[llvm] 2702fb1 - [SimplifyCFG] Restart if `removeUndefIntroducingPredecessor()` made changes

[Roman Lebedev via llvm-commits]

Author: Roman Lebedev
Date: 2021-08-13T15:45:03+03:00
New Revision: 2702fb11484a6f5d99c712a0e92c221288955c56

URL: https://github.com/llvm/llvm-project/commit/2702fb11484a6f5d99c712a0e92c221288955c56
DIFF: https://github.com/llvm/llvm-project/commit/2702fb11484a6f5d99c712a0e92c221288955c56.diff

LOG: [SimplifyCFG] Restart if `removeUndefIntroducingPredecessor()` made changes

It might changed the condition of a branch into a constant,
so we should restart and constant-fold terminator,
instead of continuing with the tautological "conditional" branch.
This fixes the issue reported at https://reviews.llvm.org/rGf30a7dff8a5b32919951dcbf92e4a9d56c4679ff

Added: 
    

Modified: 
    llvm/lib/Transforms/Utils/SimplifyCFG.cpp
    llvm/test/Transforms/SimplifyCFG/tautological-conditional-branch.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 482316c6111f9..c9d4641fd6ddd 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -6634,7 +6634,8 @@ bool SimplifyCFGOpt::simplifyOnceImpl(BasicBlock *BB) {
   Changed |= EliminateDuplicatePHINodes(BB);
 
   // Check for and remove branches that will always cause undefined behavior.
-  Changed |= removeUndefIntroducingPredecessor(BB, DTU);
+  if (removeUndefIntroducingPredecessor(BB, DTU))
+    return requestResimplify();
 
   // Merge basic blocks into their predecessor if there is only one distinct
   // pred, and if there is only one distinct successor of the predecessor, and

diff  --git a/llvm/test/Transforms/SimplifyCFG/tautological-conditional-branch.ll b/llvm/test/Transforms/SimplifyCFG/tautological-conditional-branch.ll
index 31ab0fa60dbe8..912598197080e 100644
--- a/llvm/test/Transforms/SimplifyCFG/tautological-conditional-branch.ll
+++ b/llvm/test/Transforms/SimplifyCFG/tautological-conditional-branch.ll
@@ -15,8 +15,7 @@ define void @eggs(i1 %arg, i1 %arg16, %0* %arg17, %0* %arg18, %0* %arg19) {
 ; CHECK:       bb21:
 ; CHECK-NEXT:    [[I:%.*]] = icmp eq %0* [[ARG17:%.*]], null
 ; CHECK-NEXT:    call void @hoge()
-; CHECK-NEXT:    [[I26:%.*]] = select i1 [[ARG16:%.*]], %0* null, %0* [[ARG19:%.*]]
-; CHECK-NEXT:    [[I27:%.*]] = getelementptr inbounds [[TMP0:%.*]], %0* [[I26]], i64 0, i32 0
+; CHECK-NEXT:    [[I27:%.*]] = getelementptr inbounds [[TMP0:%.*]], %0* [[ARG19:%.*]], i64 0, i32 0
 ; CHECK-NEXT:    [[I28:%.*]] = load %1*, %1** [[I27]], align 8
 ; CHECK-NEXT:    call void @pluto.1(%1* [[I28]])
 ; CHECK-NEXT:    call void @pluto()