fix(runtime): retag mimalloc OS mappings off the IOAccelerator VM tag (#6882)

[proggeramlug]

Closes #6882.

Problem

mimalloc (Perry's #[global_allocator] since #62) tags its OS mappings with VM tag 100, which macOS tooling — vmmap, Instruments' VM Tracker, footprint — decodes as IOAccelerator. Every Perry process on macOS therefore appears to hold hundreds of MB of GPU-driver memory (644 MB mapped / 422 MB resident on the churn benchmark in #6882), and the actual JS heap is invisible as a category. Anyone profiling Perry memory starts by chasing a GPU ghost.

Fix

js_gc_init — the first runtime call in every compiled binary's prelude, already home to per-platform one-time setup — retags mimalloc's mappings to VM_MEMORY_APPLICATION_SPECIFIC_1 (240) via mi_option_set(mi_option_os_tag, 240):

• Apple 64-bit only (cfg(all(target_pointer_width = "64", target_vendor = "apple"))): the tag is only consulted by Apple kernels, and mimalloc itself is already 64-bit-only.
• An explicit MIMALLOC_OS_TAG env setting wins — the retag is skipped so profilers can keep steering the tag themselves.
• Idempotent, like the rest of js_gc_init.
• Pages mapped before js_gc_init (early Rust startup) keep tag 100; the bulk of the heap (arena blocks, GC metadata) maps afterwards.

Adds libmimalloc-sys (the crate mimalloc already depends on, same version resolution) as an Apple-only dependency for its extended option API.

Also in this PR

• docs/src/internals/memory-model.md: new "Profiling Perry's memory on macOS" section — where the heap shows up (Memory Tag 240, not MALLOC_*), what pre-macOS tools attribute Perry's heap to IOAccelerator (GPU) — mimalloc VM tag 100 collision #6882 IOAccelerator regions meant, the MIMALLOC_OS_TAG override, and the MADV_FREE/RSS accounting caveat.
• Unit test gc::tests::os_tag::js_gc_init_retags_mimalloc_os_mappings pinning the retag (skips itself when MIMALLOC_OS_TAG is set, mirroring the runtime's deference).

Verification

vmmap -summary on the #6882 churn benchmark compiled with this branch (macOS 26.5, Apple Silicon):

before (main):
IOAccelerator                    644.4M   422.1M   422.1M        8

after (this branch):
IOAccelerator                     4352K      32K     32K         1   ← pre-js_gc_init residue only
Memory Tag 240                   640.1M   421.8M  387.5M         7   ← the JS heap, correctly attributed
Memory Tag 240 (reserved)        384.0M       0K      0K         1

Benchmark output unchanged (39998000000).

Unit suite: cargo test -p perry-runtime --lib gc:: — 477 passed, 2 failed; the same 2 failures (gc::tests::copying::large_object_old_born_array_slot_write_keeps_young_child_alive, gc::tests::teardown::map_set_side_allocations_release_on_thread_exit) reproduce on pristine origin/main (fd74751) with this branch's changes stashed — pre-existing on main, unrelated to this PR.

No behavior change beyond the mapping tag — allocation paths, sizes, and performance are untouched.

Summary by CodeRabbit

• Improvements

  • Improved macOS memory tagging so Perry heap allocations are classified more clearly in profiling tools.
  • Added support for extended memory-tagging behavior on 64-bit Apple platforms.

• Documentation

  • Added guidance for interpreting Perry memory usage, heap classifications, and RSS measurements in macOS profiling tools.

• Tests

  • Added coverage verifying the expected macOS memory tag during garbage collector initialization.

[coderabbitai]

Caution

Review failed

The pull request is closed.

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📥 Commits

Reviewing files that changed from the base of the PR and between fd74751 and 4e1dbe0.

⛔ Files ignored due to path filters (1)

• Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (5)

• crates/perry-runtime/Cargo.toml
• crates/perry-runtime/src/gc/mod.rs
• crates/perry-runtime/src/gc/tests/mod.rs
• crates/perry-runtime/src/gc/tests/os_tag.rs
• docs/src/internals/memory-model.md

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📝 Walkthrough

Walkthrough

Changes

macOS memory tagging

Layer / File(s)	Summary
Configure and apply mimalloc OS tagging crates/perry-runtime/Cargo.toml, crates/perry-runtime/src/gc/...	Apple 64-bit builds enable mimalloc’s extended API, and js_gc_init sets the OS memory tag to 240 when MIMALLOC_OS_TAG is unset. A platform-specific test verifies the resulting value.
Document macOS memory attribution docs/src/internals/memory-model.md	Documents mimalloc-backed heap regions, macOS VM tag labels, early mappings, and purging effects in profiling tools.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested reviewers: andrewtdiz

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/6882-mimalloc-vm-tag

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