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gh-130887: remove trailing jump in AArch64 JIT stencils #131042

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gh-130887: remove trailing jump in AArch64 JIT stencils #131042

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gh-130887: remove trailing jump in AArch64 JIT stencils
diegorusso Mar 6, 2025
460d0d3
📜🤖 Added by blurb_it.
blurb-it[bot] Mar 10, 2025
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-03-10-12-08-57.gh-issue-130887.f823Ih.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Optimize the AArch64 code generation for the JIT. Patch by Diego Russo
17 changes: 13 additions & 4 deletions Tools/jit/_stencils.py
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Expand Up @@ -209,7 +209,15 @@ def pad(self, alignment: int) -> None:
self.disassembly.append(f"{offset:x}: {' '.join(['00'] * padding)}")
self.body.extend([0] * padding)

def remove_jump(self, *, alignment: int = 1) -> None:
def add_nop(self, alignment: int) -> None:
"""Add a NOP if the offset is not aligned."""
offset = len(self.body)
nop = b"\x1f\x20\x03\xD5"
if offset % alignment:
self.disassembly.append(f"{offset:x}: d503201f\t\t nop")
self.body.extend(nop)
Comment on lines +212 to +218
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I don't like that this is ARM-specific, and only really happens to work because of the current alignment values we chose.

I wonder how messy it would be to make nop an attribute of _Target, and pass it into process_relocations, which passes it here (sort of like alignment).

I think that we should probably also make this so that it can add several nops if the alignment is larger (such as 16), and fail if this isn't possible (when adding nops overshoots the intended alignment).


def remove_jump(self) -> None:
"""Remove a zero-length continuation jump, if it exists."""
hole = max(self.holes, key=lambda hole: hole.offset)
match hole:
Expand Down Expand Up @@ -244,8 +252,9 @@ def remove_jump(self, *, alignment: int = 1) -> None:
jump = b"\x00\x00\x00\x14"
case _:
return
if self.body[offset:] == jump and offset % alignment == 0:
if self.body[offset:] == jump:
self.body = self.body[:offset]
self.disassembly = self.disassembly[:-2]
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Eh, I'd leave the disassembly alone. I get what you're doing here, but I'd rather leave it as what the preprocessed stencil used to look like and not try to do anything too clever.

self.holes.remove(hole)


Expand Down Expand Up @@ -289,8 +298,8 @@ def process_relocations(
self._trampolines.add(ordinal)
hole.addend = ordinal
hole.symbol = None
self.code.remove_jump(alignment=alignment)
self.code.pad(alignment)
self.code.remove_jump()
self.code.add_nop(alignment=alignment)
self.data.pad(8)
for stencil in [self.code, self.data]:
for hole in stencil.holes:
Expand Down
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