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

llvm/include/llvm/CodeGen/SwitchLoweringUtils.h

@@ -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

@@ -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,
-                                DAG.getBasicBlock(B.Default));
+  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

@@ -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