Handle successor's PHI node correctly when flattening CFG merges two if-regions

Summary:
FlattenCFG merges two 'if' basicblocks by inserting one basicblock
to another basicblock. The inserted basicblock can have a successor
that contains a PHI node whoes incoming basicblock is the inserted
basicblock. Since the existing code does not handle it, it becomes
a badref.

if (cond1)
  statement
if (cond2)
  statement
successor - contains PHI node whose predecessor is cond2

-->
if (cond1 || cond2)
  statement
(BB for cond2 was deleted)
successor - contains PHI node whose predecessor is cond2 --> bad ref!

Author: Jaebaek Seo

Reviewers: asbirlea, kuhar, tstellar, chandlerc, davide, dexonsmith

Reviewed By: kuhar

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68032

llvm-svn: 372989

1 files changed, 11 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/FlattenCFG.cpp b/llvm/lib/Transforms/Utils/FlattenCFG.cpp
index 41109746de5..893f23eb604 100644
--- a/llvm/lib/Transforms/Utils/FlattenCFG.cpp
+++ b/llvm/lib/Transforms/Utils/FlattenCFG.cpp
@@ -67,7 +67,7 @@ public:
 /// Before:
 ///   ......
 ///   %cmp10 = fcmp une float %tmp1, %tmp2
-///   br i1 %cmp1, label %if.then, label %lor.rhs
+///   br i1 %cmp10, label %if.then, label %lor.rhs
 ///
 /// lor.rhs:
 ///   ......
@@ -453,6 +453,16 @@ bool FlattenCFGOpt::MergeIfRegion(BasicBlock *BB, IRBuilder<> &Builder) {
   PBI->replaceUsesOfWith(CC, NC);
   Builder.SetInsertPoint(SaveInsertBB, SaveInsertPt);
 
+  // Handle PHI node to replace its predecessors to FirstEntryBlock.
+  for (BasicBlock *Succ : successors(PBI)) {
+    for (PHINode &Phi : Succ->phis()) {
+      for (unsigned i = 0, e = Phi.getNumIncomingValues(); i != e; ++i) {
+        if (Phi.getIncomingBlock(i) == SecondEntryBlock)
+          Phi.setIncomingBlock(i, FirstEntryBlock);
+      }
+    }
+  }
+
   // Remove IfTrue1
   if (IfTrue1 != FirstEntryBlock) {
     IfTrue1->dropAllReferences();