perf(allocator/vec2): reorder RawVec fields

[Dunqing]

According to len is more commonly used than cap and a is used less than any other field, reorder RawVec fields to get the best performance.

I've tried putting cap before len, the performance is not as good as the current order, see #11050 (comment)

[Dunqing]

• perf(allocator/vec2): reorder RawVec fields #11050 👈 (View in Graphite)
• feat(allocator/vec2): change len and cap fields from usize to u32 #10884
• refactor(allocator/vec2): move len field from Vec to RawVec #10883
• main

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[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #11050 will not alter performance

_{Comparing 05-15-refactor_allocator_vec2_reorder_rawvec_fields (5dcd0f1) with main (c60382d)}

Summary

✅ 36 untouched benchmarks

[Dunqing]

#[repr(C)]
pub struct RawVec<'a, T> {
    ptr: NonNull<T>,
    cap: u32,
    pub(super) len: u32,
    a: &'a Bump,
}

[Copilot]

Pull Request Overview

This pull request reorders the RawVec fields to optimize performance by placing the most frequently accessed field (len) immediately after the pointer.

• Updated the field offset constant in the raw transfer generator to reflect the new layout.
• Reordered the struct fields in RawVec so that len now directly follows the pointer.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
tasks/ast_tools/src/generators/raw_transfer.rs	Updated the constant offset for Vec's length field to match the new field ordering in RawVec.
crates/oxc_allocator/src/vec2/raw_vec.rs	Reordered RawVec fields from the previous order to a new order (ptr, len, cap, a) to improve performance.

Comments suppressed due to low confidence (2)

tasks/ast_tools/src/generators/raw_transfer.rs:30

• The VEC_LEN_FIELD_OFFSET value is updated to 8 to reflect the new field order in RawVec. Please verify that the adjusted offset aligns with all expected memory layout assumptions, especially regarding pointer size across architectures.

const VEC_LEN_FIELD_OFFSET: usize = 8;

crates/oxc_allocator/src/vec2/raw_vec.rs:70

• Reordering the RawVec fields so that 'len' is placed immediately after the pointer should enhance performance. Confirm that this reordering is consistently reflected in all related code that relies on the struct layout.

cap: u32,

[overlookmotel]

Merge activity

• May 15, 6:31 PM EDT: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• May 15, 6:33 PM EDT: overlookmotel added this pull request to the Graphite merge queue.
• May 15, 6:40 PM EDT: Merged by the Graphite merge queue.