Snapshot support for embedders - #13
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…les on this branch — the branch now builds from the download exactly like main does
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…live samples; size the location table to the sample count The thread that held the profiler lock in the parent (or in the process that built a snapshot) does not exist in the child, so the lock is re-initialized there. mi_prof_visit_live() lets an embedder walk the live sampled allocations, and mi_prof_lock_is_free() lets it assert the profiler is quiescent before freezing memory. The location table used while aggregating grows with the sample count instead of being fixed at 4096 entries.
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An embedder can freeze the current heap into a snapshot and map it into later processes. This adds what the allocator needs for that: pages captured in a snapshot get the thread id MI_THREADID_FROZEN, so frees of their blocks are dropped on the (already slow) cross-thread path and collection, purging and the idle hole sweep leave them alone; mi_theap_freeze() marks the frozen theaps; mi_arenas_seal_existing() stops new allocation from landing in frozen arenas; mi_theap_adopt_current_thread() lets the resuming thread take over the frozen main-thread state (otherwise every page it allocates carries a thread id nobody has and all of its frees go cross-thread); mi_heap_snapshot_hints_enabled() exports the deterministic-placement decision, made once, so the embedder's own reservations follow the same rule. Placement is deterministic when the host executable says it can carry a snapshot (MI_HEAP_SNAPSHOT_HOST_FN) or MIMALLOC_DETERMINISTIC_HINT is set; a capable process that is not taking a snapshot keeps its early allocations above MI_HEAP_SNAPSHOT_RESTORER_FLOOR so they never sit where the snapshot will be mapped. The hole-sweep bookkeeping moves from __thread variables onto the tld: on macOS the first touch of a thread-local from inside the allocator makes dyld allocate the TLV block with malloc, which re-enters the allocator before the variable exists. Nothing here runs in a process that neither takes nor maps a snapshot, apart from the frozen-id compare on the cross-thread free path.
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Allocator side of Bun's heap images (an embedder freezes a process's heap into a file and maps it into fresh processes). 240 lines; everything is behind new entry points or embedder-supplied defines, and the malloc/free fast paths are unchanged. Full rationale in the second commit's message.
MI_HEAP_IMAGE_HOST_FN/MI_HEAP_IMAGE_BUILD_ENV,mi_os_hint_floor()MI_THREADID_FROZEN,mi_arenas_freeze_pages()mi_arenas_seal_existing(),mi_theap_freeze(),mi_arenas_visit_free_ranges()mi_malloc_zone_process_owned_ranges()(macOS)__threadvariables onto the swepttldprof.c:mi_prof_reinit_lock,mi_prof_visit_live,mi_prof_lock_is_freeExercised by
test/js/bun/heap-imageon the Bun side (oven-sh/bun branchclaude/lowmem-cc) and by an application that has been running from an image for a day. Companion: oven-sh/WebKit#397 (JSC side). Draft until the Bun side is up for review.