GeezerVison

The World Wide Web, restored to 800 pixels of glory.

GeezerVison is showing github.com — GH-115802: Optimize JIT stencils for size by brandtbucher · Pull Request #136393 · python/cpython

Conversation

brandtbucher commented Jul 7, 2025 •

edited by bedevere-app Bot

Loading

Uh oh!

There was an error while loading. Please reload this page.

Copy link

Copy Markdown

Member

As the new comment says, upon manual review of -O3, -O2, and -Os, it seems that -Os generates the best code for the JIT's use-case. Perf impact is close to noise, but slightly positive on x86-64 Linux and AArch64 macOS, neutral on AArch64 Linux, and slightly negative on x86-64 Windows. According to the stats, the size of JIT code is down by about 1-2%: https://github.com/faster-cpython/benchmarking-public/blob/main/results/bm-20250628-3.15.0a0-33054dd-JIT/README.md

Here's an example of how skipping tail-duplication removes an extra jump and a duplicate instruction from _POP_TOP (also reducing its size by 19%):

-    // 11: 75 04                         jne     0x17 <_JIT_ENTRY+0x17>
+    // 11: 75 0f                         jne     0x22 <_JIT_ENTRY+0x22>
     // 13: ff 0f                         decl    (%rdi)
-    // 15: 74 07                         je      0x1e <_JIT_ENTRY+0x1e>
-    // 17: 4d 8b 6c 24 40                movq    0x40(%r12), %r13
-    // 1c: eb 10                         jmp     0x2e <_JIT_CONTINUE>
-    // 1e: 50                            pushq   %rax
-    // 1f: ff 15 00 00 00 00             callq   *(%rip)                 # 0x25 <_JIT_ENTRY+0x25>
-    // 0000000000000021:  R_X86_64_GOTPCRELX   _Py_Dealloc-0x4
-    // 25: 48 83 c4 08                   addq    $0x8, %rsp
-    // 29: 4d 8b 6c 24 40                movq    0x40(%r12), %r13
-    const unsigned char code_body[46] = {
+    // 15: 75 0b                         jne     0x22 <_JIT_ENTRY+0x22>
+    // 17: 50                            pushq   %rax
+    // 18: ff 15 00 00 00 00             callq   *(%rip)                 # 0x1e <_JIT_ENTRY+0x1e>
+    // 000000000000001a:  R_X86_64_GOTPCRELX   _Py_Dealloc-0x4
+    // 1e: 48 83 c4 08                   addq    $0x8, %rsp
+    // 22: 4d 8b 6c 24 40                movq    0x40(%r12), %r13
+    const unsigned char code_body[39] = {
         0x49, 0x8b, 0x7d, 0xf8, 0x49, 0x83, 0xc5, 0xf8,
         0x4d, 0x89, 0x6c, 0x24, 0x40, 0x40, 0xf6, 0xc7,
-        0x01, 0x75, 0x04, 0xff, 0x0f, 0x74, 0x07, 0x4d,
-        0x8b, 0x6c, 0x24, 0x40, 0xeb, 0x10, 0x50, 0xff,
-        0x15, 0x00, 0x00, 0x00, 0x00, 0x48, 0x83, 0xc4,
-        0x08, 0x4d, 0x8b, 0x6c, 0x24, 0x40,
+        0x01, 0x75, 0x0f, 0xff, 0x0f, 0x75, 0x0b, 0x50,
+        0xff, 0x15, 0x00, 0x00, 0x00, 0x00, 0x48, 0x83,
+        0xc4, 0x08, 0x4d, 0x8b, 0x6c, 0x24, 0x40,
     };

Full diff for the stencils here:

https://gist.github.com/brandtbucher/7340be56f2d2cf7061b5c9bf1c87939c

Issue: Improving JIT code quality #115802

Sorry, something went wrong.

Uh oh!

There was an error while loading. Please reload this page.

All reactions

brandtbucher and others added 2 commits

June 28, 2025 08:41

-Os

33054dd


          Add comment explaining -Os

dbca451

brandtbucher self-assigned this

Jul 7, 2025

brandtbucher requested a review from savannahostrowski as a code owner

July 7, 2025 18:52

brandtbucher added performance

Performance or resource usage

skip news interpreter-core

(Objects, Python, Grammar, and Parser dirs)

topic-JIT labels

Jul 7, 2025

bedevere-app Bot added the awaiting core review label

Jul 7, 2025

bedevere-app Bot mentioned this pull request

Jul 7, 2025

Improving JIT code quality #115802

Closed

8 of 13 tasks

savannahostrowski reviewed

Jul 8, 2025

View reviewed changes

Comment thread

Tools/jit/_targets.py

		f"-I{CPYTHON / 'Python'}",
		f"-I{CPYTHON / 'Tools' / 'jit'}",
		"-O3",
		# -O2 and -O3 include some optimizations that make sense for

savannahostrowski Jul 8, 2025

Copy link

Copy Markdown

Member

There was a problem hiding this comment.

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Did you investigate -Oz as well? The clang docs are fairly vague, but they say it reduces code size even further, so I'm curious if it's worth investigating as well.

Sorry, something went wrong.

Uh oh!

There was an error while loading. Please reload this page.

Fidget-Spinner reacted with thumbs up emoji

Fidget-Spinner reacted with eyes emoji

All reactions

👍 1 reaction
👀 1 reaction

brandtbucher Jul 9, 2025

Copy link

Copy Markdown

Member Author

There was a problem hiding this comment.

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Nice idea! I'm definitely down to try benchmarking it after this lands.

I suspect it may be quite a bit slower, though. My understanding is that -Os does all of the meaningful performance optimizations except those that increase size, while -Oz will actually hurt performance in pursuit of the smallest possible machine code. Our goal is to be fast, of course, but in this particular case -Os is also just giving us better code (as a side-effect of not aligning jumps or duplicating tails, etc). So smaller isn't necessarily always better.

Sorry, something went wrong.

Uh oh!

There was an error while loading. Please reload this page.

All reactions

brandtbucher Jul 9, 2025

Copy link

Copy Markdown

Member Author

There was a problem hiding this comment.

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Yeah, I'm not sure this is going to be a win. It basically turns off inlining for functions called more than once. For instance, _POP_TWO turns from this on -Os:

    // 0000000000000000 <_JIT_ENTRY>:
    // 0: 50                            pushq   %rax
    // 1: 49 8d 45 f8                   leaq    -0x8(%r13), %rax
    // 5: 49 8b 5d f0                   movq    -0x10(%r13), %rbx
    // 9: 49 8b 7d f8                   movq    -0x8(%r13), %rdi
    // d: 49 89 44 24 40                movq    %rax, 0x40(%r12)
    // 12: 40 f6 c7 01                   testb   $0x1, %dil
    // 16: 75 0a                         jne     0x22 <_JIT_ENTRY+0x22>
    // 18: ff 0f                         decl    (%rdi)
    // 1a: 75 06                         jne     0x22 <_JIT_ENTRY+0x22>
    // 1c: ff 15 00 00 00 00             callq   *(%rip)                 # 0x22 <_JIT_ENTRY+0x22>
    // 000000000000001e:  R_X86_64_GOTPCRELX   _Py_Dealloc-0x4
    // 22: 49 83 44 24 40 f8             addq    $-0x8, 0x40(%r12)
    // 28: f6 c3 01                      testb   $0x1, %bl
    // 2b: 75 0d                         jne     0x3a <_JIT_ENTRY+0x3a>
    // 2d: ff 0b                         decl    (%rbx)
    // 2f: 75 09                         jne     0x3a <_JIT_ENTRY+0x3a>
    // 31: 48 89 df                      movq    %rbx, %rdi
    // 34: ff 15 00 00 00 00             callq   *(%rip)                 # 0x3a <_JIT_ENTRY+0x3a>
    // 0000000000000036:  R_X86_64_GOTPCRELX   _Py_Dealloc-0x4
    // 3a: 4d 8b 6c 24 40                movq    0x40(%r12), %r13
    // 3f: 58                            popq    %rax

Into this on -Oz (outlining PyStackRef_CLOSE makes it 2 bytes shorter, but adds up to three additional jumps):

    // 0000000000000000 <_JIT_ENTRY>:
    // 0: 50                            pushq   %rax
    // 1: 49 8d 45 f8                   leaq    -0x8(%r13), %rax
    // 5: 49 8b 5d f0                   movq    -0x10(%r13), %rbx
    // 9: 49 8b 7d f8                   movq    -0x8(%r13), %rdi
    // d: 49 89 44 24 40                movq    %rax, 0x40(%r12)
    // 12: e8 16 00 00 00                callq   0x2d <PyStackRef_CLOSE>
    // 17: 49 83 44 24 40 f8             addq    $-0x8, 0x40(%r12)
    // 1d: 48 89 df                      movq    %rbx, %rdi
    // 20: e8 08 00 00 00                callq   0x2d <PyStackRef_CLOSE>
    // 25: 4d 8b 6c 24 40                movq    0x40(%r12), %r13
    // 2a: 58                            popq    %rax
    // 2b: eb 11                         jmp     0x3e <_JIT_CONTINUE>
    // 
    // 000000000000002d <PyStackRef_CLOSE>:
    // 2d: 40 f6 c7 01                   testb   $0x1, %dil
    // 31: 75 04                         jne     0x37 <PyStackRef_CLOSE+0xa>
    // 33: ff 0f                         decl    (%rdi)
    // 35: 74 01                         je      0x38 <PyStackRef_CLOSE+0xb>
    // 37: c3                            retq
    // 38: ff 25 00 00 00 00             jmpq    *(%rip)                 # 0x3e <_JIT_CONTINUE>
    // 000000000000003a:  R_X86_64_GOTPCRELX   _Py_Dealloc-0x4

I'll still try benchmarking it though. But I'll land this PR in the meantime since it's just a one-character change.

Sorry, something went wrong.

Uh oh!

There was an error while loading. Please reload this page.

All reactions

brandtbucher Jul 10, 2025

Copy link

Copy Markdown

Member Author

There was a problem hiding this comment.

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Yep, -Oz is about 1-2% slower across the board.

Sorry, something went wrong.

Uh oh!

There was an error while loading. Please reload this page.

All reactions

brandtbucher merged commit c49dc3b into python:main

Jul 9, 2025

bedevere-app Bot removed the awaiting core review label

Jul 9, 2025

AndPuQing pushed a commit to AndPuQing/cpython that referenced this pull request

Jul 11, 2025


          pythonGH-115802: Optimize JIT stencils for size (pythonGH-136393)

21b1d77

Pranjal095 pushed a commit to Pranjal095/cpython that referenced this pull request

Jul 12, 2025


          pythonGH-115802: Optimize JIT stencils for size (pythonGH-136393)

2e4718c

picnixz pushed a commit to picnixz/cpython that referenced this pull request

Jul 13, 2025


          pythonGH-115802: Optimize JIT stencils for size (pythonGH-136393)

02855b9

taegyunkim pushed a commit to taegyunkim/cpython that referenced this pull request

Aug 4, 2025


          pythonGH-115802: Optimize JIT stencils for size (pythonGH-136393)

46aadc8

Agent-Hellboy pushed a commit to Agent-Hellboy/cpython that referenced this pull request

Aug 19, 2025


          pythonGH-115802: Optimize JIT stencils for size (pythonGH-136393)

febc81b

Reviewers

savannahostrowski

savannahostrowski left review comments

Uh oh!

There was an error while loading. Please reload this page.

Assignees

brandtbucher

Labels

interpreter-core

(Objects, Python, Grammar, and Parser dirs)

performance

Performance or resource usage

skip news topic-JIT

Projects

None yet

Milestone

No milestone

Development

Successfully merging this pull request may close these issues.

Uh oh!

There was an error while loading. Please reload this page.

2 participants

Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.

You are visitor 01402503 — GeezerVison does not store the pages you look up.
Best viewed with Netscape Navigator 4.7 or Internet Explorer 6 at 800×600. No cookies required.

GeezerVison

Conversation

brandtbucher commented Jul 7, 2025 • edited by bedevere-app Bot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

savannahostrowski Jul 8, 2025

Choose a reason for hiding this comment

Uh oh!

brandtbucher Jul 9, 2025

Choose a reason for hiding this comment

Uh oh!

brandtbucher Jul 9, 2025

Choose a reason for hiding this comment

Uh oh!

brandtbucher Jul 10, 2025

Choose a reason for hiding this comment

Uh oh!

Reviewers

Uh oh!

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

brandtbucher commented Jul 7, 2025 •

edited by bedevere-app Bot

Loading

Uh oh!

There was an error while loading. Please reload this page.