[GVN] Don't perform scalar PRE on GEPs

Partial Redundancy Elimination of GEPs prevents CodeGenPrepare from
sinking the addressing mode computation of memory instructions back
to its uses. The problem comes from the insertion of PHIs, which
confuse CGP and make it bail.

I've autogenerated the check lines of an existing test and added a
store instruction to demonstrate the motivation behind this change.
The store is now using the gep instead of a phi.

Differential Revision: https://reviews.llvm.org/D55009

llvm-svn: 348496

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index c080c2a1813..440ea4a5bc7 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -2156,6 +2156,16 @@ bool GVN::performScalarPRE(Instruction *CurInst) {
   if (isa<CmpInst>(CurInst))
     return false;
 
+  // Don't do PRE on GEPs. The inserted PHI would prevent CodeGenPrepare from
+  // sinking the addressing mode computation back to its uses. Extending the
+  // GEP's live range increases the register pressure, and therefore it can
+  // introduce unnecessary spills.
+  //
+  // This doesn't prevent Load PRE. PHI translation will make the GEP available
+  // to the load by moving it to the predecessor block if necessary.
+  if (isa<GetElementPtrInst>(CurInst))
+    return false;
+
   // We don't currently value number ANY inline asm calls.
   if (CallInst *CallI = dyn_cast<CallInst>(CurInst))
     if (CallI->isInlineAsm())