[SimpleLoopUnswitch] Don't consider unswitching `switch` insructions with one unique successor

[DaniilSuchkov]

Summary

Only instructions with two or more unique successors should be considered for unswitching.

[skatkov]

llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp

2552

Do you really need "== nullptr"?

[DaniilSuchkov]

Addressed Serguei's comment. Serguei, could you please submit this patch? I don't have permissions to submit patches yet.

llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp

if (CollectGuards)
         if (isGuard(&I)) {
           auto *Cond = cast<IntrinsicInst>(&I)->getArgOperand(0);
           // TODO: Support AND, OR conditions and partial unswitching.
           if (!isa<Constant>(Cond) && L.isLoopInvariant(Cond))
             UnswitchCandidates.push_back({&I, {Cond}});
         }
 
     if (auto *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
+      const bool HasAtLeastTwoSuccessors = BB->getUniqueSuccessor() == nullptr;
       // We can only consider fully loop-invariant switch conditions as we need
       // to completely eliminate the switch after unswitching.
       if (!isa<Constant>(SI->getCondition()) &&
-          L.isLoopInvariant(SI->getCondition()))
+          L.isLoopInvariant(SI->getCondition()) && HasAtLeastTwoSuccessors)
         UnswitchCandidates.push_back({SI, {SI->getCondition()}});
       continue;
     }
 
     auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
     if (!BI || !BI->isConditional() || isa<Constant>(BI->getCondition()) ||
         BI->getSuccessor(0) == BI->getSuccessor(1))
       continue;

llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch-redundant-switch.ll

 ; REQUIRES: asserts
-; XFAIL: *
 ; RUN: opt -passes='unswitch<nontrivial>' -disable-output -S < %s
 ; RUN: opt -simple-loop-unswitch -enable-nontrivial-unswitch -disable-output -S < %s
 
 ; This loop shouldn't trigger asserts in SimpleLoopUnswitch.
 define void @test_redundant_switch(i1* %ptr, i32 %cond) {
 entry:
   br label %loop_begin