[DAGCombine] Disable finding better chains for stores at O0

Unoptimized IR can have linear sequences of stores to an array, where the
initial GEP for the first store is formed from the pointer to the array, and the
GEP for each store after the first is formed from the previous GEP with some
offset in an inductive fashion.

The (large) resulting DAG when analyzed by DAGCombine undergoes an excessive
number of combines as each store node is examined every time its' offset node
is combined with any child of the offset. One of the transformations is
findBetterNeighborChains which assists MergeConsecutiveStores. The former
relies on repeated chain walking to do its' work, however MergeConsecutiveStores
is disabled at O0 which makes the transformation redundant.

Any optimization level other than O0 would invoke InstCombine which would
resolve the chain of GEPs into flat base + offset GEP for each store which
does not exhibit the repeated examination of each store to the array.

Disabling this optimization fixes an excessive compile time issue (30~ minutes
for the test case provided) at O0.

Reviewers: niravd, craig.topper, t.p.northover

Differential Revision: https://reviews.llvm.org/D40193

llvm-svn: 319142

1 files changed, 8 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 6e3e5fc4646..097ff63e12b 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -17424,7 +17424,11 @@ void DAGCombiner::GatherAllAliases(SDNode *N, SDValue OriginalChain,
 /// Walk up chain skipping non-aliasing memory nodes, looking for a better chain
 /// (aliasing node.)
 SDValue DAGCombiner::FindBetterChain(SDNode *N, SDValue OldChain) {
-  SmallVector<SDValue, 8> Aliases;  // Ops for replacing token factor.
+  if (OptLevel == CodeGenOpt::None)
+    return OldChain;
+
+  // Ops for replacing token factor.
+  SmallVector<SDValue, 8> Aliases;
 
   // Accumulate all the aliases to this node.
   GatherAllAliases(N, OldChain, Aliases);
@@ -17454,6 +17458,9 @@ SDValue DAGCombiner::FindBetterChain(SDNode *N, SDValue OldChain) {
 // to go from a partially-merged state to the desired final
 // fully-merged state.
 bool DAGCombiner::findBetterNeighborChains(StoreSDNode *St) {
+  if (OptLevel == CodeGenOpt::None)
+    return false;
+
   // This holds the base pointer, index, and the offset in bytes from the base
   // pointer.
   BaseIndexOffset BasePtr = BaseIndexOffset::match(St->getBasePtr(), DAG);