[browser/wasi][coreCLR] Dedicated WASM GC PAL — replace mmap with posix_memalign and optimize memory operations

[pavelsavara]

Summary

This PR replaces the WASM GC and PAL virtual-memory paths to use posix_memalign/free instead of mmap/munmap. Emscripten's mmap implementation is fundamentally broken for the GC's needs: munmap cannot unmap partial allocations, mmap(PROT_NONE) still consumes linear memory, and MAP_FIXED doesn't work correctly. This change introduces a dedicated WASM GC OS interface (gc/wasm/gcenv.cpp) and updates the PAL layer to use the allocator-based approach.

Based on runtimelab's NativeAOT-LLVM gcenv.wasm.cpp (PR#1510, PR#3151).

Fixes #121036
Fixes #117813
Fixes #118943

Motivation

The CoreCLR GC on WASM targets (browser and WASI) previously shared the Unix gcenv.unix.cpp code path with #ifdef TARGET_WASM carve-outs. This was problematic because:

1. mmap/munmap semantics don't work on WASM — Emscripten's munmap cannot release partial mappings, mmap(PROT_NONE) reserves real linear memory (no lazy commit), and MAP_FIXED is broken.
2. Virtual memory doesn't exist on WASM — The linear memory model means reserve == commit. The existing code pretended otherwise, leading to wasted memory.
3. Page size mismatch — WASM's memory.grow granularity is 64KB, but the GC works best with a 16KB page size for alignment and thresholds.

Changes

New files

• src/coreclr/gc/wasm/gcenv.cpp — Complete WASM-specific GCToOSInterface implementation using posix_memalign/free. Includes an sbrk optimization that avoids unnecessary memset zeroing on fresh memory.grow pages (safe because single-threaded WASM has no concurrent sbrk calls).
• src/coreclr/gc/wasm/CMakeLists.txt — Build file for the WASM GC PAL module.
• src/native/minipal/wasm.h — Cross-platform minipal_getpagesize() helper that returns 16KB on WASM instead of the 64KB memory.grow granularity.

Modified files

• src/coreclr/gc/CMakeLists.txt — Routes WASM builds to the new wasm/ subdirectory instead of unix/.
• src/coreclr/gc/init.cpp — Enables the large-pages code path on WASM (to skip VirtualDecommit, since decommit is meaningless on WASM). Caps segment sizes so initial segments fit within the hard limit. Enables auto-detection of the hard limit from WASM linear memory max (treating it like a container memory limit).
• src/coreclr/gc/interface.cpp — Suppresses the 32-bit assert(!use_large_pages_p) for WASM.
• src/coreclr/gc/unix/gcenv.unix.cpp — Removes all TARGET_WASM/__EMSCRIPTEN__ conditionals (WASM now has its own file). Also fixes a pre-existing bug: nanosleep returns -1 on EINTR, not EINTR itself.
• src/coreclr/pal/src/map/virtual.cpp — Adds TARGET_WASM paths in ReserveVirtualMemory (using posix_memalign), VIRTUALCommitMemory (sentinel-based lazy zeroing), VirtualFree (decommit via sentinel or free), and VIRTUALReserveMemory (using free instead of munmap on error). Restructures the MEM_RELEASE path to check munmap return before calling VIRTUALReleaseMemory.
• src/coreclr/pal/src/misc/sysinfo.cpp — Uses minipal_getpagesize() instead of getpagesize().

Key design decisions

Sentinel-based lazy zeroing (single-threaded path)

Instead of eagerly zeroing memory on decommit, the single-threaded path writes a non-zero sentinel byte at each page boundary. On recommit, the first byte is checked — if non-zero, the range is zeroed. This avoids double-zeroing in the common case where memory is decommitted and never recommitted.

Under multithreading (FEATURE_MULTITHREADING), the code falls back to unconditional zeroing on both decommit and commit to avoid races.

sbrk optimization

When posix_memalign returns memory at or above the previous sbrk(0) break, the allocation came from memory.grow which guarantees zero-initialization per the WASM spec. Only recycled blocks below the old break need explicit zeroing. This is safe because WASM is single-threaded (no concurrent sbrk calls). The same approach is used by Mono's WASM mmap implementation.

use_large_pages_p = true on WASM

The GC's large-pages mode skips VirtualDecommit for heap segments, which is exactly what WASM needs since decommit cannot return memory. The hard-limit auto-detection (75% of emscripten_get_heap_max()) is preserved rather than being tightened to actual segment sizes, leaving room for bookkeeping allocations.

16KB page size

WASM memory.grow uses 64KB pages, but the GC's alignment and threshold calculations work better with a finer granularity. The 16KB page size (minipal_getpagesize()) is used for GC page alignment while the 64KB WasmPageSize constant is used only when converting __builtin_wasm_memory_size counts to bytes.

Code review notes

Correctness

• The nanosleep fix in gcenv.unix.cpp (checking == -1 && errno == EINTR instead of == EINTR) is a pre-existing bug fix that affects all Unix platforms, not just WASM.
• The MEM_RELEASE restructuring in virtual.cpp inverts the error-checking logic from if (munmap == 0) { if (!Release) fail } else { fail } to if (munmap != 0) fail; if (!Release) fail. This is a behavioral no-change but is clearer.
• VirtualReset on WASM returns false, forcing the GC to use the decommit+commit fallback path. The previous code returned true (pretending madvise worked), which silently did nothing.

Thread safety

• All sbrk-based optimizations are guarded by #ifndef FEATURE_MULTITHREADING.
• The sentinel trick is only used in single-threaded mode; MT mode zeroes unconditionally.

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[Copilot]

Pull request overview

This PR separates WebAssembly-specific GC OS interface behavior from the shared Unix implementation by introducing a dedicated gcenv.wasm.cpp, and adjusts the PAL virtual memory implementation on WASM to avoid relying on Emscripten’s incomplete mmap/munmap support.

Changes:

• Added a dedicated WASM GCToOSInterface implementation (gcenv.wasm.cpp) and CMake wiring for building it.
• Routed WASM GC builds to the new gc/wasm directory and removed WASM-specific #ifdef paths from gcenv.unix.cpp.
• Updated PAL virtual memory reserve/release on WASM to use posix_memalign/free instead of mmap/munmap.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/coreclr/pal/src/map/virtual.cpp	Switches WASM reserve/release behavior to posix_memalign/free and adjusts related error/cleanup paths.
src/coreclr/gc/wasm/gcenv.wasm.cpp	New WASM-specific GC OS interface implementation (virtual memory, CPU/NUMA stubs, memory stats).
src/coreclr/gc/wasm/CMakeLists.txt	Adds build definition for the WASM GC PAL object library.
src/coreclr/gc/unix/gcenv.unix.cpp	Removes WASM-specific branches and fixes nanosleep EINTR retry logic.
src/coreclr/gc/CMakeLists.txt	Routes WASM builds to gc/wasm instead of gc/unix.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 7 comments.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 7 comments.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 4 comments.

[pavelsavara]

I processed your feedback and updated the PR description above
Please let me know if I understood it well enough @janvorli @jkotas @SingleAccretion

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

[Copilot]

On TARGET_WASM, VIRTUALCommitMemory zeroes the entire aligned range whenever the first byte is non-zero. This breaks VirtualAlloc semantics for recommitting already committed memory: PAL test filemapping_memmgt/VirtualAlloc/test20 explicitly writes a value, calls VirtualAlloc(ptr, ..., MEM_COMMIT, ...) again, and expects the contents to be unchanged. With the current check (*(BYTE*)StartBoundary != 0) this will incorrectly zero valid committed data. The commit path needs a way to distinguish ‘recommit after MEM_DECOMMIT’ from ‘commit on already committed pages’ (e.g., always do the zeroing in the MEM_DECOMMIT path and make commit a no-op, or track decommitted state out-of-band instead of inspecting user data bytes).

Suggested change

	// On WASM, reserve == commit. If this range was previously decommitted,
	// sentinels were placed at each page boundary. Check the first byte and
	// zero the entire range if needed.
	#ifdef FEATURE_MULTITHREADING
	// Under MT, VirtualDecommit already zeroes the full range on decommit, and
	// reserve already zeroes on allocation - so commit is a no-op.
	(void)MemSize;
	#else
	if (MemSize && *(BYTE*)StartBoundary != 0)
	{
	ZeroMemory((LPVOID) StartBoundary, MemSize);
	}
	#endif
	#endif
	// On WASM, reserve == commit, so MEM_COMMIT must be a no-op here.
	// In particular, recommitting already committed memory must preserve its
	// contents; using user data (for example, the first byte) to infer prior
	// decommit state is incorrect and can spuriously zero valid committed data.
	// Any zeroing required to model MEM_DECOMMIT must be handled by the
	// decommit path or tracked out-of-band rather than during commit.
	(void)MemSize;
	#endif

[jkotas]

I think we want to merge #127290 first. It should make this change simpler.

[janvorli]

@pavelsavara I've just merged @cshung's PR.

[jkotas]

This should not be needed once #127290 is merged

[jkotas]

Can we delete GetOsPageSize and use this everywhere instead (it can be a separate PR)?

Also, it may be nice to cache the value here.