Fix trellis layout to avoid mis-identify triangle.

Summary:
For the following CFG:

A->B
B->C
A->C

If there is another edge B->D, then ABC should not be considered as triangle.

Reviewers: davidxl, iteratee

Reviewed By: iteratee

Subscribers: nemanjai, llvm-commits

Differential Revision: https://reviews.llvm.org/D31310

llvm-svn: 298661

1 files changed, 6 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/MachineBlockPlacement.cpp b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
index d3bf95b9a95..de0b572f2fe 100644
--- a/llvm/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
@@ -839,8 +839,13 @@ bool MachineBlockPlacement::isTrellis(
     int PredCount = 0;
     for (auto SuccPred : Succ->predecessors()) {
       // Allow triangle successors, but don't count them.
-      if (Successors.count(SuccPred))
+      if (Successors.count(SuccPred)) {
+        // Make sure that it is actually a triangle.
+        for (MachineBasicBlock *CheckSucc : SuccPred->successors())
+          if (!Successors.count(CheckSucc))
+            return false;
         continue;
+      }
       const BlockChain *PredChain = BlockToChain[SuccPred];
       if (SuccPred == BB || (BlockFilter && !BlockFilter->count(SuccPred)) ||
           PredChain == &Chain || PredChain == BlockToChain[Succ])