchBB))

    BrRange = DAG.getNode(ISD::BR, dl, MVT::Other, BrRange,

                          DAG.getBasicBlock(MBB));

    Root = DAG.getNode(ISD::BR, dl, MVT::Other, Root, DAG.getBasicBlock(MBB));



  DAG.setRoot(BrRange);

  DAG.setRoot(Root);

}



/// visitBitTestCase - this function produces one "bit test"
@@ -10164,8 +10167,6 @@ void SelectionDAGBuilder::lowerWorkItem(SwitchWorkListItem W, Value *Cond, 
        break;

      }

      case CC_BitTests: {

        // FIXME: If Fallthrough is unreachable, skip the range check.



        // FIXME: Optimize away range check based on pivot comparisons.

        BitTestBlock *BTB = &SL->BitTestCases[I->BTCasesIndex];
@@ -10186,6 +10187,11 @@ void SelectionDAGBuilder::lowerWorkItem(SwitchWorkListItem W, Value *Cond, 
          BTB->DefaultProb -= DefaultProb / 2;

        }



        if (FallthroughUnreachable) {

          // Skip the range check if the fallthrough block is unreachable.

          BTB->OmitRangeCheck = true;

        }



        // If we're in the right place, emit the bit test header right now.

        if (CurMBB == SwitchMBB) {

          visitBitTestHeader(*BTB, SwitchMBB);

llvm/test/CodeGen/X86/switch-bt.ll

+2 −3
Original line number Diff line number Diff line
@@ -157,13 +157,12 @@ sw.epilog: 
}





; TODO: Omit the range check when the default case is unreachable, see PR43129.

; Omit the range check when the default case is unreachable, see PR43129.

declare void @g(i32)

define void @test5(i32 %x) {



; CHECK-LABEL: test5

; CHECK: cmpl $8, %edi

; CHECK: ja

; CHECK-NOT: cmp



; 73 = 2^0 + 2^3 + 2^6

; CHECK:      movl $73