[SimplifyCFG] Hoist common code when succ is unreachable block (#165570)

Previously, `hoistCommonCodeFromSuccessors` returned early if one of the
succ of BB has >1 predecessors.

However, if the succ is an unreachable BB, we can relax the condition to
perform `hoistCommonCodeFromSuccessors` based on the assumption of not
reaching UB.

See discussion https://github.com/dtcxzyw/llvm-opt-benchmark/pull/2989
for details.

Alive2 proof: https://alive2.llvm.org/ce/z/OJOw0s
Promising optimization impact:
https://github.com/dtcxzyw/llvm-opt-benchmark/pull/2995

1 files changed, 12 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 90423d3..b03fb62 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1866,10 +1866,19 @@ bool SimplifyCFGOpt::hoistCommonCodeFromSuccessors(Instruction *TI,
   // If either of the blocks has it's address taken, then we can't do this fold,
   // because the code we'd hoist would no longer run when we jump into the block
   // by it's address.
-  for (auto *Succ : successors(BB))
-    if (Succ->hasAddressTaken() || !Succ->getSinglePredecessor())
+  for (auto *Succ : successors(BB)) {
+    if (Succ->hasAddressTaken())
       return false;
-
+    if (Succ->getSinglePredecessor())
+      continue;
+    // If Succ has >1 predecessors, continue to check if the Succ contains only
+    // one `unreachable` inst. Since executing `unreachable` inst is an UB, we
+    // can relax the condition based on the assumptiom that the program would
+    // never enter Succ and trigger such an UB.
+    if (isa<UnreachableInst>(*Succ->begin()))
+      continue;
+    return false;
+  }
   // The second of pair is a SkipFlags bitmask.
   using SuccIterPair = std::pair<BasicBlock::iterator, unsigned>;
   SmallVector<SuccIterPair, 8> SuccIterPairs;