Description

markshannon

opened on Apr 15, 2025

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Iteration over tuples and short lists is quite inefficient as we need to create an iterator object, only to have to destroy it again moments later. Not only that, fetching values from iterators involves additional indirection compared to fetching them from sequences.

Instead we can push a pair of values to the stack. For common sequences, like tuple, list, strings, some ranges and a few others, we push the sequence and the integer index (initially 0) to the stack. For other iterables, we push the iterator and NULL.

GET_ITER will have the signature:
iterable -- iter, index_or_null
FOR_ITER now has the signature:
iter, index_or_null -- iter, index_or_null, next.

What makes this efficient is tagged integers. By using tagged integers, no objects need to be created.

Examples

GET_ITER

[ <tuple at ...> ] -> [ <tuple at ...>, 0 ]

[ <file at ...> ] -> [ <file iterator at ...>, NULL ]

FOR_ITER

[ <tuple at ...>, 0 ] -> [ <tuple at ...>, 1, item0 ]

[ <file iterator at ...>, NULL ] -> [ <file iterator at ...>, NULL, line ]

Linked PRs

• GH-132554: "Virtual" iterators #132555
• GH-132554: Add stats for GET_ITER #132592
• GH-132554: Specialize GET_ITER and FOR_ITER for range #135063
• GH-132554: Fix tier2 FOR_ITER implementation and optimizations #135137

Activity

1. markshannon commented on Apr 15, 2025

   

   markshannon

   on Apr 15, 2025

   MemberAuthor

   More actions

   Faster CPython discussion

   Tagged ints issue
2. picnixz

   added

   type-featureA feature request or enhancementA feature request or enhancement

   performancePerformance or resource usagePerformance or resource usage

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

   on Apr 17, 2025
3. mdboom

   self-assigned this

   on Apr 21, 2025
4. mdboom commented on Apr 21, 2025

   

   mdboom

   on Apr 21, 2025

   Contributor

   More actions

   As discussed in our meeting, I'm going to take a first crack at this.
5. mdboom

   removed their assignment

   on Apr 21, 2025
6. mdboom commented on Apr 21, 2025

   

   mdboom

   on Apr 21, 2025

   Contributor

   More actions

   As discussed in our meeting, I'm going to take a first crack at this.

   Actually, realizing @markshannon already started that, I'm instead going to follow-on and do GET_ITER specializations.
7. markshannon

   added a commit that references this issue on Apr 29, 2025

   GH-132554: Add stats for GET_ITER (GH-132592)

   622300b
8. markshannon

   added a commit that references this issue on May 27, 2025

   GH-132554: "Virtual" iterators (GH-132555)

   f6f4e8a
9. markshannon

   added a commit that references this issue on Jun 5, 2025

   GH-132554: Fix tier2 `FOR_ITER` implementation and optimizations (GH-…

   b90ecea
10. Pranjal095

    added 2 commits that reference this issue on Jul 12, 2025

    pythonGH-132554: "Virtual" iterators (pythonGH-132555)

    03f4000

    pythonGH-132554: Fix tier2 `FOR_ITER` implementation and optimizations (

    3557d29
11. taegyunkim

    added 2 commits that reference this issue on Aug 4, 2025

    pythonGH-132554: "Virtual" iterators (pythonGH-132555)

    b3fb901

    pythonGH-132554: Fix tier2 `FOR_ITER` implementation and optimizations (

    dd6f0ca
12. Agent-Hellboy

    added a commit that references this issue on Aug 19, 2025

    pythonGH-132554: Fix tier2 `FOR_ITER` implementation and optimizations (

    e7ac90f
13. gostak-dd

    added a commit that references this issue on Jun 2, 2026

    pythonGH-132554: "Virtual" iterators (pythonGH-132555)

    88df9e9

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