Fix loop unswitching's assumption that a code path which either

infinite loops or exits will eventually exit. This fixes PR5373.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112745 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/LoopUnswitch.cpp

@@ -277,19 +277,18 @@ bool LoopUnswitch::processCurrentLoop() {
   return Changed;
 }
 
-/// isTrivialLoopExitBlock - Check to see if all paths from BB either:
+/// isTrivialLoopExitBlock - Check to see if all paths from BB exit the
-///   1. Exit the loop with no side effects.
+/// loop with no side effects (including infinite loops).
-///   2. Branch to the latch block with no side-effects.
 ///
-/// If these conditions are true, we return true and set ExitBB to the block we
+/// If true, we return true and set ExitBB to the block we
 /// exit through.
 ///
 static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
                                          BasicBlock *&ExitBB,
                                          std::set<BasicBlock*> &Visited) {
   if (!Visited.insert(BB).second) {
-    // Already visited and Ok, end of recursion.
+    // Already visited. Without more analysis, this could indicate an infinte loop.
-    return true;
+    return false;
   } else if (!L->contains(BB)) {
     // Otherwise, this is a loop exit, this is fine so long as this is the
     // first exit.
@@ -319,7 +318,7 @@ static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
 /// process.  If so, return the block that is exited to, otherwise return null.
 static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
   std::set<BasicBlock*> Visited;
-  Visited.insert(L->getHeader());  // Branches to header are ok.
+  Visited.insert(L->getHeader());  // Branches to header make infinite loops.
   BasicBlock *ExitBB = 0;
   if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited))
     return ExitBB;
@@ -351,8 +350,8 @@ bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val,
     if (!BI->isConditional() || BI->getCondition() != Cond)
       return false;
   
-    // Check to see if a successor of the branch is guaranteed to go to the
+    // Check to see if a successor of the branch is guaranteed to 
-    // latch block or exit through a one exit block without having any 
+    // exit through a unique exit block without having any 
     // side-effects.  If so, determine the value of Cond that causes it to do
     // this.
     if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, 

test/Transforms/LoopUnswitch/infinite-loop.ll

@@ -0,0 +1,53 @@
+; RUN: opt -loop-unswitch -disable-output -stats -info-output-file - < %s | FileCheck --check-prefix=STATS %s
+; RUN: opt -loop-unswitch -simplifycfg -S < %s | FileCheck %s
+; PR5373
+
+; Loop unswitching shouldn't trivially unswitch the true case of condition %a
+; in the code here because it leads to an infinite loop. While this doesn't
+; contain any instructions with side effects, it's still a kind of side effect.
+; It can trivially unswitch on the false cas of condition %a though.
+
+; STATS: 2 loop-unswitch - Number of branches unswitched
+; STATS: 1 loop-unswitch - Number of unswitches that are trivial
+
+; CHECK: @func_16
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br i1 %a, label %entry.split, label %abort0.split
+
+; CHECK: entry.split:
+; CHECK-NEXT: br i1 %b, label %cond.end.us, label %abort1
+
+; CHECK: cond.end.us:
+; CHECK-NEXT: br label %cond.end.us
+
+; CHECK: abort0.split:
+; CHECK-NEXT: call void @end0() noreturn nounwind
+; CHECK-NEXT: unreachable
+
+; CHECK: abort1:
+; CHECK-NEXT: call void @end1() noreturn nounwind
+; CHECK-NEXT: unreachable
+
+; CHECK: }
+
+define void @func_16(i1 %a, i1 %b) nounwind {
+entry:
+  br label %for.body
+
+for.body:
+  br i1 %a, label %cond.end, label %abort0
+
+cond.end:
+  br i1 %b, label %for.body, label %abort1
+
+abort0:
+  call void @end0() noreturn nounwind
+  unreachable
+
+abort1:
+  call void @end1() noreturn nounwind
+  unreachable
+}
+
+declare void @end0() noreturn
+declare void @end1() noreturn