Revert "[CVP] Check whether the default case is reachable (#79993)" (#81585)

This reverts commit a034e65.

Some protobuf users reported that this patch caused a significant
compile-time regression because `TailDuplicator` works poorly with a
specific pattern.

We will reland it once the codegen issue is fixed.

Author: dtcxzyw

llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp

@@ -371,7 +371,6 @@ static bool processSwitch(SwitchInst *I, LazyValueInfo *LVI,
   { // Scope for SwitchInstProfUpdateWrapper. It must not live during
     // ConstantFoldTerminator() as the underlying SwitchInst can be changed.
     SwitchInstProfUpdateWrapper SI(*I);
-    unsigned ReachableCaseCount = 0;
 
     for (auto CI = SI->case_begin(), CE = SI->case_end(); CI != CE;) {
       ConstantInt *Case = CI->getCaseValue();
@@ -408,33 +407,6 @@ static bool processSwitch(SwitchInst *I, LazyValueInfo *LVI,
 
       // Increment the case iterator since we didn't delete it.
       ++CI;
-      ++ReachableCaseCount;
-    }
-
-    BasicBlock *DefaultDest = SI->getDefaultDest();
-    if (ReachableCaseCount > 1 &&
-        !isa<UnreachableInst>(DefaultDest->getFirstNonPHIOrDbg())) {
-      ConstantRange CR = LVI->getConstantRangeAtUse(I->getOperandUse(0),
-                                                    /*UndefAllowed*/ false);
-      // The default dest is unreachable if all cases are covered.
-      if (!CR.isSizeLargerThan(ReachableCaseCount)) {
-        BasicBlock *NewUnreachableBB =
-            BasicBlock::Create(BB->getContext(), "default.unreachable",
-                               BB->getParent(), DefaultDest);
-        new UnreachableInst(BB->getContext(), NewUnreachableBB);
-
-        DefaultDest->removePredecessor(BB);
-        SI->setDefaultDest(NewUnreachableBB);
-
-        if (SuccessorsCount[DefaultDest] == 1)
-          DTU.applyUpdatesPermissive(
-              {{DominatorTree::Delete, BB, DefaultDest}});
-        DTU.applyUpdatesPermissive(
-            {{DominatorTree::Insert, BB, NewUnreachableBB}});
-
-        ++NumDeadCases;
-        Changed = true;
-      }
     }
   }
 
@@ -1255,12 +1227,6 @@ CorrelatedValuePropagationPass::run(Function &F, FunctionAnalysisManager &AM) {
   if (!Changed) {
     PA = PreservedAnalyses::all();
   } else {
-#if defined(EXPENSIVE_CHECKS)
-    assert(DT->verify(DominatorTree::VerificationLevel::Full));
-#else
-    assert(DT->verify(DominatorTree::VerificationLevel::Fast));
-#endif // EXPENSIVE_CHECKS
-
     PA.preserve<DominatorTreeAnalysis>();
     PA.preserve<LazyValueAnalysis>();
   }

llvm/test/Transforms/CorrelatedValuePropagation/basic.ll

@@ -442,7 +442,7 @@ define i32 @switch_range(i32 %cond) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[S:%.*]] = urem i32 [[COND:%.*]], 3
 ; CHECK-NEXT:    [[S1:%.*]] = add nuw nsw i32 [[S]], 1
-; CHECK-NEXT:    switch i32 [[S1]], label [[DEFAULT_UNREACHABLE:%.*]] [
+; CHECK-NEXT:    switch i32 [[S1]], label [[UNREACHABLE:%.*]] [
 ; CHECK-NEXT:      i32 1, label [[EXIT1:%.*]]
 ; CHECK-NEXT:      i32 2, label [[EXIT2:%.*]]
 ; CHECK-NEXT:      i32 3, label [[EXIT1]]
@@ -451,8 +451,6 @@ define i32 @switch_range(i32 %cond) {
 ; CHECK-NEXT:    ret i32 1
 ; CHECK:       exit2:
 ; CHECK-NEXT:    ret i32 2
-; CHECK:       default.unreachable:
-; CHECK-NEXT:    unreachable
 ; CHECK:       unreachable:
 ; CHECK-NEXT:    ret i32 0
 ;
@@ -515,9 +513,10 @@ define i8 @switch_defaultdest_multipleuse(i8 %t0) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[O:%.*]] = or i8 [[T0:%.*]], 1
 ; CHECK-NEXT:    [[R:%.*]] = srem i8 1, [[O]]
-; CHECK-NEXT:    br label [[EXIT:%.*]]
-; CHECK:       default.unreachable:
-; CHECK-NEXT:    unreachable
+; CHECK-NEXT:    switch i8 [[R]], label [[EXIT:%.*]] [
+; CHECK-NEXT:      i8 0, label [[EXIT]]
+; CHECK-NEXT:      i8 1, label [[EXIT]]
+; CHECK-NEXT:    ]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret i8 0
 ;