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Eliminate redundant refcounting in the JIT #134584

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Eliminate redundant refcounting in the JIT#134584
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corona10Fidget-Spinner
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interpreter-core(Objects, Python, Grammar, and Parser dirs)performancePerformance or resource usagetopic-JITtype-featureA feature request or enhancement

Description

@Fidget-Spinner
Fidget-Spinner
opened on May 23, 2025

Feature or enhancement

Proposal:

Thanks to Matt's work on borrowed LOAD_FAST, we can now eliminate reference counting trivially in the JIT.

Reference counting is expensive, Matt found that eliminating 90% of refcounts in LOAD_FAST meant a 2-3% speedup in the interpreter. So the speedup for the JIT should be quite a bit too.

The other problem is that reference counts block register allocation/TOS caching. As they force spills to the stack more often.

This issue has the potential to speedup up JIT benchmarks by several percent.

How to contribute.

Now that reference tracking in the JIT optimizer is in place, the first thing we need to do is to convert ops to make the decref an explicit uop.

For escaping decref ops (ie, ops that decref could run the GC), we would need to refactor them so their decrefs are eliminated via specialization of pops. For example: the following op (which is not an escaping op, but just purely for demonstration!):

macro(BINARY_OP_ADD_INT) =
_GUARD_TOS_INT + _GUARD_NOS_INT + unused/5 + _BINARY_OP_ADD_INT;

becomes

macro(BINARY_OP_ADD_INT) =
_GUARD_TOS_INT + _GUARD_NOS_INT + unused/5 + _BINARY_OP_ADD_INT + _POP_TOP_INT + _POP_TOP_INT;

Previously _BINARY_OP_ADD_INT's stack effect looked like this: (left, right -- res). The new version should look like (left, right -- res, left, right).
So for all the decref specializations, we would just need a _POP_X of their variants! This means no explosion of uop and their decref variants. We just specialize _POP_X to _POP_X_NO_DECREF in the JIT. Keeping things clean

These are open for contributors to take:

Once that's done, we can think about further ops.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

Activity

  1. Fidget-Spinner
    added
    type-featureA feature request or enhancement
    performancePerformance or resource usage
    interpreter-core(Objects, Python, Grammar, and Parser dirs)
    topic-JIT
    on May 23, 2025

  2. Fidget-Spinner commented on May 23, 2025

    @Fidget-Spinner
    Fidget-Spinner
    on May 23, 2025
    MemberAuthor

    @tomasr8 @brandtbucher this is gonna be a pretty sweet optimization :)
    Before:

        // 
    // _BINARY_OP_ADD_INT.o:  file format elf64-x86-64
    // 
    // Disassembly of section .text:
    // 
    // 0000000000000000 <_JIT_ENTRY>:
    // 0: 55                            pushq   %rbp
    // 1: 4d 8b 7d f0                   movq    -0x10(%r13), %r15
    // 5: 49 8b 6d f8                   movq    -0x8(%r13), %rbp
    // 9: 4c 89 ff                      movq    %r15, %rdi
    // c: 48 83 e7 fe                   andq    $-0x2, %rdi
    // 10: 48 89 ee                      movq    %rbp, %rsi
    // 13: 48 83 e6 fe                   andq    $-0x2, %rsi
    // 17: 4d 89 6c 24 40                movq    %r13, 0x40(%r12)
    // 1c: ff 15 00 00 00 00             callq   *(%rip)                 # 0x22 <_JIT_ENTRY+0x22>
    // 000000000000001e:  R_X86_64_GOTPCRELX   _PyLong_Add-0x4
    // 22: 48 89 c3                      movq    %rax, %rbx
    // 25: 4d 8b 6c 24 40                movq    0x40(%r12), %r13
    // 2a: 40 f6 c5 01                   testb   $0x1, %bpl
    // 2e: 75 05                         jne     0x35 <_JIT_ENTRY+0x35>
    // 30: ff 4d 00                      decl    (%rbp)
    // 33: 74 3f                         je      0x74 <_JIT_ENTRY+0x74>
    // 35: 41 f6 c7 01                   testb   $0x1, %r15b
    // 39: 75 71                         jne     0xac <_JIT_ENTRY+0xac>
    // 3b: 41 ff 0f                      decl    (%r15)
    // 3e: 75 6c                         jne     0xac <_JIT_ENTRY+0xac>
    // 40: 48 b8 00 00 00 00 00 00 00 00 movabsq $0x0, %rax
    // 0000000000000042:  R_X86_64_64  _PyRuntime+0x28e0
    // 4a: 48 8b 00                      movq    (%rax), %rax
    // 4d: 48 85 c0                      testq   %rax, %rax
    // 50: 74 17                         je      0x69 <_JIT_ENTRY+0x69>
    // 52: 48 b9 00 00 00 00 00 00 00 00 movabsq $0x0, %rcx
    // 0000000000000054:  R_X86_64_64  _PyRuntime+0x28e8
    // 5c: 48 8b 11                      movq    (%rcx), %rdx
    // 5f: 4c 89 ff                      movq    %r15, %rdi
    // 62: be 01 00 00 00                movl    $0x1, %esi
    // 67: ff d0                         callq   *%rax
    // 69: 4c 89 ff                      movq    %r15, %rdi
    // 6c: ff 15 00 00 00 00             callq   *(%rip)                 # 0x72 <_JIT_ENTRY+0x72>
    // 000000000000006e:  R_X86_64_GOTPCRELX   _PyLong_ExactDealloc-0x4
    // 72: eb 38                         jmp     0xac <_JIT_ENTRY+0xac>
    // 74: 48 b8 00 00 00 00 00 00 00 00 movabsq $0x0, %rax
    // 0000000000000076:  R_X86_64_64  _PyRuntime+0x28e0
    // 7e: 48 8b 00                      movq    (%rax), %rax
    // 81: 48 85 c0                      testq   %rax, %rax
    // 84: 74 17                         je      0x9d <_JIT_ENTRY+0x9d>
    // 86: 48 b9 00 00 00 00 00 00 00 00 movabsq $0x0, %rcx
    // 0000000000000088:  R_X86_64_64  _PyRuntime+0x28e8
    // 90: 48 8b 11                      movq    (%rcx), %rdx
    // 93: 48 89 ef                      movq    %rbp, %rdi
    // 96: be 01 00 00 00                movl    $0x1, %esi
    // 9b: ff d0                         callq   *%rax
    // 9d: 48 89 ef                      movq    %rbp, %rdi
    // a0: ff 15 00 00 00 00             callq   *(%rip)                 # 0xa6 <_JIT_ENTRY+0xa6>
    // 00000000000000a2:  R_X86_64_GOTPCRELX   _PyLong_ExactDealloc-0x4
    // a6: 41 f6 c7 01                   testb   $0x1, %r15b
    // aa: 74 8f                         je      0x3b <_JIT_ENTRY+0x3b>
    // ac: 48 85 db                      testq   %rbx, %rbx
    // af: 74 18                         je      0xc9 <_JIT_ENTRY+0xc9>
    // b1: 0f b7 43 06                   movzwl  0x6(%rbx), %eax
    // b5: 83 e0 01                      andl    $0x1, %eax
    // b8: 48 09 d8                      orq     %rbx, %rax
    // bb: 49 89 45 f0                   movq    %rax, -0x10(%r13)
    // bf: 49 83 c5 f8                   addq    $-0x8, %r13
    // c3: 5d                            popq    %rbp
    // c4: e9 00 00 00 00                jmp     0xc9 <_JIT_ENTRY+0xc9>
    // 00000000000000c5:  R_X86_64_PLT32       _JIT_CONTINUE-0x4
    // c9: 49 83 c5 f0                   addq    $-0x10, %r13
    // cd: 5d                            popq    %rbp
    // ce: e9 00 00 00 00                jmp     0xd3 <_JIT_ENTRY+0xd3>
    // 00000000000000cf:  R_X86_64_PLT32       _JIT_ERROR_TARGET-0x4

    After:

        // 
    // _BINARY_OP_ADD_INT__NO_INPUT_DECREF.o: file format elf64-x86-64
    // 
    // Disassembly of section .text:
    // 
    // 0000000000000000 <_JIT_ENTRY>:
    // 0: 50                            pushq   %rax
    // 1: 49 8b 7d f0                   movq    -0x10(%r13), %rdi
    // 5: 49 8b 75 f8                   movq    -0x8(%r13), %rsi
    // 9: 48 83 e7 fe                   andq    $-0x2, %rdi
    // d: 48 83 e6 fe                   andq    $-0x2, %rsi
    // 11: 4d 89 6c 24 40                movq    %r13, 0x40(%r12)
    // 16: ff 15 00 00 00 00             callq   *(%rip)                 # 0x1c <_JIT_ENTRY+0x1c>
    // 0000000000000018:  R_X86_64_GOTPCRELX   _PyLong_Add-0x4
    // 1c: 4d 8b 6c 24 40                movq    0x40(%r12), %r13
    // 21: 48 85 c0                      testq   %rax, %rax
    // 24: 74 18                         je      0x3e <_JIT_ENTRY+0x3e>
    // 26: 0f b7 48 06                   movzwl  0x6(%rax), %ecx
    // 2a: 83 e1 01                      andl    $0x1, %ecx
    // 2d: 48 09 c1                      orq     %rax, %rcx
    // 30: 49 89 4d f0                   movq    %rcx, -0x10(%r13)
    // 34: 49 83 c5 f8                   addq    $-0x8, %r13
    // 38: 58                            popq    %rax
    // 39: e9 00 00 00 00                jmp     0x3e <_JIT_ENTRY+0x3e>
    // 000000000000003a:  R_X86_64_PLT32       _JIT_CONTINUE-0x4
    // 3e: 49 83 c5 f0                   addq    $-0x10, %r13
    // 42: 58                            popq    %rax
    // 43: e9 00 00 00 00                jmp     0x48 <_JIT_ENTRY+0x48>
    // 0000000000000044:  R_X86_64_PLT32       _JIT_ERROR_TARGET-0x4
  3. Fidget-Spinner
    added a commit that references this issue on Jun 17, 2025

    gh-134584: Decref elimination for float ops in the JIT (GH-134588)

    fba5dde
  4. lkollar
    added a commit that references this issue on Jun 19, 2025

    pythongh-134584: Decref elimination for float ops in the JIT (pythonG…

    4c5dbeb
  5. corona10
    assigned
    corona10
    and
    Fidget-Spinner
    on Jun 22, 2025
  6. corona10
    added a commit that references this issue on Jun 22, 2025

    pythongh-134584: Elimiate redundant refcounting from _BINARY_OP_X_UNI…

    d5afe15
  7. Fidget-Spinner
    added a commit that references this issue on Jun 23, 2025

    gh-134584: Specialize POP_TOP by reference and type in JIT (GH-135761)

    569fc68
  8. corona10
    added a commit that references this issue on Jun 23, 2025

    Merge remote-tracking branch 'upstream/main' into pythongh-134584-uni…

    d95aee9

  9. Zheaoli commented on Jun 23, 2025

    @Zheaoli
    Zheaoli
    on Jun 23, 2025
    Contributor

    FYI here #131798 (comment)

    100% want to take a part into this issue. BTW I may need a little bit time to read the whole context and the previous PR made by others.

    So maybe I will send the first PR in this weekend. If this is acceptable, 100% want to join into this

  10. Fidget-Spinner commented on Jun 23, 2025

    @Fidget-Spinner
    Fidget-Spinner
    on Jun 23, 2025
    MemberAuthor

    @Zheaoli and anyone else reading this thread who wants to contribute. No worries, I'm happy to explain and answer any questions over email or video if you want to. My email is on my GH profile.

  11. Zheaoli commented on Jun 23, 2025

    @Zheaoli
    Zheaoli
    on Jun 23, 2025
    Contributor

    No worries, I'm happy to explain and answer any questions over email or video if you want to.

    Just for joking: This is 100% facts(lol. @Fidget-Spinner is a supper nice guy(lol

  12. 127 remaining items

  13. thunder-coding
    added 5 commits that reference this issue on Feb 15, 2026

    pythongh-134584: JIT: Remove redundant refcount for _BINARY_OP_SUBSCR…

    6595e71

    pythongh-134584: Eliminate redundant refcounting from `_CONTAINS_{OP|…

    6da1b3a

    pythongh-134584: Eliminate redundant refcounting in TO_BOOL_INT, TO_B…

    90c67cd

    pythongh-134584: Add more contributors for JIT refcounting work (pyth…

    66ee370

    pythongh-134584: Optimize `_BINARY_OP_SUBSCR_LIST_SLICE` (pythonGH-14…

    ab3b7fe
  14. brijkapadia
    added a commit that references this issue on Feb 28, 2026

    pythongh-134584: Optimize `_BINARY_OP_SUBSCR_LIST_SLICE` (pythonGH-14…

    7f4312b
  15. Fidget-Spinner
    added a commit that references this issue on Mar 14, 2026

    gh-134584: Eliminate redundant refcounting in JIT for `MATCH_CLASS` (G…

    798070d
  16. Fidget-Spinner
    added a commit that references this issue on Apr 9, 2026

    gh-134584 : Optimize and eliminate redundant ref-counting for `MAKE_F…

    38d3aef
  17. ljfp
    added 3 commits that reference this issue on Apr 25, 2026

    pythongh-134584: Optimize `_BINARY_OP_SUBSCR_LIST_SLICE` (pythonGH-14…

    5afbfef

    pythongh-134584: Eliminate redundant refcounting in JIT for `MATCH_CL…

    525c670

    pythongh-134584 : Optimize and eliminate redundant ref-counting for `…

    b6dbcbb
  18. eendebakpt
    added 4 commits that reference this issue on May 22, 2026

    gh-NNNNN: Make _STORE_ATTR_SLOT lock-free on the free-threaded build

    73d3dc3

    gh-NNNNN: Make _STORE_ATTR_SLOT lock-free on the free-threaded build

    2cf7c01

    gh-NNNNN: Make _STORE_ATTR_SLOT lock-free on the free-threaded build

    17cca5e

    gh-NNNNN: Make _STORE_ATTR_SLOT lock-free on the free-threaded build

    e72e080
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