[JumpThreading] Fold br(freeze(undef))

This patch makes JumpThreading fold br(freeze(undef)) if the freeze instruction
is only used by the branch.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D84818

diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
index 7399c7a..37a588a 100644 (file)
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -1057,9 +1057,11 @@ bool JumpThreadingPass::ProcessBlock(BasicBlock *BB) {
     }
   }
 
-  // If the terminator is branching on an undef, we can pick any of the
-  // successors to branch to.  Let GetBestDestForJumpOnUndef decide.
-  if (isa<UndefValue>(Condition)) {
+  // If the terminator is branching on an undef or freeze undef, we can pick any
+  // of the successors to branch to.  Let GetBestDestForJumpOnUndef decide.
+  auto *FI = dyn_cast<FreezeInst>(Condition);
+  if (isa<UndefValue>(Condition) ||
+      (FI && isa<UndefValue>(FI->getOperand(0)) && FI->hasOneUse())) {
     unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
     std::vector<DominatorTree::UpdateType> Updates;
 
@@ -1078,6 +1080,8 @@ bool JumpThreadingPass::ProcessBlock(BasicBlock *BB) {
     BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
     BBTerm->eraseFromParent();
     DTU->applyUpdatesPermissive(Updates);
+    if (FI)
+      FI->eraseFromParent();
     return true;
   }
 

diff --git a/llvm/test/Transforms/JumpThreading/freeze.ll b/llvm/test/Transforms/JumpThreading/freeze.ll
index 8f8f3f5..99df46f 100644 (file)
--- a/llvm/test/Transforms/JumpThreading/freeze.ll
+++ b/llvm/test/Transforms/JumpThreading/freeze.ll
@@ -128,19 +128,13 @@ F2:
 
 define i32 @test1_undef(i1 %cond) {
 ; CHECK-LABEL: @test1_undef(
-; CHECK-NEXT:    br i1 [[COND:%.*]], label [[MERGE_THREAD:%.*]], label [[MERGE:%.*]]
-; CHECK:       Merge.thread:
-; CHECK-NEXT:    [[V1:%.*]] = call i32 @f1()
-; CHECK-NEXT:    br label [[T2:%.*]]
-; CHECK:       Merge:
-; CHECK-NEXT:    [[V2:%.*]] = call i32 @f2()
-; CHECK-NEXT:    [[A_FR:%.*]] = freeze i1 undef
-; CHECK-NEXT:    br i1 [[A_FR]], label [[T2]], label [[F2:%.*]]
+; CHECK-NEXT:    br i1 [[COND:%.*]], label [[T2:%.*]], label [[F2:%.*]]
 ; CHECK:       T2:
-; CHECK-NEXT:    [[B4:%.*]] = phi i32 [ [[V1]], [[MERGE_THREAD]] ], [ [[V2]], [[MERGE]] ]
+; CHECK-NEXT:    [[V1:%.*]] = call i32 @f1()
 ; CHECK-NEXT:    call void @f3()
-; CHECK-NEXT:    ret i32 [[B4]]
+; CHECK-NEXT:    ret i32 [[V1]]
 ; CHECK:       F2:
+; CHECK-NEXT:    [[V2:%.*]] = call i32 @f2()
 ; CHECK-NEXT:    ret i32 [[V2]]
 ;
   br i1 %cond, label %T1, label %F1