8264945: Optimize the code-gen for Math.pow(x, 0.5) #3404

DamonFool · 2021-04-09T02:19:10Z

Hi all,

I'd like to optimize the code-gen for Math.pow(x, 0.5).
And 7x ~ 14x performance improvement is observed by the jmh micro-benchmarks.

While I was optimizing a machine learning program, I found both Math.pow(x, 2) and Math.pow(x, 0.5) are used.
To my surprise, C2 just optimizes the case for Math.pow(x, 2) [1], but still not for Math.pow(x, 0.5) yet.

The patch just replace Math.pow(x, 0.5) with Math.sqrt(x).

Before:

Benchmark                     (seed)   Mode  Cnt      Score    Error   Units
MathBench.powDouble0Dot5           0  thrpt    8  45525.117 ? 11.686  ops/ms
MathBench.powDouble0Dot5Loop       0  thrpt    8      0.031 ?  0.001  ops/ms

Benchmark                     (seed)   Mode  Cnt      Score    Error   Units
MathBench.powDouble0Dot5           0  thrpt    8  45509.317 ?  6.581  ops/ms
MathBench.powDouble0Dot5Loop       0  thrpt    8      0.031 ?  0.001  ops/ms

After:

Benchmark                     (seed)   Mode  Cnt       Score     Error   Units
MathBench.powDouble0Dot5           0  thrpt    8  343354.892 ? 362.900  ops/ms
MathBench.powDouble0Dot5Loop       0  thrpt    8       0.457 ?   0.001  ops/mso

Benchmark                     (seed)   Mode  Cnt       Score    Error   Units
MathBench.powDouble0Dot5           0  thrpt    8  343421.559 ? 49.326  ops/ms
MathBench.powDouble0Dot5Loop       0  thrpt    8       0.457 ?  0.001  ops/ms

Testing:

tier1~3 on Linux/x64

Thanks,
Best regards,
Jie

[1] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/library_call.cpp#L1680

Progress

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Issue

JDK-8264945: Optimize the code-gen for Math.pow(x, 0.5)

Reviewers

Nils Eliasson (@neliasso - Reviewer) ⚠️ Review applies to 1adb8c7
Vladimir Kozlov (@vnkozlov - Reviewer)

Reviewing

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Checkout this PR locally:
$ git fetch https://git.openjdk.java.net/jdk pull/3404/head:pull/3404
$ git checkout pull/3404

Update a local copy of the PR:
$ git checkout pull/3404
$ git pull https://git.openjdk.java.net/jdk pull/3404/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 3404

View PR using the GUI difftool:
$ git pr show -t 3404

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Download this PR as a diff file:
https://git.openjdk.java.net/jdk/pull/3404.diff

DamonFool · 2021-04-09T02:19:23Z

/test
/label add hotspot-compiler
/cc hotspot-compiler

bridgekeeper · 2021-04-09T02:20:09Z

👋 Welcome back jiefu! 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 · 2021-04-09T02:21:48Z

@DamonFool
The hotspot-compiler label was successfully added.

openjdk · 2021-04-09T02:23:11Z

@DamonFool The hotspot-compiler label was already applied.

mlbridge · 2021-04-09T02:26:45Z

Webrevs

01: Full (beab8e79)
00: Full (1adb8c79)

huishi-hs · 2021-04-09T03:37:52Z

Does float vector needs same optimization， convert pow to sqrt？

DamonFool · 2021-04-09T03:56:32Z

Does float vector needs same optimization， convert pow to sqrt？

Thanks @huishi-hs for your review.

The same optimization for the vector api is in progress.
Let's see what the reviewers think of this opt.

Thanks.

neliasso

Looks good.

openjdk · 2021-04-09T11:57:53Z

@DamonFool 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:

8264945: Optimize the code-gen for Math.pow(x, 0.5)

Reviewed-by: neliasso, kvn

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 11 new commits pushed to the master branch:

3990713: 8265463: ProblemList vmTestbase/vm/mlvm/mixed/stress/regression/b6969574/INDIFY_Test.java on Win-X64 -Xcomp
5b43b39: 8263154: [macos] DMG builds have finder errors
54cb388: 8252600: [JVMCI] remove mx configuration
b703e0a: 8264569: Remove obsolete error messages from CDSTestUtils.java
235daea: 8265285: Unnecessary inclusion of bytecodeHistogram.hpp
8bec6fe: 8264277: java.xml.crypto module should be granted FilePermission and SocketPermission
5303ccb: 8265135: Reduce work initializing VarForms
d9e40dd: 8199407: Skip Rebuild Remset Phase if there are no rebuild candidates
8dbf7aa: 8241306: Add SignatureMethodParameterSpec subclass for RSASSA-PSS params
b14e0ee: 8265371: Change to Visual Studio 2019 16.9.3 for building on Windows at Oracle
... and 1 more: https://git.openjdk.java.net/jdk/compare/b64a3fb946a2855c612458aca9d62ef008f8f9be...master

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DamonFool · 2021-04-09T12:38:25Z

Looks good.

Thanks @neliasso .

vnkozlov

Please, verify that result is the same when run with -Xint (Interpreter only) and (-XX:TieredStopAtLevel=1) C1 only. May be they need the same optimization.

DamonFool · 2021-04-12T07:00:50Z

Hi @vnkozlov @neliasso and @huishi-hs ,

Thanks for your review and comments.
While I was implementing the opt for C1 and interpreter, I found Math.pow(x, 0.5) and Math.sqrt(x) would compute different values for x={-0.0, Double.NEGATIVE_INFINITY} [1].
Let's hold on this issue until we have a conclusion about that question.
Thanks.

[1] https://mail.openjdk.java.net/pipermail/core-libs-dev/2021-April/076195.html

DamonFool · 2021-04-16T07:06:10Z

Hi @vnkozlov @neliasso and @huishi-hs ,

Thanks for your review and comments.
While I was implementing the opt for C1 and interpreter, I found Math.pow(x, 0.5) and Math.sqrt(x) would compute different values for x={-0.0, Double.NEGATIVE_INFINITY} [1].
Let's hold on this issue until we have a conclusion about that question.
Thanks.

[1] https://mail.openjdk.java.net/pipermail/core-libs-dev/2021-April/076195.html

Hi all,

According to the discussion [1], we can still perform pow(x, 0.5) => sqrt(x) for x >= 0.0 or x is +Inf/NaN.

To better help the code review, the whole optimization has been split into JDK-8265325 and JDK-8264945.

1. JDK-8265325: Optimize StubRoutines::dpow() for Math.pow(x, 0.5)
1. JDK-8264945: Optimize the code-gen for Math.pow(x, 0.5)

I'll update this pr once JDK-8265325 is finished since it depends on JDK-8265325.

Thanks.
Best regards,
Jie

[1] https://mail.openjdk.java.net/pipermail/core-libs-dev/2021-April/076220.html

DamonFool · 2021-04-19T15:06:15Z

Hi all,

This is the follow-up of JDK-8265325, which optimizes the code-gen of C2 for pow(x, 0.5).
Instead of calling into the StubRoutines::dpow(), the compiler can directly generate sqrt for all x >= 0.0.
1.5x ~ 4.2x performance improvement is observed after the opt.

Before

Benchmark                      (seed)   Mode  Cnt       Score       Error   Units
MathBench.powDouble0Dot5Const       0  thrpt    8  203004.302 ?   684.072  ops/ms
MathBench.powDouble0Dot5Loop        0  thrpt    8       0.121 ?     0.001  ops/ms

After

Benchmark                      (seed)   Mode  Cnt       Score      Error   Units
MathBench.powDouble0Dot5Const       0  thrpt    8  308771.237 ? 1604.567  ops/ms   <-- 1.5x
MathBench.powDouble0Dot5Loop        0  thrpt    8       0.508 ?    0.001  ops/ms   <-- 4.2x

Testing:
tier1 ~ tier3 on Linux/x64

Thanks.
Best regards,
Jie

DamonFool · 2021-04-19T15:06:24Z

/test

openjdk · 2021-04-19T15:06:49Z

@DamonFool you need to get approval to run the tests in tier1 for commits up until beab8e7

vnkozlov

Good.

DamonFool · 2021-04-20T09:29:52Z

Good.

Thanks @vnkozlov .

Hi @neliasso , are you OK with the latest change?
Thanks.

neliasso · 2021-04-20T09:38:22Z

Hi @neliasso , are you OK with the latest change?

Yes. Looks good.

DamonFool · 2021-04-20T09:41:46Z

Hi @neliasso , are you OK with the latest change?

Yes. Looks good.

Thanks @neliasso .

Will push it tomorrow if there is no objection.

theRealAph · 2021-04-20T16:49:58Z

This may be unsafe. AFAIK there is no guarantee about the monotonicity of Math.pow(x, y) as y moves from Math.pow(x, nextAfter(0.5, -1) to Math.pow(x, 0.5) to Math.pow(x, nextAfter(0.5, 1) for all x. Did anyone look?

DamonFool · 2021-04-20T22:39:50Z

This may be unsafe. AFAIK there is no guarantee about the monotonicity of Math.pow(x, y) as y moves from Math.pow(x, nextAfter(0.5, -1) to Math.pow(x, 0.5) to Math.pow(x, nextAfter(0.5, 1) for all x. Did anyone look?

Thanks @theRealAph for your review.

What did you mean by Math.pow(x, nextAfter(0.5, -1) and Math.pow(x, nextAfter(0.5, 1) ?

DamonFool · 2021-04-21T08:38:23Z

This may be unsafe. AFAIK there is no guarantee about the monotonicity of Math.pow(x, y) as y moves from Math.pow(x, nextAfter(0.5, -1) to Math.pow(x, 0.5) to Math.pow(x, nextAfter(0.5, 1) for all x. Did anyone look?

Hi @theRealAph ,

I didn't get your point.

Could you please make it more clearer and give us an example?
Thanks.

theRealAph · 2021-04-21T08:44:15Z

This may be unsafe. AFAIK there is no guarantee about the monotonicity of Math.pow(x, y) as y moves from Math.pow(x, nextAfter(0.5, -1) to Math.pow(x, 0.5) to Math.pow(x, nextAfter(0.5, 1) for all x. Did anyone look?

Thanks @theRealAph for your review.

What did you mean by Math.pow(x, nextAfter(0.5, -1) and Math.pow(x, nextAfter(0.5, 1) ?

From the spec, "errors are required to be semi-monotonic: whenever the mathematical function is non-decreasing, so is the floating-point approximation, likewise, whenever the mathematical function is non-increasing, so is the floating-point approximation."

nextAfter "Returns the floating-point number adjacent to the first argument in the direction of the second argument."

Using sqrt() rather than pow() for pow(x,0.5) will return different results for some arguments. For this optimization you're proposing to be safe, you need to prove that in such cases the "Results must be semi-monotonic" requirement is still satisfied.

There may exist values of pow(x,0.5) where, if we substitute sqrt() this requirement is not met. I don't immediately know how to prove this does not happen. I think that to do so would require careful analysis.

DamonFool · 2021-04-21T09:10:08Z

This may be unsafe. AFAIK there is no guarantee about the monotonicity of Math.pow(x, y) as y moves from Math.pow(x, nextAfter(0.5, -1) to Math.pow(x, 0.5) to Math.pow(x, nextAfter(0.5, 1) for all x. Did anyone look?

Thanks @theRealAph for your review.
What did you mean by Math.pow(x, nextAfter(0.5, -1) and Math.pow(x, nextAfter(0.5, 1) ?

From the spec, "errors are required to be semi-monotonic: whenever the mathematical function is non-decreasing, so is the floating-point approximation, likewise, whenever the mathematical function is non-increasing, so is the floating-point approximation."

nextAfter "Returns the floating-point number adjacent to the first argument in the direction of the second argument."

Using sqrt() rather than pow() for pow(x,0.5) will return different results for some arguments. For this optimization you're proposing to be safe, you need to prove that in such cases the "Results must be semi-monotonic" requirement is still satisfied.

There may exist values of pow(x,0.5) where, if we substitute sqrt() this requirement is not met. I don't immediately know how to prove this does not happen. I think that to do so would require careful analysis.

Hi @theRealAph ,

Thanks for your clarification.

Actually, this optimization has been long used for Java.
And we don't see any problems of it, right?
So I don't think it will cause problems in the future.

What do you think of the same optimization in the HotSpot which has been applied more than 10 years ago?
https://github.com/openjdk/jdk/blob/master/src/hotspot/share/runtime/sharedRuntimeTrans.cpp#L499

And also the same optimization in the Java lib?

jdk/src/java.base/share/classes/java/lang/FdLibm.java

Line 366 in d84a7e5

return Math.sqrt(x + 0.0); // Add 0.0 to properly handle x == -0.0

Thanks.

theRealAph · 2021-04-21T09:46:18Z

Actually, this optimization has been long used for Java.
And we don't see any problems of it, right?
So I don't think it will cause problems in the future.

What do you think of the same optimization in the HotSpot which has been applied more than 10 years ago?
https://github.com/openjdk/jdk/blob/master/src/hotspot/share/runtime/sharedRuntimeTrans.cpp#L499

And also the same optimization in the Java lib?

OK, fair enough. I still don't know if this optimization is safe, but I guess (?) the analysis was done at the time. In any case, given that this optimization is already done elsewhere, your patch won't make anything any worse.

DamonFool · 2021-04-21T10:09:23Z

/integrate

openjdk · 2021-04-21T10:11:09Z

@DamonFool Since your change was applied there have been 45 commits pushed to the master branch:

7146104: 8265421: java/lang/String/StringRepeat.java test is missing a memory requirement
b5c92ca: 8265106: IGV: Enforce en-US locale while parsing ideal graph
3de0dcb: 8265483: All-caps “JAVA” in the top navigation bar
739769c: 8265411: Avoid unnecessary Method::init_intrinsic_id calls
a22ad03: 8264983: Add gtest for JDK-8264008
91b08b7: 8261779: JCK test api/javax_crypto/EncryptedPrivateKeyInfo/Ctor4.html is failing with assertion error when assertions enabled
2fcd920: 8261183: Follow on to Make lists of normal filenames
40ef00c: 8258457: testlibrary_tests/ctw/JarDirTest.java fails with InvalidPathException on windows
3f0da35: 8261392: Exclude testlibrary_tests/ctw/JarDirTest.java
8d0faaf: 8257001: Improve Http Client Support
... and 35 more: https://git.openjdk.java.net/jdk/compare/b64a3fb946a2855c612458aca9d62ef008f8f9be...master

Your commit was automatically rebased without conflicts.

Pushed as commit ed477da.

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

DamonFool · 2021-04-21T10:11:10Z

Actually, this optimization has been long used for Java.
And we don't see any problems of it, right?
So I don't think it will cause problems in the future.
What do you think of the same optimization in the HotSpot which has been applied more than 10 years ago?
https://github.com/openjdk/jdk/blob/master/src/hotspot/share/runtime/sharedRuntimeTrans.cpp#L499
And also the same optimization in the Java lib?

OK, fair enough. I still don't know if this optimization is safe, but I guess (?) the analysis was done at the time. In any case, given that this optimization is already done elsewhere, your patch won't make anything any worse.

Thanks @theRealAph .

And thank you all again.
Much has been learned while discussing with you.

Best regards,
Jie

8264945: Optimize the code-gen for Math.pow(x, 0.5)

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DamonFool added 2 commits April 19, 2021 18:59

Revert old changes

b902506

Merge branch 'master' into JDK-8264945

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Optimize C2

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8264945: Optimize the code-gen for Math.pow(x, 0.5) #3404

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8264945: Optimize the code-gen for Math.pow(x, 0.5) #3404

8264945: Optimize the code-gen for Math.pow(x, 0.5) #3404

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