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8265917: Different values computed by C2 and interpreter/C1 for Math.pow(x, 2.0) on x86_32 #3677

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@DamonFool DamonFool commented Apr 25, 2021

Hi all,

C2 may produce different results for Math.pow(x, 2.0) compared with interpreter/C1 on x86_32.

E.g., for Math.pow(1.0 / 2047, 2.0):

interpreter: 2.38651580386563E-7 
         C2: 2.3865158038656307E-7

The reason is that C2 will replace Math.pow(x, 2.0) with x*x.
However, there is no such optimization for interpreter/C1 on x86_32.

The fix just disables C2's opt for Math.pow(x, 2.0) on x86_32 since nobody (or very few people) would run x86_32.
And we don't have a plan to implement such opt on x86_32.

Another reason to fix this bug is that we need this patch to extend C2's opt for Math.pow(x, 0.5) on other platforms.

Thanks.
Best regards,
Jie


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  • JDK-8265917: Different values computed by C2 and interpreter/C1 for Math.pow(x, 2.0) on x86_32

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👋 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.

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@DamonFool DamonFool commented Apr 25, 2021

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

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@DamonFool The hotspot-compiler label was already applied.

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@mlbridge mlbridge bot commented Apr 25, 2021

Webrevs

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@vnkozlov vnkozlov commented Apr 25, 2021

I think you should fix 32-bit version of dpow stub because 64-bit stub has this optimization:
https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/x86/macroAssembler_x86_pow.cpp#L828

Also add pow(x, 0.5) as well.

C dpow code in VM has this optimization which works in 32- and in 64-bit when stub is not used (for example, on other platforms):
https://github.com/openjdk/jdk/blob/master/src/hotspot/share/runtime/sharedRuntimeTrans.cpp#L496

Java lib code also has this optimization:
https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/lang/FdLibm.java#L362

Only 32-bit x86 stub is missing it:
https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/x86/macroAssembler_x86_pow.cpp#L2544

The stub is generated only on x86:

$ grep -r StubRoutines::_dpow src/hotspot/cpu/
src/hotspot/cpu//x86/stubGenerator_x86_64.cpp:        StubRoutines::_dpow = generate_libmPow();
src/hotspot/cpu//x86/stubGenerator_x86_32.cpp:        StubRoutines::_dpow = generate_libmPow();

I suggest to fix it to be consistent with the rest of VM and Java lib code.

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@DamonFool DamonFool commented Apr 26, 2021

I think you should fix 32-bit version of dpow stub because 64-bit stub has this optimization:
https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/x86/macroAssembler_x86_pow.cpp#L828

Also add pow(x, 0.5) as well.

C dpow code in VM has this optimization which works in 32- and in 64-bit when stub is not used (for example, on other platforms):
https://github.com/openjdk/jdk/blob/master/src/hotspot/share/runtime/sharedRuntimeTrans.cpp#L496

Java lib code also has this optimization:
https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/lang/FdLibm.java#L362

Only 32-bit x86 stub is missing it:
https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/x86/macroAssembler_x86_pow.cpp#L2544

The stub is generated only on x86:

$ grep -r StubRoutines::_dpow src/hotspot/cpu/
src/hotspot/cpu//x86/stubGenerator_x86_64.cpp:        StubRoutines::_dpow = generate_libmPow();
src/hotspot/cpu//x86/stubGenerator_x86_32.cpp:        StubRoutines::_dpow = generate_libmPow();

I suggest to fix it to be consistent with the rest of VM and Java lib code.

Hi @vnkozlov ,

Math.pow(x, 2.0) has been optimized for x86_32.
As for the Math.pow(x, 0.5), it will be handled in JDK-8265940 after this issue.

Thanks.

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@DamonFool DamonFool commented Apr 26, 2021

Testing:
-tier1 ~ tier3 on Linux/x86_32, no regression

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@vnkozlov vnkozlov left a comment

Good.

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@vnkozlov vnkozlov commented Apr 26, 2021

As for the Math.pow(x, 0.5), it will be handled in JDK-8265940 after this issue.

Okay.

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@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:

8265917: Different values computed by C2 and interpreter/C1 for Math.pow(x, 2.0) on x86_32

Reviewed-by: kvn, thartmann

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 58 new commits pushed to the master branch:

  • e144104: 8262908: JFR: Allow JFR to stream events from a known repository path
  • 30b1354: 8266153: mark hotspot compiler/onSpinWait tests which ignore VM flags
  • eeddb30: 8266150: mark hotspot compiler/arguments tests which ignore VM flags
  • feb18d2: 8266149: mark hotspot compiler/startup tests which ignore VM flags
  • eb72950: 8266154: mark hotspot compiler/oracle tests which ignore VM flags
  • 7f4a9f6: 8266088: compiler/arguments/TestPrintOptoAssemblyLineNumbers test should user driver mode
  • f560b89: 8264873: Dependencies: Split ClassHierarchyWalker
  • 0a88f0a: 8255915: jdk/incubator/vector/AddTest.java timed out
  • ab2aec2: 8265938: C2's conditional move optimization does not handle top Phi
  • 5634f20: 8265932: Move safepoint related fields from class Thread to JavaThread
  • ... and 48 more: https://git.openjdk.java.net/jdk/compare/f1f2afda5a0ab8f213e8a1b5324a251928c8d81a...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this 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.

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@openjdk openjdk bot added the ready label Apr 26, 2021
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@DamonFool DamonFool commented Apr 26, 2021

Good.

Thanks @vnkozlov for your review.

Let's wait for a second review of this change.
Thanks.

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assert_different_registers(tmp, eax, ecx, edx);

address static_const_table_pow = (address)_static_const_table_pow;
static double DOUBLE2 = 2.0;
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@TobiHartmann TobiHartmann Apr 27, 2021

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Constants in the 64-bit code and _static_const_table_pow above use ATTRIBUTE_ALIGNED. Should we add that for DOUBLE2 as well?

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Hi @TobiHartmann ,

Thanks for your review.

Yes, it seem better to be with ATTRIBUTE_ALIGNED.
Updated.

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bind(start);
subl(rsp, 120);
movl(Address(rsp, 64), tmp);
lea(tmp, ExternalAddress(static_const_table_pow));
movsd(xmm0, Address(rsp, 128));
movsd(xmm1, Address(rsp, 136));

// Special case: pow(x, 2.0) => x * x
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64-bit code also handles Special case: pow(x, 0.5) => sqrt(x). Don't we need to add that as well for consistency?

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64-bit code also handles Special case: pow(x, 0.5) => sqrt(x). Don't we need to add that as well for consistency?

This is the bug fix for C2's optimization of pow(x, 2.0).

pow(x, 0.5) will be handled in JDK-8265940 .
Thanks.

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@TobiHartmann TobiHartmann left a comment

Looks good to me.

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@DamonFool DamonFool commented Apr 27, 2021

Looks good to me.

Thanks @TobiHartmann .

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

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@vnkozlov vnkozlov left a comment

Latest version with alignment is good.

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@DamonFool DamonFool commented Apr 28, 2021

Latest version with alignment is good.

Thanks @vnkozlov .
/integrate

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@openjdk openjdk bot closed this Apr 28, 2021
@openjdk openjdk bot added integrated and removed ready rfr labels Apr 28, 2021
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@openjdk openjdk bot commented Apr 28, 2021

@DamonFool Since your change was applied there have been 58 commits pushed to the master branch:

  • e144104: 8262908: JFR: Allow JFR to stream events from a known repository path
  • 30b1354: 8266153: mark hotspot compiler/onSpinWait tests which ignore VM flags
  • eeddb30: 8266150: mark hotspot compiler/arguments tests which ignore VM flags
  • feb18d2: 8266149: mark hotspot compiler/startup tests which ignore VM flags
  • eb72950: 8266154: mark hotspot compiler/oracle tests which ignore VM flags
  • 7f4a9f6: 8266088: compiler/arguments/TestPrintOptoAssemblyLineNumbers test should user driver mode
  • f560b89: 8264873: Dependencies: Split ClassHierarchyWalker
  • 0a88f0a: 8255915: jdk/incubator/vector/AddTest.java timed out
  • ab2aec2: 8265938: C2's conditional move optimization does not handle top Phi
  • 5634f20: 8265932: Move safepoint related fields from class Thread to JavaThread
  • ... and 48 more: https://git.openjdk.java.net/jdk/compare/f1f2afda5a0ab8f213e8a1b5324a251928c8d81a...master

Your commit was automatically rebased without conflicts.

Pushed as commit 889d246.

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

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@DamonFool DamonFool deleted the JDK-8265917 branch Apr 28, 2021
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