[llvm] 5d4f37e - [NFCI][SimplifyCFG] Rewrite `createUnreachableSwitchDefault()`

[Craig Topper via llvm-commits]

Nevermind, you're not deleting the original block just disconnecting it.

~Craig


On Fri, Aug 20, 2021 at 9:40 AM Craig Topper <craig.topper at gmail.com> wrote:

> Is it possible the unreachable block has instructions that are used by its
> original successors?
>
> ~Craig
>
>
> On Fri, Aug 20, 2021 at 3:28 AM Roman Lebedev via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
>>
>> Author: Roman Lebedev
>> Date: 2021-08-20T13:28:08+03:00
>> New Revision: 5d4f37e895487408e61498ec42e545ec5f778b5b
>>
>> URL:
>> https://github.com/llvm/llvm-project/commit/5d4f37e895487408e61498ec42e545ec5f778b5b
>> DIFF:
>> https://github.com/llvm/llvm-project/commit/5d4f37e895487408e61498ec42e545ec5f778b5b.diff
>>
>> LOG: [NFCI][SimplifyCFG] Rewrite `createUnreachableSwitchDefault()`
>>
>> The only thing that function should do as per it's semantic,
>> is to ensure that the switch's default is a block consisting only of
>> an `unreachable` terminator.
>>
>> So let's just create such a block and update switch's default
>> to point to it. There should be no need for all this weird dance
>> around predecessors/successors.
>>
>> Added:
>>
>>
>> Modified:
>>     llvm/lib/Transforms/Utils/SimplifyCFG.cpp
>>     llvm/test/Transforms/SimplifyCFG/switch-dead-default.ll
>>
>> Removed:
>>
>>
>>
>>
>> ################################################################################
>> diff  --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
>> b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
>> index 847fdd760d2fe..ba3129f5581fe 100644
>> --- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
>> +++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
>> @@ -4747,23 +4747,20 @@ static void
>> createUnreachableSwitchDefault(SwitchInst *Switch,
>>                                             DomTreeUpdater *DTU) {
>>    LLVM_DEBUG(dbgs() << "SimplifyCFG: switch default is dead.\n");
>>    auto *BB = Switch->getParent();
>> -  BasicBlock *NewDefaultBlock = SplitBlockPredecessors(
>> -      Switch->getDefaultDest(), Switch->getParent(), "", DTU);
>>    auto *OrigDefaultBlock = Switch->getDefaultDest();
>> +  OrigDefaultBlock->removePredecessor(BB);
>> +  BasicBlock *NewDefaultBlock = BasicBlock::Create(
>> +      BB->getContext(), BB->getName() + ".unreachabledefault",
>> BB->getParent(),
>> +      OrigDefaultBlock);
>> +  new UnreachableInst(Switch->getContext(), NewDefaultBlock);
>>    Switch->setDefaultDest(&*NewDefaultBlock);
>> -  if (DTU)
>> -    DTU->applyUpdates({{DominatorTree::Insert, BB, &*NewDefaultBlock},
>> -                       {DominatorTree::Delete, BB, OrigDefaultBlock}});
>> -  SplitBlock(&*NewDefaultBlock, &NewDefaultBlock->front(), DTU);
>> -  SmallVector<DominatorTree::UpdateType, 2> Updates;
>> -  if (DTU)
>> -    for (auto *Successor : successors(NewDefaultBlock))
>> -      Updates.push_back({DominatorTree::Delete, NewDefaultBlock,
>> Successor});
>> -  auto *NewTerminator = NewDefaultBlock->getTerminator();
>> -  new UnreachableInst(Switch->getContext(), NewTerminator);
>> -  EraseTerminatorAndDCECond(NewTerminator);
>> -  if (DTU)
>> +  if (DTU) {
>> +    SmallVector<DominatorTree::UpdateType, 2> Updates;
>> +    Updates.push_back({DominatorTree::Insert, BB, &*NewDefaultBlock});
>> +    if (!is_contained(successors(BB), OrigDefaultBlock))
>> +      Updates.push_back({DominatorTree::Delete, BB, &*OrigDefaultBlock});
>>      DTU->applyUpdates(Updates);
>> +  }
>>  }
>>
>>  /// Turn a switch with two reachable destinations into an integer range
>>
>> diff  --git a/llvm/test/Transforms/SimplifyCFG/switch-dead-default.ll
>> b/llvm/test/Transforms/SimplifyCFG/switch-dead-default.ll
>> index b42024ee48995..154ecb2166310 100644
>> --- a/llvm/test/Transforms/SimplifyCFG/switch-dead-default.ll
>> +++ b/llvm/test/Transforms/SimplifyCFG/switch-dead-default.ll
>> @@ -33,7 +33,7 @@ default:
>>
>>  define void @test2(i2 %a) {
>>  ; CHECK-LABEL: @test2(
>> -; CHECK-NEXT:    switch i2 [[A:%.*]], label [[DEFAULT1:%.*]] [
>> +; 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:%.*]]
>> @@ -53,7 +53,7 @@ define void @test2(i2 %a) {
>>  ; CHECK:       case3:
>>  ; CHECK-NEXT:    call void @foo(i32 3)
>>  ; CHECK-NEXT:    br label [[COMMON_RET]]
>> -; CHECK:       default1:
>> +; CHECK:       .unreachabledefault:
>>  ; CHECK-NEXT:    unreachable
>>  ;
>>    switch i2 %a, label %default [i2 0, label %case0
>>
>>
>>
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>
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