[llvm-commits] [PATCH] Allow SelectionDAGBuilder to reorder loads past stores

[Owen Anderson]

Hal,

How does this compare to the -combiner-alias-analysis and -combiner-global-alias-analysis already in SelectionDAG?  I gave the former a try about a year ago, and found that it at least didn't cause things to fail.

--Owen


On Dec 19, 2011, at 9:42 AM, Hal Finkel wrote:

> The current SelectionDAGBuilder does not allow loads to be reordered
> past stores, and does not allow stores to be reordered. This is a side
> effect of the way the critical chain is constructed: there is a queue of
> pending loads that is flushed (in parallel) to the root of the chain
> upon encountering any store (and that store is also appended to the root
> of the chain). Among other things, loop unrolling is far less effective
> than it otherwise could be.
> 
> The attached patch allows SelectionDAGBuilder to use the available alias
> analysis to reorder independent loads and stores. It changes the queue
> of pending loads into a more general queue of pending memory operations,
> and flushes, in parallel, all potentially-conflicting loads and stores
> as necessary.
> 
> This can result in a significant performance boost. On my x86_64
> machine, the average percentage decrease in execution time is ~8% (to
> calculate my performance numbers from the test suite, I've included only
> the 174 tests with a base execution time of at least 0.1s; the times of
> the shorter tests seem noisy on my machine). Of these, 131 showed a
> performance increase and 36 showed a performance decrease.
> 
> The top-5 winners were:
> MultiSource/Benchmarks/MiBench/consumer-typeset/consumer-typeset - 92%
> performance increase ( = runtime decrease)
> MultiSource/Benchmarks/llubenchmark/llu - 47% performance increase
> MultiSource/Applications/minisat/minisat - 47% performance increase
> MultiSource/Benchmarks/sim/sim - 40% performance increase
> MultiSource/Benchmarks/Prolangs-C++/life/life - 35.7% performance
> increase
> The top-5 losers were:
> MultiSource/Benchmarks/MiBench/consumer-lame/consumer-lame - 88%
> performance decrease
> MultiSource/Benchmarks/VersaBench/beamformer/beamformer - 49%
> performance decrease
> MultiSource/Benchmarks/MallocBench/espresso/espresso 47% performance
> decrease
> MultiSource/Benchmarks/MiBench/automotive-bitcount/automotive-bitcount -
> 21% performance decrease
> MultiSource/Benchmarks/MiBench/network-patricia/network-patricia - 20%
> performance decrease
> 
> The patch adds a few new options:
> max-parallel-chains - replaces the old MaxParallelChains constant)
> max-load-store-reorder - the maximum size of the reorder buffer -
> previously it was unlimited, but contained only stores
> no-reordering-past-stores - invokes the previous behavior
> 
> Some of the regression tests had to be updated because the order of some
> stores changed. For most of these, I just updated the test to reflect
> the new instruction sequence. The following tests I've marked as XFAIL
> because they would require larger changes (and I'd like someone with
> more experience than me to make sure that they really are okay and make
> any necessary adjustments):
> CodeGen/X86/2008-02-22-LocalRegAllocBug.ll
> CodeGen/X86/2010-09-17-SideEffectsInChain.ll
> CodeGen/X86/lea-recursion.ll
> 
> Also, there is one test-suite runtime failure on x86_64:
> MultiSource/Benchmarks/Ptrdist/ft/ft
> 
> And several test-suite runtime failures on i686:
> MultiSource/Benchmarks/tramp3d-v4/tramp3d-v4
> SingleSource/Benchmarks/Misc-C++/Large/ray
> SingleSource/Benchmarks/Misc-C++/stepanov_container
> SingleSource/Benchmarks/Shootout-C++/lists
> SingleSource/Benchmarks/Shootout-C++/lists1
> SingleSource/Benchmarks/Shootout-C++/sieve
> 
> Please review (and help with the test-suite failures).
> 
> Thank you in advance,
> Hal
> 
> -- 
> Hal Finkel
> Postdoctoral Appointee
> Leadership Computing Facility
> Argonne National Laboratory
> 
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