8296545: C2 Blackholes should allow load optimizations #11041

shipilev · 2022-11-08T15:48:01Z

If you look at generated code for the JMH benchmark like:

public class ArrayRead {
    @Param({"1", "100", "10000", "1000000"})
    int size;

    int[] is;

    @Setup
    public void setup() {
        is = new int[size];
        for (int c = 0; c < size; c++) {
            is[c] = c;
        }
    }

    @Benchmark
    public void test(Blackhole bh) {
        for (int i = 0; i < is.length; i++) {
            bh.consume(is[i]);
        }
    }
}

...then you would notice that the loop always re-reads is, is.length, does the range check, etc. -- all the things we would otherwise expect to be hoisted out of the loop.

This is because C2 blackholes are modeled as membars that pinch both control and memory slices (like you would expect from the opaque non-inlined call), therefore every iteration has to re-read the referenced memory contents and recompute everything dependent on those loads. This behavior is not new -- the old, non-compiler blackholes were doing the same thing, accidentally -- but it was drowned in blackhole overheads. Now, these effects are clearly visible.

We can try to do this a bit better: allow load optimizations to work across the blackholes, leaving only "prevent dead code elimination" part, as minimally required by blackhole semantics.

Motivational improvements on the test above:

Benchmark        (size)  Mode  Cnt        Score   Error     Units

# Before, full Java blackholes
ArrayRead.test        1  avgt    9        5.422 ±    0.023  ns/op
ArrayRead.test      100  avgt    9      460.619 ±    0.421  ns/op
ArrayRead.test    10000  avgt    9    44697.909 ± 1964.787  ns/op
ArrayRead.test  1000000  avgt    9  4332723.304 ± 2791.324  ns/op

# Before, compiler blackholes
ArrayRead.test        1  avgt    9        1.791 ±    0.007  ns/op
ArrayRead.test      100  avgt    9      114.103 ±    1.677  ns/op
ArrayRead.test    10000  avgt    9     8528.544 ±   52.010  ns/op
ArrayRead.test  1000000  avgt    9  1005139.070 ± 2883.011  ns/op

# After, compiler blackholes
ArrayRead.test        1  avgt    9        1.686 ±    0.006  ns/op  ; ~1.1x better
ArrayRead.test      100  avgt    9       16.249 ±    0.019  ns/op  ; ~7.0x better
ArrayRead.test    10000  avgt    9     1375.265 ±    2.420  ns/op  ; ~6.2x better
ArrayRead.test  1000000  avgt    9   136862.574 ± 1057.100  ns/op  ; ~7.3x better

-prof perfasm shows the reason for these improvements clearly:

Before:

          ↗  0x00007f0b54498360:   mov    0xc(%r12,%r10,8),%edx    ; range check 1
   7.97%  │  0x00007f0b54498365:   cmp    %edx,%r11d               
   1.27%  │  0x00007f0b54498368:   jae    0x00007f0b5449838f
          │  0x00007f0b5449836a:   shl    $0x3,%r10
   0.03%  │  0x00007f0b5449836e:   mov    0x10(%r10,%r11,4),%r10d  ; get "is[i]"
   7.76%  │  0x00007f0b54498373:   mov    0x10(%r9),%r10d          ; restore "is"
   0.24%  │  0x00007f0b54498377:   mov    0x3c0(%r15),%rdx         ; safepoint poll, part 1
  17.48%  │  0x00007f0b5449837e:   inc    %r11d                    ; i++
   0.17%  │  0x00007f0b54498381:   test   %eax,(%rdx)              ; safepoint poll, part 2  
  53.26%  │  0x00007f0b54498383:   mov    0xc(%r12,%r10,8),%edx    ; loop index check
   4.84%  │  0x00007f0b54498388:   cmp    %edx,%r11d
   0.31%  ╰  0x00007f0b5449838b:   jl     0x00007f0b54498360

After:

          ↗  0x00007fa06c49a8b0:   mov    0x2c(%rbp,%r10,4),%r9d   ; stride read
  19.66%  │  0x00007fa06c49a8b5:   mov    0x28(%rbp,%r10,4),%edx
   0.14%  │  0x00007fa06c49a8ba:   mov    0x10(%rbp,%r10,4),%ebx
  22.09%  │  0x00007fa06c49a8bf:   mov    0x14(%rbp,%r10,4),%ebx
   0.21%  │  0x00007fa06c49a8c4:   mov    0x18(%rbp,%r10,4),%ebx
  20.19%  │  0x00007fa06c49a8c9:   mov    0x1c(%rbp,%r10,4),%ebx
   0.04%  │  0x00007fa06c49a8ce:   mov    0x20(%rbp,%r10,4),%ebx
  24.02%  │  0x00007fa06c49a8d3:   mov    0x24(%rbp,%r10,4),%ebx
   0.21%  │  0x00007fa06c49a8d8:   add    $0x8,%r10d               ; i += 8
          │  0x00007fa06c49a8dc:   cmp    %esi,%r10d
   0.07%  ╰  0x00007fa06c49a8df:   jl     0x00007fa06c49a8b0

Additional testing:

Eyeballing JMH Samples -prof perfasm
Linux x86_64 fastdebug, compiler/blackhole, compiler/c2/irTests/blackhole
Linux x86_64 fastdebug, JDK benchmark corpus

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Issue

JDK-8296545: C2 Blackholes should allow load optimizations

Reviewers

Vladimir Kozlov (@vnkozlov - Reviewer) ⚠️ Review applies to 5a91ed9a
Vladimir Ivanov (@iwanowww - Reviewer) ⚠️ Review applies to 66247f75

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$ git fetch https://git.openjdk.org/jdk pull/11041/head:pull/11041
$ git checkout pull/11041

Update a local copy of the PR:
$ git checkout pull/11041
$ git pull https://git.openjdk.org/jdk pull/11041/head

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Checkout this PR locally:
$ git pr checkout 11041

View PR using the GUI difftool:
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https://git.openjdk.org/jdk/pull/11041.diff

bridgekeeper · 2022-11-08T15:50:05Z

👋 Welcome back shade! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2022-11-08T15:52:14Z

@shipilev The following label will be automatically applied to this pull request:

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mlbridge · 2022-11-08T22:12:21Z

Webrevs

adinn · 2022-11-08T22:24:39Z

Ooh, nice work. Unfortunately the VerifyGraphEdges test seems to be failing.

vnkozlov

Looks good.
It is just copy of insert_mem_bar() without setting memory to BH.

openjdk · 2022-11-08T23:58:46Z

@shipilev This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8296545: C2 Blackholes should allow load optimizations

Reviewed-by: kvn, vlivanov

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been no new commits pushed to the master branch. If another commit should be pushed before you perform the /integrate command, your PR will be automatically rebased. If you prefer to avoid any potential automatic rebasing, please check the documentation for the /integrate command for further details.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

vnkozlov · 2022-11-09T00:38:06Z

Ooh, nice work. Unfortunately the VerifyGraphEdges test seems to be failing.

https://bugs.openjdk.org/browse/JDK-8295867

src/hotspot/share/opto/library_call.cpp

vnkozlov

Looks good for me with one comment.

src/hotspot/share/opto/cfgnode.hpp

iwanowww

Looks good.

(hs-tier1 - hs-tier2 testing passed.)

shipilev · 2022-11-15T13:36:12Z

Thanks for reviews. I'll sit on it a bit, while doing more extensive benchmarking with these new blackholes.

shipilev · 2022-12-05T11:59:20Z

Experiments look fine. I am integrating to get this to JDK 20.

/integrate

openjdk · 2022-12-05T12:00:42Z

Going to push as commit eab0ada.
Since your change was applied there have been 6 commits pushed to the master branch:

dea2161: 8297959: Provide better descriptions for some Operating System JFR events
d523d9d: 8297864: Dead code elimination
777fb52: 8297974: ClassCastException in com.sun.tools.javac.comp.AttrRecover.doRecovery
17666fb: 8297794: Deprecate JMX Management Applets for Removal
619b68c: 8294540: Remove Opaque2Node: it is broken and triggers assert
82561de: 8296384: [TESTBUG] sun/security/provider/SecureRandom/AbstractDrbg/SpecTest.java intermittently timeout

Your commit was automatically rebased without conflicts.

openjdk · 2022-12-05T12:01:05Z

@shipilev Pushed as commit eab0ada.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

Fix

5a91ed9

openjdk bot added the hotspot-compiler hotspot-compiler-dev@openjdk.org label Nov 8, 2022

shipilev marked this pull request as ready for review November 8, 2022 22:06

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vnkozlov approved these changes Nov 8, 2022

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iwanowww reviewed Nov 9, 2022

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src/hotspot/share/opto/library_call.cpp Outdated Show resolved Hide resolved

Do not touch memory at all

1ca2feb

iwanowww reviewed Nov 9, 2022

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shipilev added 3 commits November 14, 2022 12:10

Blackhole should be AliasIdxTop

d8d287f

Merge branch 'master' into JDK-8296545-blackhole-effects

f592bcb

Blackhole as CFG node

66247f7

vnkozlov reviewed Nov 14, 2022

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Add comment in cfgnode.hpp

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shipilev added 3 commits November 29, 2022 17:45

Merge branch 'master' into JDK-8296545-blackhole-effects

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Merge branch 'master' into JDK-8296545-blackhole-effects

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Merge branch 'master' into JDK-8296545-blackhole-effects

e847a8b

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shipilev deleted the JDK-8296545-blackhole-effects branch January 4, 2023 10:22

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