[llvm] r321653 - [BasicBlockUtils] Check for unreachable preds before updating LI in UpdateAnalysisInformation

Anna Thomas via llvm-commits llvm-commits at lists.llvm.org
Tue Jan 2 08:25:50 PST 2018

Author: annat
Date: Tue Jan  2 08:25:50 2018
New Revision: 321653

URL: http://llvm.org/viewvc/llvm-project?rev=321653&view=rev
[BasicBlockUtils] Check for unreachable preds before updating LI in UpdateAnalysisInformation

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


Modified: llvm/trunk/lib/Transforms/Utils/BasicBlockUtils.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/BasicBlockUtils.cpp?rev=321653&r1=321652&r2=321653&view=diff
--- llvm/trunk/lib/Transforms/Utils/BasicBlockUtils.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/BasicBlockUtils.cpp Tue Jan  2 08:25:50 2018
@@ -323,6 +323,12 @@ static void UpdateAnalysisInformation(Ba
   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)

Modified: llvm/trunk/test/Transforms/LoopSimplify/unreachable-loop-pred.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopSimplify/unreachable-loop-pred.ll?rev=321653&r1=321652&r2=321653&view=diff
--- llvm/trunk/test/Transforms/LoopSimplify/unreachable-loop-pred.ll (original)
+++ llvm/trunk/test/Transforms/LoopSimplify/unreachable-loop-pred.ll Tue Jan  2 08:25:50 2018
@@ -18,3 +18,52 @@ while.body115:
   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 {
+  br label %outerHeader
+  invoke void @foo() 
+          to label %innerPreheader unwind label %innerLoopExit
+  br label %innerH
+  %tmp50 = invoke i8 * undef()
+          to label %innerLatch unwind label %innerLoopExit
+  %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.
+  %tmp65 = landingpad { i8*, i32 }
+          cleanup
+  invoke void @foo() 
+          to label %outerHeader unwind label %unwindblock
+  %tmp67 = landingpad { i8*, i32 }
+          cleanup
+  ret void
+  ret void
+; Function Attrs: nounwind
+declare i32* @wobble()
+; Function Attrs: uwtable
+declare void @foo()

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