[DAG, x86] allow store merging before and after legalization (PR34217)

rL310710 allowed store merging to occur after legalization to catch stores that are created late,
but this exposes a logic hole seen in PR34217:
https://bugs.llvm.org/show_bug.cgi?id=34217

We will miss merging stores if the target lowers vector extracts into target-specific operations.
This patch allows store merging to occur both before and after legalization if the target chooses
to get maximum merging.

I don't think the potential regressions in the other tests are relevant. The tests are for
correctness of weird IR constructs rather than perf tests, and I think those are still correct.

Differential Revision: https://reviews.llvm.org/D37987

llvm-svn: 313564

1 files changed, 4 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index f81a144de82..b69a5f4738c 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -13565,10 +13565,10 @@ SDValue DAGCombiner::visitSTORE(SDNode *N) {
                              Ptr, ST->getMemoryVT(), ST->getMemOperand());
   }
 
-  // Only perform this optimization before the types are legal, because we
-  // don't want to perform this optimization on every DAGCombine invocation.
-  if ((TLI.mergeStoresAfterLegalization()) ? Level == AfterLegalizeDAG
-                                           : !LegalTypes) {
+  // Always perform this optimization before types are legal. If the target
+  // prefers, also try this after legalization to catch stores that were created
+  // by intrinsics or other nodes.
+  if (!LegalTypes || (TLI.mergeStoresAfterLegalization())) {
     for (;;) {
       // There can be multiple store sequences on the same chain.
       // Keep trying to merge store sequences until we are unable to do so