[JumpThreading] Thread jumps through two basic blocks

[kazu]

Summary

This patch teaches JumpThreading.cpp to thread through two basic
blocks like:

bb3:
  %var = phi i32* [ null, %bb1 ], [ @a, %bb2 ]
  %tobool = icmp eq i32 %cond, 0
  br i1 %tobool, label %bb4, label ...

bb4:
  %cmp = icmp eq i32* %var, null
  br i1 %cmp, label bb5, label bb6

by duplicating basic blocks like bb3 above. Once we duplicate bb3 as
bb3.dup and redirect edge bb2->bb3 to bb2->bb3.dup, we have:

bb3:
  %var = phi i32* [ @a, %bb2 ]
  %tobool = icmp eq i32 %cond, 0
  br i1 %tobool, label %bb4, label ...

bb3.dup:
  %var = phi i32* [ null, %bb1 ]
  %tobool = icmp eq i32 %cond, 0
  br i1 %tobool, label %bb4, label ...

bb4:
  %cmp = icmp eq i32* %var, null
  br i1 %cmp, label bb5, label bb6

Then the existing code in JumpThreading.cpp can thread edge
bb3.dup->bb4 through bb4 and eventually create bb3.dup->bb5.

[wmi]

llvm/lib/Transforms/Scalar/JumpThreading.cpp

1609–1610

MaybeCloneThreadableBasicBlock may clone the predecessor of BB, return true and leaves the real threading job to the next call of ProcessBlock(BB). Is it possible that predecessor of BB is cloned but somehow threading doesn't happen in the next call of ProcessBlock(BB), and the clone is wasted? I know there are many checks in MaybeCloneThreadableBasicBlock to ensure most cases it won't happen, but maybe it is helpful to add some assertion to catch unexpected case?

For example, because ProcessBlock(BB) is called in a loop, we can save the information about whether any clone has happend in the loop, and any threading has happened, and add an assertion after the loop to check if clone has happened, threading must happen as well.

2119–2122

Why we care Cst is one or zero? Can we use CstCount instead?

[kazu]

llvm/lib/Transforms/Scalar/JumpThreading.cpp

1609–1610

I understand your concern.

I am not sure if I like to save information for the next iteration. That would get messy. If you would like to see jump threading always happen after MaybeCloneThreadableBasicBlock, should I perform actual jump threading inside MaybeCloneThreadableBasicBlock? (Maybe we should change the function name at that point.)

I am wondering if it's an option to just "hope" that jump threading happens after MaybeCloneThreadableBasicBlock. A couple of other helper transformations like TryToUnfoldSelectInCurrBB and ProcessBranchOnPHI also "hope" that they will increase jump threading opportunities.

2119–2122

The reason why I care about specific constants is that I would like to set up the CFG so that after calling MaybeCloneThreadableBasicBlock, taking the edge from NewBB to BB will necessarily imply which successor edge we take out of BB.

By CstCount, I assume you are proposing to count the number of incoming edges into PredBB with known successor edges out of BB. If we do so, we could end up with CstCount == 2, where one incoming edge into PredBB takes the "then" edge out of BB, and the other takes the "else" edge. That doesn't tell us which edge into PredBB we should redirect to NewBB.

Note that for simplicity, I would like to redirect exactly one incoming edge into PredBB to NewBB when no other incoming edge into PredBB is known to always take the same successor edge out of BB, which is why I care about ZeroCount == 1 or OneCount == 1.

[wmi]

llvm/lib/Transforms/Scalar/JumpThreading.cpp

2119–2122

Note that for simplicity, I would like to redirect exactly one incoming edge into PredBB to NewBB when no other incoming edge into PredBB is known to always take the same successor edge out of BB, which is why I care about ZeroCount == 1 or OneCount == 1.

I see, thanks. It is good to put the above in the comment and add a TODO there, if you plan to handle the case with multiple PredBBs threading into the same SuccBB later on.

[kazu]

I've updated the patch so that we commit to jump threading across two
basic blocks once we are done with analysis. That is, we don't
transform the IR partially only to see jump threading proper failing.

[wmi]

llvm/lib/Transforms/Scalar/JumpThreading.cpp

1552

BB->getSingleSuccessor() => BB->getSinglePredecessor() ?

2068–2069

Now the predecessor of BB will be cloned only if threading through two bb is meant to happen, so it does not just increase the chance of jump threading. Better update the comment to reflect the change.

2088–2089

Update the comment here.

2182

Can we wrap the following code into a function saying ThreadThroughTwoBasicBlocks?

[kazu]

Updated the patch, incorporating feedback from Wei.

[kazu]

Please take a look. Thanks!

[wmi]

LGTM.

[vitalybuka]

This patch is causing problems on Window bot: http://lab.llvm.org:8011/builders/sanitizer-windows/builds/56324

[kazu]

I've reverted the change for now. I'll take a look at the failure on Windows.

[kazu]

I'm reopening this so that I can upload a revision containing a fix for Windows build.

[kazu]

PTAL Thanks!

This revision fixes Windows builds and adds a testcase for the problem.

[wmi]

llvm/test/Transforms/JumpThreading/thread-two-bbs3.ll

9

We usually use CHECK instead of CHECK-LABEL for BasicBlock label.

13

Can we make the check more robust? It is possible the same word "thread" appears in somewhere else or in the comment.

[kazu]

I've further simplied thread-two-bbs3.ll, the testcase for Windows,
and made it a little more robust with:

CHECK-NOT: bb_{{[^ ]*}}.thread:

[kazu]

PTAL Thanks!

[wmi]

LGTM.

[kazu]

I am re-opening this so that I can check in a revised version.

[kazu]

This patch is work-in-progress, but I am uploading it now so I can
share it with wider audience. I am still working on fixing loose
ends.

[kazu]

I've fixed several bugs. PTAL Thanks!

[xbolva00]

llvm/lib/Transforms/Scalar/JumpThreading.cpp

2110

Can you allow this if ThreadAcrossLoopHeaders is set?

[kazu]

llvm/lib/Transforms/Scalar/JumpThreading.cpp

2110

Actually, we already allow the transformation if ThreadAcrossLoopHeaders is set.

If ThreadAcrossLoopHeaders is set, we don't call FindLoopHeaders in JumpThreadingPass::runImpl, so LoopHeaders remains empty.

[xbolva00]

llvm/lib/Transforms/Scalar/JumpThreading.cpp

2110

Oh, okay, thanks!

[wmi]

LGTM.

llvm/lib/Transforms/Scalar/JumpThreading.cpp

2180–2184

You may want to dump the cost in a better way.

For example: if BBCost is (unsigned)~0 and PredBBCost is 1.

(unsigned)~0 + 1 = 0. Then the message "Cost is too high: 0" will be confusing.

[kazu]

I've fixed the confusing debug message that Wei has pointed out.

[thakis]

Heads-up: I'm guessing this causes https://bugs.chromium.org/p/chromium/issues/detail?id=1049434 . I'll try to verify it's this CL and then get you a repo.

[thakis]

My guess was wrong and it's not due to this change.

[maxim-kuvyrkov]

Hi @kazu ,
This patch increases code-size of 444.namd by 1% on arm-linux-gnueabihf:

• when compiling with -Os: from 171653 to 173645
• when compiling with -Os -flto: from 137772 to 139708

Could you take a look if these regressions can be easily avoided?
Let me know if you need help reproducing the regressions. I have pre-processed source and generated assembly handy.
Thanks!

[maxim-kuvyrkov]

This commit 4698bf145d583e26ed438026ef7fde031ef322b1

Author: Kazu Hirata <[e-mail]>
Date: Wed Feb 5 08:24:01 2020 -0800

Resubmit^2: [JumpThreading] Thread jumps through two basic blocks

also regresses performance of 482.sphinx3 by 2% on arm-linux-gnueabihf at -O2 in -marm mode. The regression comes from subvq_mgau_shortlist() becoming 28% slower:

482.sphinx3,sphinx_livepretend_base.default ,102,100,17299,17607,157416,157688
482.sphinx3,[.] mgau_eval ,98,100,6725,6624,904,904
482.sphinx3,[.] vector_gautbl_eval_logs3 ,99,100,5802,5759,456,456
482.sphinx3,[.] subvq_mgau_shortlist ,128,100,1353,1731,528,528

@kazu , subvq_mgau_shortlist() is only 528 bytes; would you please investigate what's going on there?

[kazu]

Hi @maxim-kuvyrkov, would you mind filing two separate issues -- the size and performance -- preferably with preprocessed source code? Thanks!

llvm/include/llvm/Transforms/Scalar/JumpThreading.h

ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
                                   jumpthreading::PredValueInfo &Result,
                                   jumpthreading::ConstantPreference Preference,
                                   Instruction *CxtI = nullptr) {
     DenseSet<std::pair<Value *, BasicBlock *>> RecursionSet;
     return ComputeValueKnownInPredecessorsImpl(V, BB, Result, Preference,
                                                RecursionSet, CxtI);
   }
 
+  Constant *EvaluateOnPredecessorEdge(BasicBlock *BB, BasicBlock *PredPredBB,
+                                      Value *cond);
+  bool MaybeThreadThroughTwoBasicBlocks(BasicBlock *BB, Value *Cond);
+  void ThreadThroughTwoBasicBlocks(BasicBlock *PredPredBB, BasicBlock *PredBB,
+                                   BasicBlock *BB, BasicBlock *SuccBB);
   bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
                               jumpthreading::ConstantPreference Preference,
                               Instruction *CxtI = nullptr);
 
   bool ProcessBranchOnPHI(PHINode *PN);
   bool ProcessBranchOnXOR(BinaryOperator *BO);
   bool ProcessImpliedCondition(BasicBlock *BB);
 

llvm/lib/Transforms/Scalar/JumpThreading.cpp

for (unsigned i = 0; ; ++i) {
       break;
     }
   }
 
   // Okay, we have finally picked the most popular destination.
   return MostPopularDest;
 }
 
+// Try to evaluate the value of V when the control flows from PredPredBB to
+// BB->getSinglePredecessor() and then on to BB.
+Constant *JumpThreadingPass::EvaluateOnPredecessorEdge(BasicBlock *BB,
+                                                       BasicBlock *PredPredBB,
+                                                       Value *V) {
+  BasicBlock *PredBB = BB->getSinglePredecessor();
+  assert(PredBB && "Expected a single predecessor");
+
+  if (Constant *Cst = dyn_cast<Constant>(V)) {
+    return Cst;
+  }
+
+  // Consult LVI if V is not an instruction in BB or PredBB.
+  Instruction *I = dyn_cast<Instruction>(V);
+  if (!I || (I->getParent() != BB && I->getParent() != PredBB)) {
+    if (DTU->hasPendingDomTreeUpdates())
+      LVI->disableDT();
+    else
+      LVI->enableDT();
+    return LVI->getConstantOnEdge(V, PredPredBB, PredBB, nullptr);
+  }
+
+  // Look into a PHI argument.
+  if (PHINode *PHI = dyn_cast<PHINode>(V)) {
+    if (PHI->getParent() == PredBB)
+      return dyn_cast<Constant>(PHI->getIncomingValueForBlock(PredPredBB));
+    return nullptr;
+  }
+
+  // If we have a CmpInst, try to fold it for each incoming edge into PredBB.
+  if (CmpInst *CondCmp = dyn_cast<CmpInst>(V)) {
+    if (CondCmp->getParent() == BB) {
+      Constant *Op0 =
+          EvaluateOnPredecessorEdge(BB, PredPredBB, CondCmp->getOperand(0));
+      Constant *Op1 =
+          EvaluateOnPredecessorEdge(BB, PredPredBB, CondCmp->getOperand(1));
+      if (Op0 && Op1) {
+        return ConstantExpr::getCompare(CondCmp->getPredicate(), Op0, Op1);
+      }
+    }
+    return nullptr;
+  }
+
+  return nullptr;
+}
+
 bool JumpThreadingPass::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
                                                ConstantPreference Preference,
                                                Instruction *CxtI) {
   // If threading this would thread across a loop header, don't even try to
   // thread the edge.
   if (LoopHeaders.count(BB))
     return false;
 
   PredValueInfoTy PredValues;
-  if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference, CxtI))
-    return false;
+  if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference,
+                                       CxtI)) {
+    // We don't have known values in predecessors.  See if we can thread through
+    // BB and its sole predecessor.
+    return MaybeThreadThroughTwoBasicBlocks(BB, Cond);
+  }
 
   assert(!PredValues.empty() &&
          "ComputeValueKnownInPredecessors returned true with no values");
 
   LLVM_DEBUG(dbgs() << "IN BB: " << *BB;
              for (const auto &PredValue : PredValues) {
                dbgs() << "  BB '" << BB->getName()
                       << "': FOUND condition = " << *PredValue.first
for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
         if (I != ValueMapping.end())
           New->setOperand(i, I->second);
       }
   }
 
   return ValueMapping;
 }
 
+/// Attempt to thread through two successive basic blocks.
+bool JumpThreadingPass::MaybeThreadThroughTwoBasicBlocks(BasicBlock *BB,
+                                                         Value *Cond) {
+  // Consider:
+  //
+  // PredBB:
+  //   %var = phi i32* [ null, %bb1 ], [ @a, %bb2 ]
+  //   %tobool = icmp eq i32 %cond, 0
+  //   br i1 %tobool, label %BB, label ...
+  //
+  // BB:
+  //   %cmp = icmp eq i32* %var, null
+  //   br i1 %cmp, label ..., label ...
+  //
+  // We don't know the value of %var at BB even if we know which incoming edge
+  // we take to BB.  However, once we duplicate PredBB for each of its incoming
+  // edges (say, PredBB1 and PredBB2), we know the value of %var in each copy of
+  // PredBB.  Then we can thread edges PredBB1->BB and PredBB2->BB through BB.
+
+  // Require that BB end with a Branch for simplicity.
+  BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
+  if (!CondBr)
+    return false;
+
+  // BB must have exactly one predecessor.
+  BasicBlock *PredBB = BB->getSinglePredecessor();
+  if (!PredBB)
+    return false;
+
+  // Require that PredBB end with a Branch.  If PredBB ends with an
+  // unconditional branch, we should be merging PredBB and BB instead.  For
+  // simplicity, we don't deal with a switch.
+  BranchInst *PredBBBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
+  if (!PredBBBranch)
+    return false;
+
+  // If PredBB has exactly one incoming edge, we don't gain anything by copying
+  // PredBB.
+  if (PredBB->getSinglePredecessor())
+    return false;
+
+  // Don't thread across a loop header.
+  if (LoopHeaders.count(PredBB))
+    return false;
+
+  // Avoid complication with duplicating EH pads.
+  if (PredBB->isEHPad())
+    return false;
+
+  // Find a predecessor that we can thread.  For simplicity, we only consider a
+  // successor edge out of BB to which we thread exactly one incoming edge into
+  // PredBB.
+  unsigned ZeroCount = 0;
+  unsigned OneCount = 0;
+  BasicBlock *ZeroPred = nullptr;
+  BasicBlock *OnePred = nullptr;
+  for (BasicBlock *P : predecessors(PredBB)) {
+    if (Constant *Cst = EvaluateOnPredecessorEdge(BB, P, Cond)) {
+      if (Cst->isZeroValue()) {
+        ZeroCount++;
+        ZeroPred = P;
+      } else {
+        OneCount++;
+        OnePred = P;
+      }
+    }
+  }
+
+  // Disregard complicated cases where we have to thread multiple edges.
+  BasicBlock *PredPredBB;
+  if (ZeroCount == 1) {
+    PredPredBB = ZeroPred;
+  } else if (OneCount == 1) {
+    PredPredBB = OnePred;
+  } else {
+    return false;
+  }
+
+  BasicBlock *SuccBB = CondBr->getSuccessor(PredPredBB == ZeroPred);
+
+  // If threading to the same block as we come from, we would infinite loop.
+  if (SuccBB == BB) {
+    LLVM_DEBUG(dbgs() << "  Not threading across BB '" << BB->getName()
+                      << "' - would thread to self!\n");
+    return false;
+  }
+
+  // If threading this would thread across a loop header, don't thread the edge.
+  // See the comments above FindLoopHeaders for justifications and caveats.
+  if (LoopHeaders.count(BB) || LoopHeaders.count(SuccBB)) {
+    LLVM_DEBUG({
+      bool BBIsHeader = LoopHeaders.count(BB);
+      bool SuccIsHeader = LoopHeaders.count(SuccBB);
+      dbgs() << "  Not threading across "
+             << (BBIsHeader ? "loop header BB '" : "block BB '")
+             << BB->getName() << "' to dest "
+             << (SuccIsHeader ? "loop header BB '" : "block BB '")
+             << SuccBB->getName()
+             << "' - it might create an irreducible loop!\n";
+    });
+    return false;
+  }
+
+  // Check the cost of duplicating BB and PredBB.
+  unsigned JumpThreadCost =
+      getJumpThreadDuplicationCost(BB, BB->getTerminator(), BBDupThreshold);
+  JumpThreadCost += getJumpThreadDuplicationCost(
+      PredBB, PredBB->getTerminator(), BBDupThreshold);
+  if (JumpThreadCost > BBDupThreshold) {
+    LLVM_DEBUG(dbgs() << "  Not threading BB '" << BB->getName()
+                      << "' - Cost is too high: " << JumpThreadCost << "\n");
+    return false;
+  }
+
+  // Now we are ready to duplicate PredBB.
+  ThreadThroughTwoBasicBlocks(PredPredBB, PredBB, BB, SuccBB);
+  return true;
+}
+
+void JumpThreadingPass::ThreadThroughTwoBasicBlocks(BasicBlock *PredPredBB,
+                                                    BasicBlock *PredBB,
+                                                    BasicBlock *BB,
+                                                    BasicBlock *SuccBB) {
+  LLVM_DEBUG(dbgs() << "  Threading through '" << PredBB->getName() << "' and '"
+                    << BB->getName() << "'\n");
+
+  BranchInst *CondBr = cast<BranchInst>(BB->getTerminator());
+  BranchInst *PredBBBranch = cast<BranchInst>(PredBB->getTerminator());
+
+  BasicBlock *NewBB =
+      BasicBlock::Create(PredBB->getContext(), PredBB->getName() + ".thread",
+                         PredBB->getParent(), PredBB);
+  NewBB->moveAfter(PredBB);
+
+  // Set the block frequency of NewBB.
+  if (HasProfileData) {
+    auto NewBBFreq = BFI->getBlockFreq(PredPredBB) *
+                     BPI->getEdgeProbability(PredPredBB, PredBB);
+    BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency());
+  }
+
+  // We are going to have to map operands from the original BB block to the new
+  // copy of the block 'NewBB'.  If there are PHI nodes in PredBB, evaluate them
+  // to account for entry from PredPredBB.
+  DenseMap<Instruction *, Value *> ValueMapping =
+      CloneInstructions(PredBB->begin(), PredBB->end(), NewBB, PredPredBB);
+
+  // Update the terminator of PredPredBB to jump to NewBB instead of PredBB.
+  // This eliminates predecessors from PredPredBB, which requires us to simplify
+  // any PHI nodes in PredBB.
+  Instruction *PredPredTerm = PredPredBB->getTerminator();
+  for (unsigned i = 0, e = PredPredTerm->getNumSuccessors(); i != e; ++i)
+    if (PredPredTerm->getSuccessor(i) == PredBB) {
+      PredBB->removePredecessor(PredPredBB, true);
+      PredPredTerm->setSuccessor(i, NewBB);
+    }
+
+  AddPHINodeEntriesForMappedBlock(PredBBBranch->getSuccessor(0), PredBB, NewBB,
+                                  ValueMapping);
+  AddPHINodeEntriesForMappedBlock(PredBBBranch->getSuccessor(1), PredBB, NewBB,
+                                  ValueMapping);
+
+  DTU->applyUpdatesPermissive(
+      {{DominatorTree::Insert, NewBB, CondBr->getSuccessor(0)},
+       {DominatorTree::Insert, NewBB, CondBr->getSuccessor(1)},
+       {DominatorTree::Insert, PredPredBB, NewBB},
+       {DominatorTree::Delete, PredPredBB, PredBB}});
+
+  UpdateSSA(PredBB, NewBB, ValueMapping);
+
+  // Clean up things like PHI nodes with single operands, dead instructions,
+  // etc.
+  SimplifyInstructionsInBlock(NewBB, TLI);
+  SimplifyInstructionsInBlock(PredBB, TLI);
+
+  SmallVector<BasicBlock *, 1> PredsToFactor;
+  PredsToFactor.push_back(NewBB);
+  ThreadEdge(BB, PredsToFactor, SuccBB);
+}
+
 /// TryThreadEdge - Thread an edge if it's safe and profitable to do so.
 bool JumpThreadingPass::TryThreadEdge(
     BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs,
     BasicBlock *SuccBB) {
   // If threading to the same block as we come from, we would infinite loop.
   if (SuccBB == BB) {
     LLVM_DEBUG(dbgs() << "  Not threading across BB '" << BB->getName()
                       << "' - would thread to self!\n");

llvm/test/Transforms/JumpThreading/thread-two-bbs1.ll

+; RUN: opt < %s -jump-threading -S -verify | FileCheck %s
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@a = global i32 0, align 4
+
+define void @foo(i32 %cond1, i32 %cond2) {
+; CHECK-LABEL: @foo
+; CHECK-LABEL: entry
+entry:
+  %tobool = icmp eq i32 %cond1, 0
+  br i1 %tobool, label %bb.cond2, label %bb.f1
+
+bb.f1:
+  call void @f1()
+  br label %bb.cond2
+; Verify that we branch on cond2 without checking ptr.
+; CHECK:      call void @f1()
+; CHECK-NEXT: icmp eq i32 %cond2, 0
+; CHECK-NEXT: label %bb.f4, label %bb.f2
+
+bb.cond2:
+  %ptr = phi i32* [ null, %bb.f1 ], [ @a, %entry ]
+  %tobool1 = icmp eq i32 %cond2, 0
+  br i1 %tobool1, label %bb.file, label %bb.f2
+; Verify that we branch on cond2 without checking ptr.
+; CHECK:      icmp eq i32 %cond2, 0
+; CHECK-NEXT: label %bb.f3, label %bb.f2
+
+bb.f2:
+  call void @f2()
+  br label %exit
+
+; Verify that we eliminate this basic block.
+; CHECK-NOT: bb.file:
+bb.file:
+  %cmp = icmp eq i32* %ptr, null
+  br i1 %cmp, label %bb.f4, label %bb.f3
+
+bb.f3:
+  call void @f3()
+  br label %exit
+
+bb.f4:
+  call void @f4()
+  br label %exit
+
+exit:
+  ret void
+}
+
+declare void @f1()
+
+declare void @f2()
+
+declare void @f3()
+
+declare void @f4()

llvm/test/Transforms/JumpThreading/thread-two-bbs2.ll

+; RUN: opt < %s -jump-threading -S -verify | FileCheck %s
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @foo(i32 %cond1, i32 %cond2) {
+; CHECK-LABEL: @foo
+; CHECK-LABEL: entry
+entry:
+  %tobool = icmp ne i32 %cond1, 0
+  br i1 %tobool, label %bb.f1, label %bb.f2
+
+bb.f1:
+  call void @f1()
+  br label %bb.cond2
+; Verify that we branch on cond2 without checking tobool again.
+; CHECK:      call void @f1()
+; CHECK-NEXT: icmp eq i32 %cond2, 0
+; CHECK-NEXT: label %exit, label %bb.f3
+
+bb.f2:
+  call void @f2()
+  br label %bb.cond2
+; Verify that we branch on cond2 without checking tobool again.
+; CHECK:      call void @f2()
+; CHECK-NEXT: icmp eq i32 %cond2, 0
+; CHECK-NEXT: label %exit, label %bb.f4
+
+bb.cond2:
+  %tobool1 = icmp eq i32 %cond2, 0
+  br i1 %tobool1, label %exit, label %bb.cond1again
+
+; Verify that we eliminate this basic block.
+; CHECK-NOT: bb.cond1again:
+bb.cond1again:
+  br i1 %tobool, label %bb.f3, label %bb.f4
+
+bb.f3:
+  call void @f3()
+  br label %exit
+
+bb.f4:
+  call void @f4()
+  br label %exit
+
+exit:
+  ret void
+}
+
+declare void @f1() local_unnamed_addr
+
+declare void @f2() local_unnamed_addr
+
+declare void @f3() local_unnamed_addr
+
+declare void @f4() local_unnamed_addr

llvm/test/Transforms/JumpThreading/thread-two-bbs3.ll

+; RUN: opt < %s -jump-threading -S -verify | FileCheck %s
+
+target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc19.16.27026"
+
+; Verify that we do *not* thread any edge.  On Windows, we used to
+; improperly duplicate EH pads like bb_cleanup below, resulting in an
+; assertion failure later down the pass pipeline.
+define void @foo([2 x i8]* %0) personality i8* bitcast (i32 ()* @baz to i8*) {
+; CHECK-LABEL: @foo
+; CHECK-NOT: bb_{{[^ ]*}}.thread:
+entry:
+  invoke void @bar()
+          to label %bb_invoke unwind label %bb_cleanuppad
+
+bb_invoke:
+  invoke void @bar()
+          to label %bb_exit unwind label %bb_cleanuppad
+
+bb_cleanuppad:
+  %index = phi i64 [ 1, %bb_invoke ], [ 0, %entry ]
+  %cond1 = phi i1 [ false, %bb_invoke ], [ true, %entry ]
+  %1 = cleanuppad within none []
+  br i1 %cond1, label %bb_action, label %bb_cleanupret
+
+bb_action:
+  %cond2 = icmp eq i64 %index, 0
+  br i1 %cond2, label %bb_cleanupret, label %bb_exit
+
+bb_exit:
+  call void @bar()
+  ret void
+
+bb_cleanupret:
+  cleanupret from %1 unwind to caller
+}
+
+declare void @bar()
+declare i32 @baz()