[llvm] [JumpThreading] Remove deleted BB from Unreachable (PR #126984)

[via llvm-commits]

https://github.com/weiguozhi created https://github.com/llvm/llvm-project/pull/126984

Although an unreachable BB is skipped by processBlock, its successor can still be handled by processBlock, and maybeMergeBasicBlockIntoOnlyPred may merge the two BBs and delete the unreachable BB. Then the garbage pointer is left in Unreachable set. This patch removes the invalid BB pointer from the Unreachable set.

>From 73ce9611e854d0ed7a27b561e2b02678da0392a5 Mon Sep 17 00:00:00 2001
From: Guozhi Wei <carrot at google.com>
Date: Wed, 12 Feb 2025 14:51:21 -0800
Subject: [PATCH] [JumpThreading] Remove deleted BB from Unreachable

Although an unreachable BB is skipped by processBlock, its successor can
still be handled by processBlock, and maybeMergeBasicBlockIntoOnlyPred
may merge the two BBs and delete the unreachable BB. Then the garbage
pointer is left in Unreachable set. This patch removes the invalid BB
pointer from the Unreachable set.
---
 llvm/include/llvm/Transforms/Scalar/JumpThreading.h | 4 ++++
 llvm/lib/Transforms/Scalar/JumpThreading.cpp        | 8 +++++---
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/llvm/include/llvm/Transforms/Scalar/JumpThreading.h b/llvm/include/llvm/Transforms/Scalar/JumpThreading.h
index 7d11fc0ad6938..84292c716a0a9 100644
--- a/llvm/include/llvm/Transforms/Scalar/JumpThreading.h
+++ b/llvm/include/llvm/Transforms/Scalar/JumpThreading.h
@@ -94,6 +94,10 @@ class JumpThreadingPass : public PassInfoMixin<JumpThreadingPass> {
   SmallPtrSet<const BasicBlock *, 16> LoopHeaders;
 #endif
 
+  // JumpThreading must not processes blocks unreachable from entry. It's a
+  // waste of compute time and can potentially lead to hangs.
+  SmallPtrSet<BasicBlock *, 16> Unreachable;
+
   unsigned BBDupThreshold;
   unsigned DefaultBBDupThreshold;
 
diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
index 7b221a814aabd..dfa49ba183779 100644
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -307,12 +307,11 @@ bool JumpThreadingPass::runImpl(Function &F_, FunctionAnalysisManager *FAM_,
   else
     BBDupThreshold = DefaultBBDupThreshold;
 
-  // JumpThreading must not processes blocks unreachable from entry. It's a
-  // waste of compute time and can potentially lead to hangs.
-  SmallPtrSet<BasicBlock *, 16> Unreachable;
   assert(DTU && "DTU isn't passed into JumpThreading before using it.");
   assert(DTU->hasDomTree() && "JumpThreading relies on DomTree to proceed.");
   DominatorTree &DT = DTU->getDomTree();
+
+  Unreachable.clear();
   for (auto &BB : *F)
     if (!DT.isReachableFromEntry(&BB))
       Unreachable.insert(&BB);
@@ -1902,6 +1901,9 @@ bool JumpThreadingPass::maybeMergeBasicBlockIntoOnlyPred(BasicBlock *BB) {
   LVI->eraseBlock(SinglePred);
   MergeBasicBlockIntoOnlyPred(BB, DTU.get());
 
+  if (Unreachable.count(SinglePred) && DTU->isBBPendingDeletion(SinglePred))
+    Unreachable.erase(SinglePred);
+
   // Now that BB is merged into SinglePred (i.e. SinglePred code followed by
   // BB code within one basic block `BB`), we need to invalidate the LVI
   // information associated with BB, because the LVI information need not be