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GeezerVison is showing github.com — GH-135379: Support limited scalar replacement for replicated uops in the JIT code generator. by markshannon · Pull Request #135563 · python/cpython

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@markshannon

@markshannon markshannon commented Jun 16, 2025 •
edited by bedevere-app Bot
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This PR does three things:

  • Performs limited replacement of array stack items with scalars for uops.
  • Modify replicate to support ranges starting from values other than zero
  • Replicates COPY and SWAP to take advantage of that.

With TOS caching this should allow COPY and SWAP to be implemented as register-to-register moves.

  • Issue: Top-of-stack caching in the JIT #135379

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@markshannon
Support limited scalar replacement for replicated uops in the code ge…
18c78b1 
…nerator. Use it to support efficient specializations of COPY and SWAP in the JIT.
@bedevere-app bedevere-app Bot added the awaiting core review label Jun 16, 2025
@markshannon markshannon added skip news and removed awaiting core review labels Jun 16, 2025
@bedevere-app bedevere-app Bot mentioned this pull request Jun 16, 2025
Top-of-stack caching in the JIT #135379
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@Fidget-Spinner

Fidget-Spinner commented Jun 16, 2025

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Can you call the PR something else instead of "scalar replacement"? Scalar replacement is the other term for escape analysis in the JVM https://devblogs.microsoft.com/java/improving-openjdk-scalar-replacement-part-1-3/

I know CS has a lot of overlapping terms, but I think it would be clearer to call it "change replicates to ranges" or something.
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@Fidget-Spinner

Fidget-Spinner commented Jun 16, 2025

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scalarize works too.
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Fidget-Spinner
Fidget-Spinner reviewed Jun 16, 2025
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Comment thread Tools/cases_generator/analyzer.py Outdated
] + items[index+1:]

def scalarize_stack(stack: StackEffect, oparg: int) -> StackEffect:
# Only scalarize if no more than one input or output is array

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This comment doesn't seem to belong to this code segment?

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@markshannon markshannon Jun 16, 2025

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Yes, I refactored the code but didn't move the comment.

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@markshannon

markshannon commented Jun 16, 2025

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This is scalar replacement. We are replacing a composite object (an array in this case) with a set of scalar values.

In the JVM, that means breaking down an object into its parts. The JVM version is a lot more complex and powerful, but it is doing the same thing, and the motivation is the same: register allocation.
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@Fidget-Spinner

Fidget-Spinner commented Jun 16, 2025

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This is scalar replacement. We are replacing a composite object (an array in this case) with a set of scalar values.

In the JVM, that means breaking down an object into its parts. The JVM version is a lot more complex and powerful, but it is doing the same thing, and the motivation is the same: register allocation.

It's somewhat confusing because it's scalar replacement in the JIT, but not in the default interpreter. Though you subtly implied it with the "uops".
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@markshannon markshannon changed the title GH-135379: Support limited scalar replacement for replicated uops in the code generator. GH-135379: Support limited scalar replacement for replicated uops in the JIT code generator. Jun 16, 2025
@markshannon
Update comment
ce421ad
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Comment thread Tools/cases_generator/analyzer.py Resolved

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@markshannon @Fidget-Spinner
Update Tools/cases_generator/analyzer.py
72aa4a6 
Co-authored-by: Ken Jin <[email protected]>
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Fidget-Spinner approved these changes Jun 16, 2025
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@bedevere-app bedevere-app Bot added the awaiting merge label Jun 16, 2025
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Comment thread Tools/cases_generator/analyzer.py
Comment on lines +873 to +880
index = -1
for i, item in enumerate(items):
if "oparg" in item.size:
if index >= 0:
return items
index = i
if index < 0:
return items

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This entire part is somewhat confusing, but I'll let it slide since there's a comment up top :).

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@markshannon
markshannon merged commit 8dd8b5c into python:main Jun 17, 2025
@bedevere-app bedevere-app Bot removed the awaiting merge label Jun 17, 2025
lkollar pushed a commit to lkollar/cpython that referenced this pull request Jun 19, 2025
@markshannon @lkollar
pythonGH-135379: Support limited scalar replacement for replicated uo…
a7fe5be 
…ps in the JIT code generator. (pythonGH-135563)

* Use it to support efficient specializations of COPY and SWAP in the JIT.
Pranjal095 pushed a commit to Pranjal095/cpython that referenced this pull request Jul 12, 2025
@markshannon @Pranjal095
pythonGH-135379: Support limited scalar replacement for replicated uo…
fee436c 
…ps in the JIT code generator. (pythonGH-135563)

* Use it to support efficient specializations of COPY and SWAP in the JIT.
taegyunkim pushed a commit to taegyunkim/cpython that referenced this pull request Aug 4, 2025
@markshannon @taegyunkim
pythonGH-135379: Support limited scalar replacement for replicated uo…
1134fb2 
…ps in the JIT code generator. (pythonGH-135563)

* Use it to support efficient specializations of COPY and SWAP in the JIT.
Agent-Hellboy pushed a commit to Agent-Hellboy/cpython that referenced this pull request Aug 19, 2025
@markshannon @Agent-Hellboy
pythonGH-135379: Support limited scalar replacement for replicated uo…
b00b926 
…ps in the JIT code generator. (pythonGH-135563)

* Use it to support efficient specializations of COPY and SWAP in the JIT.
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@markshannon @Fidget-Spinner
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