fix two significant issues with FoldTwoEntryPHINode:

first, it can kick in on blocks whose conditions have been
folded to a constant, even though one of the edges will be
trivially folded.

second, it doesn't clean up the "if diamond" that it just 
eliminated away.  This is a problem because other simplifycfg
xforms kick in depending on the order of block visitation,
causing pointless work.

llvm-svn: 121762

2 files changed, 16 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index f0c97fd4a04..1fbbb42b60e 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1148,7 +1148,10 @@ static bool FoldTwoEntryPHINode(PHINode *PN, const TargetData *TD) {
   BasicBlock *BB = PN->getParent();
   BasicBlock *IfTrue, *IfFalse;
   Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse);
-  if (!IfCond) return false;
+  if (!IfCond ||
+      // Don't bother if the branch will be constant folded trivially.
+      isa<ConstantInt>(IfCond))
+    return false;
   
   // Okay, we found that we can merge this two-entry phi node into a select.
   // Doing so would require us to fold *all* two entry phi nodes in this block.
@@ -1243,6 +1246,13 @@ static bool FoldTwoEntryPHINode(PHINode *PN, const TargetData *TD) {
     NV->takeName(PN);
     PN->eraseFromParent();
   }
+  
+  // At this point, IfBlock1 and IfBlock2 are both empty, so our if statement
+  // has been flattened.  Change DomBlock to jump directly to our new block to
+  // avoid other simplifycfg's kicking in on the diamond.
+  TerminatorInst *OldTI = DomBlock->getTerminator();
+  BranchInst::Create(BB, OldTI);
+  OldTI->eraseFromParent();
   return true;
 }
 
diff --git a/llvm/test/Transforms/SimplifyCFG/PhiBlockMerge.ll b/llvm/test/Transforms/SimplifyCFG/PhiBlockMerge.ll
index a648efd1740..c28d0bac375 100644
--- a/llvm/test/Transforms/SimplifyCFG/PhiBlockMerge.ll
+++ b/llvm/test/Transforms/SimplifyCFG/PhiBlockMerge.ll
@@ -1,12 +1,14 @@
 ; Test merging of blocks that only have PHI nodes in them
 ;
-; RUN: opt < %s -simplifycfg -S | not grep N:
+; RUN: opt < %s -simplifycfg -S | FileCheck %s
 ;
 
 define i32 @test(i1 %a, i1 %b) {
-; <label>:0
+; CHECK: br i1 %a
         br i1 %a, label %M, label %O
 O:              ; preds = %0
+; CHECK: select i1 %b, i32 0, i32 1
+; CHECK-NOT: phi
         br i1 %b, label %N, label %Q
 Q:              ; preds = %O
         br label %N
@@ -15,6 +17,7 @@ N:              ; preds = %Q, %O
         %Wp = phi i32 [ 0, %O ], [ 1, %Q ]              ; <i32> [#uses=1]
         br label %M
 M:              ; preds = %N, %0
+; CHECK: %W = phi i32
         %W = phi i32 [ %Wp, %N ], [ 2, %0 ]             ; <i32> [#uses=1]
         %R = add i32 %W, 1              ; <i32> [#uses=1]
         ret i32 %R