[DAGCombiner] Poor DAG combining with alias analysis turned on.

[Hal Finkel]

Hi Jonas,

Why don't we want to add TF users to the worklist always? Is it generically useless unless AA is used?

> + // Don't add users to work list, unless alias analysis is used.

If nothing else, this comment needs to explain why.

 -Hal

----- Original Message -----
> From: "Jonas Paulsson" <jonas.paulsson at ericsson.com>
> To: llvm-commits at cs.uiuc.edu
> Sent: Monday, February 9, 2015 9:38:18 AM
> Subject: [DAGCombiner] Poor DAG combining with alias analysis turned on.
> 
> Hi,
> 
> I have a test case with an unrolled loop body, consisting of a long
> sequence of non-overlapping stores. I found that with alias analysis
> it became very compile time expensive (53 seconds, without AA it was
> 0.02 sec). The program spent a lot of time in WalkChainUsers()
> (isel), and it was due to all the token factor nodes introduced by
> the combiner.
> 
> 
> 
> I spent some time looking into it, and found that nearly all of the
> compile time disappeared if the combiner added a new token factor
> node to the worklist. The case was that there was a mess of token
> factors at the bottom of the DAG, that disappeared after reiteration
> over those nodes.
> 
> 
> 
> Test case supplied, to run: llc -mtriple=arm -combiner-alias-analysis
> unrolledloop.opt.ll
> 
> 
> 
> I supply a patch for this. Let me know if this seems reasonable to
> use and if I can commit.
> 
> 
> 
> /Jonas Paulsson
> 
> 
> 
> 
> 
> Patch:
> 
> 
> 
> From 0123157edefc2aea3137b702b1fa24ebd41e7709 Mon Sep 17 00:00:00
> 2001
> 
> From: Jonas Paulsson <jonas.paulsson at ericsson.com>
> 
> Date: Mon, 9 Feb 2015 16:19:53 +0100
> 
> Subject: [PATCH] Fix SelectionDAG compile time issue with alias
> analysis.
> 
> 
> 
> Add new token factor node to worklist if alias analysis is turned on,
> 
> in DAGCombiner::visitTokenFactor().
> 
> ---
> 
> lib/CodeGen/SelectionDAG/DAGCombiner.cpp | 6 ++++--
> 
> 1 file changed, 4 insertions(+), 2 deletions(-)
> 
> 
> 
> diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
> b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
> 
> index 8a4b602..dbf6ef7 100644
> 
> --- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
> 
> +++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
> 
> @@ -1539,8 +1539,10 @@ SDValue DAGCombiner::visitTokenFactor(SDNode
> *N) {
> 
> Result = DAG.getNode(ISD::TokenFactor, SDLoc(N), MVT::Other, Ops);
> 
> }
> 
> 
> 
> - // Don't add users to work list.
> 
> - return CombineTo(N, Result, false);
> 
> + // Don't add users to work list, unless alias analysis is used.
> 
> + bool UseAA = CombinerAA.getNumOccurrences() > 0 ? CombinerAA
> 
> + : DAG.getSubtarget().useAA();
> 
> + return CombineTo(N, Result, UseAA /*add to worklist*/);
> 
> }
> 
> 
> 
> return Result;
> 
> --
> 
> 1.8.4.2
> 
> 
> 
> 
> 
> 
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> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory