[llvm-dev] [RFC] Delaying phi-to-select transformation until later in the pass pipeline
Hal Finkel via llvm-dev
llvm-dev at lists.llvm.org
Wed Aug 15 14:57:38 PDT 2018
On 08/15/2018 02:38 PM, Philip Reames via llvm-dev wrote:
>
> I'm concerned that we're focusing on one side of this. Let me point
> out a few concerns w/changing the canonical form here:
>
> 1. LICM does not know how to hoist or sink regions. It does know how
> to hoist and sink selects.
> 2. InstCombine has limited support for triangles/diamonds, but fairly
> extensive support for selects.
> 3. EarlyCSE and GVN do not know how to eliminate fully redundant
> triangles/diamonds. PRE *may* get some of these cases, but that
> is by no means guaranteed or likely to be robust.
>
> My personal opinion is that selects are the appropriate canonical form.
>
> For the one of the specific cases mentioned, teaching GVN to do FRE
> and PRE for loads from selects of pointers just doesn't seem that
> painful. I would be really tempted to do that instead. Similarly,
> walking facts back from select uses in CVP seems generally useful
> since we have use dependent facts in a bunch of contexts, not just
> selects. (Call arguments for instance, non-null from unconditional
> deref, etc..)
>
> To be clear, I am raising concerns, not actively objecting to this.
> If you want to move forward and commit to work through all of the
> issues identified I wont actively stand in the way.
>
As I've expressed in the past, I think that not using select as part of
our canonical form is potentially a superior design. However, it would
be a major change. In addition to the issues that Philip mentions,
there's also the fact that we'll just have more basic blocks and that,
in itself, could lead to an increase in compile time. However, working
through these issues will likely leave us with a more-robust optimizer.
See also: https://bugs.llvm.org/show_bug.cgi?id=34603#c19
-Hal
> Philip
>
>
> On 08/14/2018 12:39 PM, Sanjay Patel via llvm-dev wrote:
>> I didn't look closely at the new patch, but this appears to be a
>> small extension to:
>> https://reviews.llvm.org/D38566
>> ...and the GVN-based reasons for delaying transformation to 'select'
>> are discussed in detail in the motivating bug for that patch:
>> https://bugs.llvm.org/show_bug.cgi?id=34603
>>
>> So this sounds like the right direction to me. Note that there was
>> objection to the implementation (a pile of pass options vs. uniquely
>> named passes).
>>
>> Here's another motivating bug where early transform to select
>> prevents optimization:
>> https://bugs.llvm.org/show_bug.cgi?id=36760
>>
>> Is that case affected by this patch?
>>
>>
>> On Tue, Aug 14, 2018 at 11:17 AM, John Brawn via llvm-dev
>> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>> Summary
>> =======
>>
>> I'm planning on adjusting SimplifyCFG so that it doesn't turn
>> two-entry phi
>> nodes into selects until later in the pass pipeline, to give
>> passes which can
>> understand phis but not selects more opportunity to optimize. The
>> thing I'm
>> trying to do which made me think of doing this is described
>> below, but from the
>> benchmarking I've done it looks like this is overall a good idea
>> regardless of
>> if I manage to get that done or not.
>>
>> Motivation
>> ==========
>>
>> My goal is to get clang to optimize some code containing a call to
>> std::min_element which is dereferenced, so something like:
>>
>> float min_element_example(float *data, int size)
>> {
>> return *std::min_element(data, data+size);
>> }
>>
>> which, after inlining a specialization, looks like:
>>
>> float min_element_example_inlined(float *first, float *last)
>> {
>> for (float *p = first; p != last; ++p)
>> {
>> if (*p < *first)
>> first = p;
>> }
>> return *first;
>> }
>>
>> There are two loads in the loop, *p and *first, but actually the
>> load *p can be
>> eliminated by using either the previous load *p or the previous
>> *first,
>> depending on if the if-condition was taken or not. However the
>> "if (*p < *first) first = p" gets turned by simplifycfg into a
>> select and this
>> makes optimizing this a lot harder because you no longer have
>> distinct paths
>> through the CFG.
>>
>> I have some ideas on how to do the optimization (see my previous
>> RFC "Making GVN
>> able to visit the same block more than once" posted in April,
>> though I've
>> decided that the specific idea presented there isn't the right
>> way to do it),
>> but I think the first step is to make sure we don't have a select
>> when we try
>> to optimise this.
>>
>> Approach
>> ========
>>
>> I've posted a patch to https://reviews.llvm.org/D50723
>> <https://reviews.llvm.org/D50723> showing what I'm
>> intending to do. An extra parameter is added to SimplifyCFG to
>> control whether
>> two-entry phi nodes are converted into select, and this is set to
>> false in all
>> instances before the end of module simplification. At the end of
>> module
>> simplification we do SimplifyCFG, then Instcombine to optimise
>> the selects that
>> are introduced, then EarlyCSE to eliminate common subexpressions
>> introduced by
>> instcombine.
>>
>> Benchmark Results
>> =================
>>
>> These are performance differences reported by LNT when running
>> llvm-test-suite,
>> spec2000, and spec2006 at -O3 with and without the patch linked
>> above (using
>> trunk llvm from a week or so ago).
>>
>> AArch64 results on ARM Cortex-A72:
>>
>> Performance Regressions - execution_time
>> Change
>> SingleSource/Benchmarks/Shootout/Shootout-ary3
>> 9.48%
>> MultiSource/Benchmarks/TSVC/Packing-flt/Packing-flt
>> 3.79%
>> SingleSource/Benchmarks/CoyoteBench/huffbench
>> 1.40%
>>
>> Performance Improvements - execution_time
>> Change
>> MultiSource/Benchmarks/TSVC/Searching-dbl/Searching-dbl
>> -23.74%
>> External/SPEC/CINT2000/256.bzip2/256.bzip2
>> -9.82%
>> MultiSource/Benchmarks/TSVC/Searching-flt/Searching-flt
>> -9.57%
>> MultiSource/Benchmarks/TSVC/Equivalencing-flt/Equivalencing-flt
>> -4.38%
>> MultiSource/Benchmarks/TSVC/LinearDependence-flt/LinearDependence-flt
>> -3.94%
>> MultiSource/Benchmarks/TSVC/Packing-dbl/Packing-dbl
>> -3.44%
>> External/SPEC/CFP2006/453.povray/453.povray
>> -2.50%
>> SingleSource/Benchmarks/Adobe-C++/stepanov_vector
>> -1.49%
>>
>> X86_64 results on Intel Xeon E5-2690:
>>
>> Performance Regressions - execution_time Change
>> MultiSource/Benchmarks/Ptrdist/yacr2/yacr2 5.62%
>>
>> Performance Improvements - execution_time Change
>> SingleSource/Benchmarks/Misc-C++/Large/sphereflake -4.43%
>> External/SPEC/CINT2006/456.hmmer/456.hmmer -2.50%
>> External/SPEC/CINT2006/464.h264ref/464.h264ref -1.60%
>> MultiSource/Benchmarks/nbench/nbench -1.19%
>> SingleSource/Benchmarks/Adobe-C++/functionobjects -1.07%
>>
>> I had a brief look at the regressions and they all look to be
>> caused by
>> getting bad luck with branch mispredictions: I looked into the
>> Shootout-ary3 and
>> yacr2 cases and in both the hot code path was the same with and
>> without the
>> patch, but with more mispredictions probably caused by changes
>> elsewhere.
>>
>> John
>>
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>>
>>
>>
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>
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--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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