Check to see if a two-entry PHI block can be simplified

before trying to merge the block into its predecessors.
This allows two-entry-phi-return.ll to be simplified
into a single basic block.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@48252 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 21 insertions, 6 deletions

diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index ac5c2ec..b9a20c1 100644
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1224,6 +1224,12 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
   // away...
   Changed |= ConstantFoldTerminator(BB);
 
+  // If there is a trivial two-entry PHI node in this basic block, and we can
+  // eliminate it, do so now.
+  if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
+    if (PN->getNumIncomingValues() == 2)
+      Changed |= FoldTwoEntryPHINode(PN); 
+
   // If this is a returning block with only PHI nodes in it, fold the return
   // instruction into any unconditional branch predecessors.
   //
@@ -1915,11 +1921,5 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
         }
       }
 
-  // If there is a trivial two-entry PHI node in this basic block, and we can
-  // eliminate it, do so now.
-  if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
-    if (PN->getNumIncomingValues() == 2)
-      Changed |= FoldTwoEntryPHINode(PN); 
-
   return Changed;
 }
diff --git a/test/Transforms/SimplifyCFG/two-entry-phi-return.ll b/test/Transforms/SimplifyCFG/two-entry-phi-return.ll
new file mode 100644
index 0000000..19814ad
--- /dev/null
+++ b/test/Transforms/SimplifyCFG/two-entry-phi-return.ll
@@ -0,0 +1,15 @@
+; RUN: llvm-as < %s | opt -simplifycfg | llvm-dis | not grep br
+
+define i1 @qux(i8* %m, i8* %n, i8* %o, i8* %p) nounwind  {
+entry:
+        %tmp7 = icmp eq i8* %m, %n
+        br i1 %tmp7, label %bb, label %UnifiedReturnBlock
+
+bb:
+        %tmp15 = icmp eq i8* %o, %p
+        br label %UnifiedReturnBlock
+
+UnifiedReturnBlock:
+        %result = phi i1 [ 0, %entry ], [ %tmp15, %bb ]
+        ret i1 %result
+}