[InstCombine] Teach instcombine not to create extra PHI nodes when folding GEPs

Summary:
InstCombine tries to transform GEP(PHI(GEP1, GEP2, ..)) into GEP(GEP(PHI(...))
when possible. However, this may leave the old PHI node around. Even if we
do end up folding the GEPs, having an extra PHI node might not be beneficial.

This change makes the transformation more conservative. We now only do this if
the PHI has only one use, and can therefore be removed after the transformation.

Reviewers: jmolloy, majnemer

Subscribers: mcrosier, mssimpso, llvm-commits

Differential Revision: http://reviews.llvm.org/D13887

llvm-svn: 251281

1 files changed, 6 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 69a2661f031..b51ec291f91 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1453,8 +1453,13 @@ Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
       }
     }
 
-    GetElementPtrInst *NewGEP = cast<GetElementPtrInst>(Op1->clone());
+    // If not all GEPs are identical we'll have to create a new PHI node.
+    // Check that the old PHI node has only one use so that it will get
+    // removed.
+    if (DI != -1 && !PN->hasOneUse())
+      return nullptr;
 
+    GetElementPtrInst *NewGEP = cast<GetElementPtrInst>(Op1->clone());
     if (DI == -1) {
       // All the GEPs feeding the PHI are identical. Clone one down into our
       // BB so that it can be merged with the current GEP.