Revert "[SimplifyCFG] When only one case value is missing, replace default with that case (#76669)"

This reverts commit 7d81e07, which introduces a
compiler memory usage regression. See
#76669 (comment)

Author: alexfh

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -5414,13 +5414,11 @@ static bool CasesAreContiguous(SmallVectorImpl<ConstantInt *> &Cases) {
 }
 
 static void createUnreachableSwitchDefault(SwitchInst *Switch,
-                                           DomTreeUpdater *DTU,
-                                           bool RemoveOrigDefaultBlock = true) {
+                                           DomTreeUpdater *DTU) {
   LLVM_DEBUG(dbgs() << "SimplifyCFG: switch default is dead.\n");
   auto *BB = Switch->getParent();
   auto *OrigDefaultBlock = Switch->getDefaultDest();
-  if (RemoveOrigDefaultBlock)
-    OrigDefaultBlock->removePredecessor(BB);
+  OrigDefaultBlock->removePredecessor(BB);
   BasicBlock *NewDefaultBlock = BasicBlock::Create(
       BB->getContext(), BB->getName() + ".unreachabledefault", BB->getParent(),
       OrigDefaultBlock);
@@ -5429,8 +5427,7 @@ static void createUnreachableSwitchDefault(SwitchInst *Switch,
   if (DTU) {
     SmallVector<DominatorTree::UpdateType, 2> Updates;
     Updates.push_back({DominatorTree::Insert, BB, &*NewDefaultBlock});
-    if (RemoveOrigDefaultBlock &&
-        !is_contained(successors(BB), OrigDefaultBlock))
+    if (!is_contained(successors(BB), OrigDefaultBlock))
       Updates.push_back({DominatorTree::Delete, BB, &*OrigDefaultBlock});
     DTU->applyUpdates(Updates);
   }
@@ -5612,28 +5609,10 @@ static bool eliminateDeadSwitchCases(SwitchInst *SI, DomTreeUpdater *DTU,
       Known.getBitWidth() - (Known.Zero | Known.One).popcount();
   assert(NumUnknownBits <= Known.getBitWidth());
   if (HasDefault && DeadCases.empty() &&
-      NumUnknownBits < 64 /* avoid overflow */) {
-    uint64_t AllNumCases = 1ULL << NumUnknownBits;
-    if (SI->getNumCases() == AllNumCases) {
-      createUnreachableSwitchDefault(SI, DTU);
-      return true;
-    }
-    // When only one case value is missing, replace default with that case.
-    // Eliminating the default branch will provide more opportunities for
-    // optimization, such as lookup tables.
-    if (SI->getNumCases() == AllNumCases - 1) {
-      assert(NumUnknownBits > 1 && "Should be canonicalized to a branch");
-      uint64_t MissingCaseVal = 0;
-      for (const auto &Case : SI->cases())
-        MissingCaseVal ^= Case.getCaseValue()->getValue().getLimitedValue();
-      auto *MissingCase =
-          cast<ConstantInt>(ConstantInt::get(Cond->getType(), MissingCaseVal));
-      SwitchInstProfUpdateWrapper SIW(*SI);
-      SIW.addCase(MissingCase, SI->getDefaultDest(), SIW.getSuccessorWeight(0));
-      createUnreachableSwitchDefault(SI, DTU, /*RemoveOrigDefaultBlock*/ false);
-      SIW.setSuccessorWeight(0, 0);
-      return true;
-    }
+      NumUnknownBits < 64 /* avoid overflow */ &&
+      SI->getNumCases() == (1ULL << NumUnknownBits)) {
+    createUnreachableSwitchDefault(SI, DTU);
+    return true;
   }
 
   if (DeadCases.empty())

llvm/test/Transforms/SimplifyCFG/switch-dead-default-lookup-table.ll

@@ -79,15 +79,15 @@ default:
   ret void
 }
 
-; We can replace the default branch with case 3 since it is the only case that is missing.
+; This one is a negative test - we know the value of the default,
+; but that's about it
 define void @test3(i2 %a) {
 ; CHECK-LABEL: define void @test3(
 ; CHECK-SAME: i2 [[A:%.*]]) {
-; CHECK-NEXT:    switch i2 [[A]], label [[DOTUNREACHABLEDEFAULT:%.*]] [
+; CHECK-NEXT:    switch i2 [[A]], label [[DEFAULT:%.*]] [
 ; CHECK-NEXT:      i2 0, label [[CASE0:%.*]]
 ; CHECK-NEXT:      i2 1, label [[CASE1:%.*]]
 ; CHECK-NEXT:      i2 -2, label [[CASE2:%.*]]
-; CHECK-NEXT:      i2 -1, label [[DEFAULT:%.*]]
 ; CHECK-NEXT:    ]
 ; CHECK:       common.ret:
 ; CHECK-NEXT:    ret void
@@ -100,8 +100,6 @@ define void @test3(i2 %a) {
 ; CHECK:       case2:
 ; CHECK-NEXT:    call void @foo(i32 2)
 ; CHECK-NEXT:    br label [[COMMON_RET]]
-; CHECK:       .unreachabledefault:
-; CHECK-NEXT:    unreachable
 ; CHECK:       default:
 ; CHECK-NEXT:    call void @foo(i32 3)
 ; CHECK-NEXT:    br label [[COMMON_RET]]
@@ -124,50 +122,6 @@ default:
   ret void
 }
 
-define void @test3_prof(i2 %a) {
-; CHECK-LABEL: define void @test3_prof(
-; CHECK-SAME: i2 [[A:%.*]]) {
-; CHECK-NEXT:    switch i2 [[A]], label [[DOTUNREACHABLEDEFAULT:%.*]] [
-; CHECK-NEXT:      i2 0, label [[CASE0:%.*]]
-; CHECK-NEXT:      i2 1, label [[CASE1:%.*]]
-; CHECK-NEXT:      i2 -2, label [[CASE2:%.*]]
-; CHECK-NEXT:      i2 -1, label [[DEFAULT:%.*]]
-; CHECK-NEXT:    ], !prof [[PROF0:![0-9]+]]
-; CHECK:       common.ret:
-; CHECK-NEXT:    ret void
-; CHECK:       case0:
-; CHECK-NEXT:    call void @foo(i32 0)
-; CHECK-NEXT:    br label [[COMMON_RET:%.*]]
-; CHECK:       case1:
-; CHECK-NEXT:    call void @foo(i32 1)
-; CHECK-NEXT:    br label [[COMMON_RET]]
-; CHECK:       case2:
-; CHECK-NEXT:    call void @foo(i32 2)
-; CHECK-NEXT:    br label [[COMMON_RET]]
-; CHECK:       .unreachabledefault:
-; CHECK-NEXT:    unreachable
-; CHECK:       default:
-; CHECK-NEXT:    call void @foo(i32 3)
-; CHECK-NEXT:    br label [[COMMON_RET]]
-;
-  switch i2 %a, label %default [i2 0, label %case0
-  i2 1, label %case1
-  i2 2, label %case2], !prof !0
-
-case0:
-  call void @foo(i32 0)
-  ret void
-case1:
-  call void @foo(i32 1)
-  ret void
-case2:
-  call void @foo(i32 2)
-  ret void
-default:
-  call void @foo(i32 3)
-  ret void
-}
-
 ; Negative test - check for possible overflow when computing
 ; number of possible cases.
 define void @test4(i128 %a) {
@@ -313,7 +267,3 @@ default:
 
 declare void @llvm.assume(i1)
 
-!0 = !{!"branch_weights", i32 8, i32 4, i32 2, i32 1}
-;.
-; CHECK: [[PROF0]] = !{!"branch_weights", i32 0, i32 4, i32 2, i32 1, i32 8}
-;.