8359386: Fix incorrect value for max_size of C2CodeStub when APX is used #25787

vamsi-parasa · 2025-06-12T19:41:01Z

The goal of this PR is to fix the value of max_size of the C2CodeStub hardcoded in the C2_MacroAssembler::convertF2I() function when Intel APX instrucitons are used. Currently, max_size is hardcoded to 23 (introduced in JDK-8306706) . However, this value is incorrect when Intel APX instructions with extended general-purpose registers (EGPRs) are used in the code stub as using EGPRs with APX instructions leads to an increase in the instruction encoding size by additional 1 byte.

Without this fix, we see the following error for the C2 compiler tests below:

compiler/vectorization/runner/ArrayTypeConvertTest.java
compiler/intrinsics/zip/TestFpRegsABI.java

# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/src/hotspot/share/opto/c2_CodeStubs.cpp:50), pid=3961123, tid=3961332
# assert(max_size >= actual_size) failed: Expected stub size (23) must be larger than or equal to actual stub size (24)
#
# JRE version: OpenJDK Runtime Environment (26.0) (fastdebug build 26-internal-adhoc.parasa.jdkdemotion)
# Java VM: OpenJDK 64-Bit Server VM (fastdebug 26-internal-adhoc.parasa.jdkdemotion, mixed mode, sharing, compressed oops, compressed class ptrs, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x955a77] C2CodeStubList::emit(C2_MacroAssembler&)+0x227
#

This PR fixes the errors in the above-mentioned tests.

Currently, the ConvertF2I macro works as follows:

vcvttss2si %xmm1,%eax
    cmp    $0x80000000,%eax
    je     STUB
CONTINUE:

STUB:
    sub    $0x8,%rsp
    vmovss %xmm1,(%rsp)
    call   Stub::f2i_fixup              ;   {runtime_call StubRoutines (initial stubs)}
    pop    %rax
    jmp    CONTINUE

The maximum size (max_size) of the stub is precomputed as 23. However, as seen in the convertF2I_slowpath implementation (below), the usage of pop(dst) instruction increases the instruction encoding size by 1 byte if dst is an extended general-purpose register (R16-R31) .

For example, pop (r15) is encoded as 41 5f, whereas pop(r21) is encoded as d5 10 5d.

static void convertF2I_slowpath(C2_MacroAssembler& masm, C2GeneralStub<Register, XMMRegister, address>& stub) {
#define __ masm.
  Register dst = stub.data<0>();
  XMMRegister src = stub.data<1>();
  address target = stub.data<2>();
  __ bind(stub.entry());
  __ subptr(rsp, 8);
  __ movdbl(Address(rsp), src);
  __ call(RuntimeAddress(target));
  __ pop(dst); // <-------- APX REX2 encoding for pop(dst) increases the stub size by 1 byte.
  __ jmp(stub.continuation());
#undef __
}

This PR fixes the issue with max_size hardcoding by adding 1 byte to the max_size when APX is used.

Progress

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Change must not contain extraneous whitespace
Commit message must refer to an issue

Issue

JDK-8359386: Fix incorrect value for max_size of C2CodeStub when APX is used (Bug - P3)(⚠️ The fixVersion in this issue is [25] but the fixVersion in .jcheck/conf is 26, a new backport will be created when this pr is integrated.)

Reviewers

Sandhya Viswanathan (@sviswa7 - Reviewer) Review applies to 0eca30db
Tobias Hartmann (@TobiHartmann - Reviewer)
Aleksey Shipilev (@shipilev - Reviewer)
Jatin Bhateja (@jatin-bhateja - Reviewer)

Reviewing

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Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/25787/head:pull/25787
$ git checkout pull/25787

Update a local copy of the PR:
$ git checkout pull/25787
$ git pull https://git.openjdk.org/jdk.git pull/25787/head

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$ git pr checkout 25787

View PR using the GUI difftool:
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https://git.openjdk.org/jdk/pull/25787.diff

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bridgekeeper · 2025-06-12T19:41:55Z

👋 Welcome back sparasa! 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 · 2025-06-12T19:42:05Z

@vamsi-parasa 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:

8359386: Fix incorrect value for max_size of C2CodeStub when APX is used

Reviewed-by: thartmann, shade, jbhateja, sviswanathan

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 70 new commits pushed to the master branch:

984d7f9: 8359895: JFR: method-timing view doesn't work
42d3604: 8359923: Const accessors for the Deferred class
cf78925: 8358891: Remove the PerfDataSamplingIntervalFunc code
... and 67 more: https://git.openjdk.org/jdk/compare/99829950f680936489048a1ad4ce12f166805e4d...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.

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openjdk · 2025-06-12T19:42:43Z

@vamsi-parasa The following label will be automatically applied to this pull request:

hotspot-compiler

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

mlbridge · 2025-06-12T19:46:38Z

Webrevs

sviswa7

Looks good to me.

shipilev · 2025-06-13T09:44:14Z

src/hotspot/cpu/x86/c2_MacroAssembler_x86.cpp

@@ -4687,7 +4687,9 @@ void C2_MacroAssembler::convertF2I(BasicType dst_bt, BasicType src_bt, Register
    }
  }

-  auto stub = C2CodeStub::make<Register, XMMRegister, address>(dst, src, slowpath_target, 23, convertF2I_slowpath);
+  // Using the APX extended general purpose registers increases the instruction encoding size by 4 bytes.
+  int max_size = dst->encoding() <= 15 ? 23 : 27;


Do you want to write it in a more explicit way then?

int max_size = 23 + (UseAPX ? 4 : 0);

Thank you for the suggestion! Please see suggested change incorporated in the updated code.

vamsi-parasa · 2025-06-17T04:22:34Z

Please see the updated version which fixes the error in the initial PR. While it's true that the size of the convert with truncation and the compare instruction increases by 4 bytes in the main common path, it does not contribute to the increase in the stub size. As explained in the updated PR description, the increase in size by 1 byte is being caused by pop(dst) instruction in the stub.

vamsi-parasa · 2025-06-17T04:24:21Z

Hello Jatin (@jatin-bhateja),

Please see the updated PR description which fixed the error in the initial PR.

Thanks,
Vamsi

TobiHartmann

Looks good to me.

jatin-bhateja

My testing passed.

LGTM

vamsi-parasa · 2025-06-18T14:47:13Z

Thank you Tobias (@TobiHartmann), Aleksey (@shipilev) and Jatin (@jatin-bhateja) for approving the changes!

vamsi-parasa · 2025-06-18T14:47:23Z

/integrate

openjdk · 2025-06-18T14:48:46Z

@vamsi-parasa
Your change (at version 4fe56be) is now ready to be sponsored by a Committer.

sviswa7 · 2025-06-18T18:29:23Z

/sponsor

openjdk · 2025-06-18T18:30:07Z

Going to push as commit b52af18.
Since your change was applied there have been 72 commits pushed to the master branch:

7bc0d82: 8359227: Code cache/heap size options should be size_t
b7fcd0b: 8359889: java/awt/MenuItem/SetLabelTest.java inadvertently triggers clicks on items pinned to the taskbar
984d7f9: 8359895: JFR: method-timing view doesn't work
... and 69 more: https://git.openjdk.org/jdk/compare/99829950f680936489048a1ad4ce12f166805e4d...master

Your commit was automatically rebased without conflicts.

openjdk · 2025-06-18T18:30:16Z

@sviswa7 @vamsi-parasa Pushed as commit b52af18.

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

8359386: Fix incorrect value for max_size of C2CodeStub when APX is used

0eca30d

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update max_size computation to be explicit

37dede6

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vamsi-parasa mentioned this pull request Jun 16, 2025

8306706: Support out-of-line code generation for MachNodes #13602

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Fix the change in the stub size by 1 byte

4fe56be

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8359386: Fix incorrect value for max_size of C2CodeStub when APX is used #25787

8359386: Fix incorrect value for max_size of C2CodeStub when APX is used #25787

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