[all-commits] [llvm/llvm-project] fcb51d: [InstCombine] PHI-of-extractvalues -> extractvalue...

Tue Aug 25 23:09:23 PDT 2020

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: fcb51d8c2460faa23b71e06abb7e826243887dd6
      https://github.com/llvm/llvm-project/commit/fcb51d8c2460faa23b71e06abb7e826243887dd6
  Author: Roman Lebedev <lebedev.ri at gmail.com>
  Date:   2020-08-26 (Wed, 26 Aug 2020)

  Changed paths:
    M llvm/lib/Transforms/InstCombine/InstCombineInternal.h
    M llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp
    M llvm/test/Transforms/InstCombine/phi-aware-aggregate-reconstruction.ll
    M llvm/test/Transforms/InstCombine/phi-of-extractvalues.ll

  Log Message:
  -----------
  [InstCombine] PHI-of-extractvalues -> extractvalue-of-PHI, aka invokes are bad

While since D86306 we do it's sibling fold for `insertvalue`,
we should also do this for `extractvalue`'s.

And unlike that one, the results here are, quite honestly, shocking,
as it can be observed here on vanilla llvm test-suite + RawSpeed results:

```
| statistic name                                     | baseline  | proposed  |       Δ |       % |    |%| |
|----------------------------------------------------|-----------|-----------|--------:|--------:|-------:|
| asm-printer.EmittedInsts                           | 7945095   | 7942507   |   -2588 |  -0.03% |  0.03% |
| assembler.ObjectBytes                              | 273209920 | 273069800 | -140120 |  -0.05% |  0.05% |
| early-cse.NumCSE                                   | 2183363   | 2183398   |      35 |   0.00% |  0.00% |
| early-cse.NumSimplify                              | 541847    | 550017    |    8170 |   1.51% |  1.51% |
| instcombine.NumAggregateReconstructionsSimplified  | 2139      | 108       |   -2031 | -94.95% | 94.95% |
| instcombine.NumCombined                            | 3601364   | 3635448   |   34084 |   0.95% |  0.95% |
| instcombine.NumConstProp                           | 27153     | 27157     |       4 |   0.01% |  0.01% |
| instcombine.NumDeadInst                            | 1694521   | 1765022   |   70501 |   4.16% |  4.16% |
| instcombine.NumPHIsOfExtractValues                 | 0         | 37546     |   37546 |   0.00% |  0.00% |
| instcombine.NumSunkInst                            | 63158     | 63686     |     528 |   0.84% |  0.84% |
| instcount.NumBrInst                                | 874304    | 871857    |   -2447 |  -0.28% |  0.28% |
| instcount.NumCallInst                              | 1757657   | 1758402   |     745 |   0.04% |  0.04% |
| instcount.NumExtractValueInst                      | 45623     | 11483     |  -34140 | -74.83% | 74.83% |
| instcount.NumInsertValueInst                       | 4983      | 580       |   -4403 | -88.36% | 88.36% |
| instcount.NumInvokeInst                            | 61018     | 59478     |   -1540 |  -2.52% |  2.52% |
| instcount.NumLandingPadInst                        | 35334     | 34215     |   -1119 |  -3.17% |  3.17% |
| instcount.NumPHIInst                               | 344428    | 331116    |  -13312 |  -3.86% |  3.86% |
| instcount.NumRetInst                               | 100773    | 100772    |      -1 |   0.00% |  0.00% |
| instcount.TotalBlocks                              | 1081154   | 1077166   |   -3988 |  -0.37% |  0.37% |
| instcount.TotalFuncs                               | 101443    | 101442    |      -1 |   0.00% |  0.00% |
| instcount.TotalInsts                               | 8890201   | 8833747   |  -56454 |  -0.64% |  0.64% |
| instsimplify.NumSimplified                         | 75822     | 75707     |    -115 |  -0.15% |  0.15% |
| simplifycfg.NumHoistCommonCode                     | 24203     | 24197     |      -6 |  -0.02% |  0.02% |
| simplifycfg.NumHoistCommonInstrs                   | 48201     | 48195     |      -6 |  -0.01% |  0.01% |
| simplifycfg.NumInvokes                             | 2785      | 4298      |    1513 |  54.33% | 54.33% |
| simplifycfg.NumSimpl                               | 997332    | 1018189   |   20857 |   2.09% |  2.09% |
| simplifycfg.NumSinkCommonCode                      | 7088      | 6464      |    -624 |  -8.80% |  8.80% |
| simplifycfg.NumSinkCommonInstrs                    | 15117     | 14021     |   -1096 |  -7.25% |  7.25% |
```
... which tells us that this new fold fires whopping 38k times,
increasing the amount of SimplifyCFG's `invoke`->`call` transforms by +54% (+1513) (again, D85787 did that last time),
decreasing total instruction count by -0.64% (-56454),
and sharply decreasing count of `insertvalue`'s (-88.36%, i.e. 9 times less)
and `extractvalue`'s (-74.83%, i.e. four times less).

This causes geomean -0.01% binary size decrease
http://llvm-compile-time-tracker.com/compare.php?from=4d5ca22b8adfb6643466e4e9f48ba14bb48938bc&to=97dacca0111cb2ae678204e52a3cee00e3a69208&stat=size-text
and, ignoring `O0-g`, is a geomean -0.01%..-0.05% compile-time improvement
http://llvm-compile-time-tracker.com/compare.php?from=4d5ca22b8adfb6643466e4e9f48ba14bb48938bc&to=97dacca0111cb2ae678204e52a3cee00e3a69208&stat=instructions

The other thing that tells is, is that while this is a massive win for `invoke`->`call` transform
`InstCombinerImpl::foldAggregateConstructionIntoAggregateReuse()` fold,
which is supposed to be dealing with such aggregate reconstructions,
fires a lot less now. There are two reasons why:
1. After this fold, as it can be seen in tests, we may (will) end up with trivially redundant PHI nodes.
   We don't CSE them in InstCombine presently, which means that EarlyCSE needs to run and then InstCombine rerun.
2. But then, EarlyCSE not only manages to fold such redundant PHI's,
   it also sees that the extract-insert chain recreates the original aggregate,
   and replaces it with the original aggregate.

The take-aways are
1. We maybe should do most trivial, same-BB PHI CSE in InstCombine
2. I need to check if what other patterns remain, and how they can be resolved.
   (i.e. i wonder if `foldAggregateConstructionIntoAggregateReuse()` might go away)

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D86530