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GH-115802: Optimize JIT stencils for size #136393

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brandtbucher merged 2 commits into from
Jul 9, 2025
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GH-115802: Optimize JIT stencils for size #136393

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-Os
brandtbucher Jun 28, 2025
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brandtbucher Jul 6, 2025
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10 changes: 9 additions & 1 deletion Tools/jit/_targets.py
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Original file line number Diff line number Diff line change
Expand Up @@ -137,7 +137,15 @@ async def _compile(
f"-I{CPYTHON / 'Include' / 'internal' / 'mimalloc'}",
f"-I{CPYTHON / 'Python'}",
f"-I{CPYTHON / 'Tools' / 'jit'}",
"-O3",
# -O2 and -O3 include some optimizations that make sense for
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@savannahostrowski savannahostrowski Jul 8, 2025

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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.
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@brandtbucher brandtbucher Jul 9, 2025

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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.
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@brandtbucher brandtbucher Jul 9, 2025

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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.
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@brandtbucher brandtbucher Jul 10, 2025

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Yep, -Oz is about 1-2% slower across the board.
# standalone functions, but not for snippets of code that are going
# to be laid out end-to-end (like ours)... common examples include
# passes like tail-duplication, or aligning jump targets with nops.
# -Os is equivalent to -O2 with many of these problematic passes
# disabled. Based on manual review, for *our* purposes it usually
# generates better code than -O2 (and -O2 usually generates better
# code than -O3). As a nice benefit, it uses less memory too:
"-Os",
"-S",
# Shorten full absolute file paths in the generated code (like the
# __FILE__ macro and assert failure messages) for reproducibility:
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