Description

markshannon

opened on Feb 14, 2024

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Splitting these micro-ops will improve performance by reducing the number of branches, the size of code generated, and the number of holes in the JIT stencils. There is no real downside; the increase in complexity at runtime is negligible and there isn't much increased complexity in the tooling.

Taking _LOAD_ATTR_INSTANCE_VALUE as an example, as it is the dynamically most common.

    op(_LOAD_ATTR_INSTANCE_VALUE, (index/1, owner -- attr, null if (oparg & 1))) {
        ...

can be split into

    op(_LOAD_ATTR_INSTANCE_VALUE_0, (index/1, owner -- attr)) {
        assert((oparg & 1) == 0);
        ...

and

    op(_LOAD_ATTR_INSTANCE_VALUE_1, (index/1, owner -- attr, null)) {
        assert((oparg & 1) == 1);
        ...

Each of these is simpler, thus smaller and faster than the base version.
We can always choose one of the two split version when projecting the trace, so we don't need an implementation of the base version at all. This means that the tier 2 interpreter and stencils aren't much bigger than before.

Linked PRs

• GH-115457: Support splitting and replication of micro ops.  #115558

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1. markshannon

   added

   performancePerformance or resource usagePerformance or resource usage

   on Feb 14, 2024
2. erlend-aasland

   added

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   on Feb 14, 2024
3. markshannon

   self-assigned this

   on Feb 15, 2024
4. markshannon commented on Feb 16, 2024

   

   markshannon

   on Feb 16, 2024

   MemberAuthor

   More actions

   It makes sense to do replication at the same time as splitting.

   By replication, I mean creating a copy of the replicated uop for each oparg in a given set.
   This is not such an obvious win, as we need multiple stencils, but each stencil can be significantly smaller than the original.

   The best example is _INIT_CALL_PY_EXACT_ARGS which has a stencil of 753 bytes, whereas the stencil for _INIT_CALL_PY_EXACT_ARGS_0 is only 308 bytes, a ~60% saving.

   _LOAD_FAST shows a smaller saving, from 45 bytes down to 31 for LOAD_FAST_0.

   Reacted by stonebig and Sergey Miryanov
5. markshannon

   added a commit that references this issue on Feb 20, 2024

   GH-115457: Support splitting and replication of micro ops. (GH-115558)

   626c414
6. markshannon

   closed this as completedon Feb 21, 2024
7. woodruffw

   added a commit that references this issue on Mar 4, 2024

   pythonGH-115457: Support splitting and replication of micro ops. (pyt…

   12ba88b
8. diegorusso

   added a commit that references this issue on Apr 17, 2024

   pythonGH-115457: Support splitting and replication of micro ops. (pyt…

   567e3ca
9. LukasWoodtli

   added a commit that references this issue on Jan 22, 2025

   pythonGH-115457: Support splitting and replication of micro ops. (pyt…

   c863aee

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