[llvm-dev] large slowdown in DAGCombiner::MergeConsecutiveStores

[Jeff Bezanson via llvm-dev]

Hello all,

We are seeing a large compiler performance regression in moving from LLVM
6.0.1 to 8.0.1. We have a long function (~50000 instructions) that used to
compile in about a minute but now takes at least an hour. All the time is
in MergeConsecutiveStores, I believe due to super-linear behavior in
analyzing very long chains of stores. For example, this change makes the
problem go away:

```
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -16011,6 +16011,9 @@ bool
DAGCombiner::MergeConsecutiveStores(StoreSDNode *St) {
                        StoreNodes.begin() + NumConsecutiveStores);
       continue;
     }
+    else if (NumConsecutiveStores > 128) {
+      return RV;
+    }
```

Does anybody have any advice on what a good way to fix this would be? Does
it make sense to have some kind of limit on how many stores we try to
merge? (Even if so, I don't know whether the above is the right way to
implement such a limit.)

Thanks in advance for any suggestions.

-Jeff
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