Add an optimization that does CSE in a group of similar GEPs.

This optimization merges the common part of a group of GEPs, so we can compute
each pointer address by adding a simple offset to the common part.

The optimization is currently only enabled for the NVPTX backend, where it has
a large payoff on some benchmarks.

Review: http://reviews.llvm.org/D3462

Patch by Jingyue Wu.

llvm-svn: 207783

1 files changed, 17 insertions, 4 deletions

diff --git a/llvm/lib/Target/NVPTX/NVPTXTargetMachine.cpp b/llvm/lib/Target/NVPTX/NVPTXTargetMachine.cpp
index 0cc5c516292..26a4f840520 100644
--- a/llvm/lib/Target/NVPTX/NVPTXTargetMachine.cpp
+++ b/llvm/lib/Target/NVPTX/NVPTXTargetMachine.cpp
@@ -147,10 +147,23 @@ void NVPTXPassConfig::addIRPasses() {
   addPass(createNVPTXAssignValidGlobalNamesPass());
   addPass(createGenericToNVVMPass());
   addPass(createNVPTXFavorNonGenericAddrSpacesPass());
-  // The FavorNonGenericAddrSpaces pass may remove instructions and leave some
-  // values unused. Therefore, we run a DCE pass right afterwards. We could
-  // remove unused values in an ad-hoc manner, but it requires manual work and
-  // might be error-prone.
+  addPass(createSeparateConstOffsetFromGEPPass());
+  // The SeparateConstOffsetFromGEP pass creates variadic bases that can be used
+  // by multiple GEPs. Run GVN or EarlyCSE to really reuse them. GVN generates
+  // significantly better code than EarlyCSE for some of our benchmarks.
+  if (getOptLevel() == CodeGenOpt::Aggressive)
+    addPass(createGVNPass());
+  else
+    addPass(createEarlyCSEPass());
+  // Both FavorNonGenericAddrSpaces and SeparateConstOffsetFromGEP may leave
+  // some dead code.  We could remove dead code in an ad-hoc manner, but that
+  // requires manual work and might be error-prone.
+  //
+  // The FavorNonGenericAddrSpaces pass shortcuts unnecessary addrspacecasts,
+  // and leave them unused.
+  //
+  // SeparateConstOffsetFromGEP rebuilds a new index from the old index, and the
+  // old index and some of its intermediate results may become unused.
   addPass(createDeadCodeEliminationPass());
 }