[BasicBlockUtils] Check for unreachable preds before updating LI in UpdateAnalysisInf...

Summary:
We are incorrectly updating the LI when loop-simplify generates
dedicated exit blocks for a loop. The issue is that there's an implicit
assumption that the Preds passed into UpdateAnalysisInformation are
reachable. However, this is not true and breaks LI by incorrectly
updating the header of a loop.

One such case is when we generate dedicated exits when the exit block is
a landing pad (through SplitLandingPadPredecessors). There maybe other
cases as well, since we do not guarantee that Preds passed in are
reachable basic blocks.

The added test case shows how loop-simplify breaks LI for the outer loop (and DT in turn)
after we try to generate the LoopSimplifyForm.

Reviewers: davide, chandlerc, sanjoy

Reviewed By: davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41519

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@321653 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/Utils/BasicBlockUtils.cpp b/lib/Transforms/Utils/BasicBlockUtils.cpp
index 5f04f14518da38a460bd93f4d8e40c328886798f..8f59913e14bbf76a98f924c76ffb4a87d4027c2f 100644 (file)
--- a/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -323,6 +323,12 @@ static void UpdateAnalysisInformation(BasicBlock *OldBB, BasicBlock *NewBB,
   bool IsLoopEntry = !!L;
   bool SplitMakesNewLoopHeader = false;
   for (BasicBlock *Pred : Preds) {
+    // Preds that are not reachable from entry should not be used to identify if
+    // OldBB is a loop entry or if SplitMakesNewLoopHeader. Unreachable blocks
+    // are not within any loops, so we incorrectly mark SplitMakesNewLoopHeader
+    // as true and make the NewBB the header of some loop. This breaks LI.
+    if (!DT->isReachableFromEntry(Pred))
+      continue;
     // If we need to preserve LCSSA, determine if any of the preds is a loop
     // exit.
     if (PreserveLCSSA)

diff --git a/test/Transforms/LoopSimplify/unreachable-loop-pred.ll b/test/Transforms/LoopSimplify/unreachable-loop-pred.ll
index 76b7bb21e468f238ee328e7724acd1cd4f517b67..1e92ee4ee3bc6631120e76cc10e5e97b915f9698 100644 (file)
--- a/test/Transforms/LoopSimplify/unreachable-loop-pred.ll
+++ b/test/Transforms/LoopSimplify/unreachable-loop-pred.ll
@@ -18,3 +18,52 @@ while.body115:                                    ; preds = %9, %if.end192, %if.
 foo:
   br label %while.body115
 }
+
+; When loopsimplify generates dedicated exit block for blocks that are landing
+; pads (i.e. innerLoopExit in this test), we should not get confused with the
+; unreachable pred (unreachableB) to innerLoopExit.
+define align 8 void @baz(i32 %trip) personality i32* ()* @wobble {
+entry:
+  br label %outerHeader
+
+outerHeader:
+  invoke void @foo() 
+          to label %innerPreheader unwind label %innerLoopExit
+
+innerPreheader:
+  br label %innerH
+
+innerH:
+  %tmp50 = invoke i8 * undef()
+          to label %innerLatch unwind label %innerLoopExit
+
+innerLatch:
+  %cmp = icmp slt i32 %trip, 42
+  br i1 %cmp, label %innerH, label %retblock
+
+unreachableB:                                             ; No predecessors!
+  %tmp62 = invoke i8 * undef()
+          to label %retblock unwind label %innerLoopExit
+
+; undedicated exit block (preds from inner and outer loop)
+; Also has unreachableB as pred.
+innerLoopExit:
+  %tmp65 = landingpad { i8*, i32 }
+          cleanup
+  invoke void @foo() 
+          to label %outerHeader unwind label %unwindblock
+
+unwindblock:
+  %tmp67 = landingpad { i8*, i32 }
+          cleanup
+  ret void
+
+retblock:
+  ret void
+}
+
+; Function Attrs: nounwind
+declare i32* @wobble()
+
+; Function Attrs: uwtable
+declare void @foo()