This is a rewrite of the Apple PAL that implements the AlignedAllocation, Entropy, and LazyCommit features through native Apple APIs.

It adds a dependency on Security.framework via SecRandomCopyBytes. Apple actively discourages use of getentropy and the symbol is not allowed on the App Store.

TODO / Discussion

USE_POSIX_COMMIT_CHECKS

I removed support for USE_POSIX_COMMIT_CHECKS because the POSIX memory management APIs are no longer used but I'd like to avoid a regression. I need some context about what the memset and mprotect calls in pal_bsd.h and pal_posix.h are supposed to accomplish. Is it supposed to emulate Windows' decommit behavior? If so, why do the pages need to be decommitted? What are the invariants?

I hope to use the mach APIs to implement the behavior if possible and prevent any M1-related bugs:

• #278
• #280
• #284

Can the size parameter of reserve_aligned be greater than 4GiB?

There are Apple equivalents of ReclaimVirtualMemory and OfferVirtualMemory, but they only work on regions that are 4GiB and smaller. (These APIs have existed since macOS 10.6 and are present in the iOS SDK.)

The 4GiB limit is a hard limit in the XNU kernel even on 64-bit platforms.

~~LowMemoryNotification~~

~~I have a working implementation behind the feature SNMALLOC_APPLE_LOWMEMORYNOTIFICATION. I'll push the change after we resolve USE_POSIX_COMMIT_CHECKS.~~

@aganea has enabled snmalloc, mimalloc, and rpmalloc to be the allocator for lld-link. He has benchmarked this with performing ThinLTO on a clang build. The results taken from https://reviews.llvm.org/D71786 are

| Allocator | Wall clock | Page ranges commited/decommited | Total touched pages | Peak Mem | ---------------------------------|----------------|-----|----|-----------| | Windows 10 version 2004 | 38 min 47 sec | | | 14.9 GB | | mimalloc | 2 min 22 sec | 1,449,501 | 174,3 GB | 19,8 GB | | rpmalloc | 2 min 15 sec | 270,796 | 45,9 GB | 31,9 GB | | snmalloc | 2 min 19 sec | 102,839 | 47,0 GB | 42,0 GB |

The time is pretty comparable, but this shows snmalloc on Windows as committing considerably more memory than other allocators.

Experiments to try

• Different "chunk" size, 16MiB versus 1MiB.
• Sub chunk, commit/decommit operations

Some portability fixes and some changes to pointer provenance throughout. No explicit MIPS or CHERI support yet (on this branch), but perhaps subsets of this are ready to merge and others deserve a chance to receive commentary. :)

I am trying to bind snmalloc to rust to have a comprehensive malloc-bench (WIP).

Basically, I have already created several bindings: snmalloc,snmalloc-sys, snmallocator.

Yet, there is a issue to provide efficient realloc with alignment. I added one following the code in malloc.h, but I do not know whether it is sound and safe. So I guess I need some guidance here.

Hi! I've only learned about the existence of this library today. I am always curious about new high performance allocators, so I tried to create an implementation of snmalloc for Unreal Engine. Unreal Engine has their own memory allocator abstraction, so it should be fairly easy to do based on your header-only library mode. I also based it on their implementation of mimalloc ( https://github.com/EpicGames/UnrealEngine/blob/4.27/Engine/Source/Runtime/Core/Private/HAL/MallocMimalloc.cpp ).

Unreal Engine has some pecularities with how it uses allocation functions, so it might use Realloc with a size 0 to Free, Realloc without an existing pointer, or Malloc with a size 0 expecting nullptr back. This wrapper takes care of all of these.

This is the result - seems pretty straight forward, but do let me know if I am using something clearly wrong.

using UnrealBuildTool;
using System.IO;

public class snmalloc : ModuleRules
{
	public snmalloc(ReadOnlyTargetRules Target) : base(Target)
	{
		Type = ModuleType.External;

		if (Target.Platform.IsInGroup(UnrealPlatformGroup.Windows))
		{
			PublicSystemIncludePaths.Add(Target.UEThirdPartySourceDirectory + "snmalloc\\src");
		}
	}
}

#pragma once

#include "CoreTypes.h"
#include "HAL/PlatformMemory.h"
#include "HAL/MemoryBase.h"

#if PLATFORM_WINDOWS

class FMallocSnmalloc final
	: public FMalloc
{
public:
	// FMalloc interface.
	virtual void* Malloc(SIZE_T Size, uint32 Alignment) override;
	virtual void* TryMalloc(SIZE_T Size, uint32 Alignment) override;
	virtual void* Realloc(void* Ptr, SIZE_T NewSize, uint32 Alignment) override;
	virtual void* TryRealloc(void* Ptr, SIZE_T NewSize, uint32 Alignment) override;
	virtual void Free(void* Ptr) override;
	virtual bool GetAllocationSize(void* Original, SIZE_T &SizeOut) override;
	virtual void Trim(bool bTrimThreadCaches) override;
	virtual SIZE_T QuantizeSize(SIZE_T Count, uint32 Alignment) override;

	virtual bool IsInternallyThreadSafe() const override
	{ 
		return true;
	}

	virtual const TCHAR* GetDescriptiveName() override
	{
		return TEXT("Snmalloc");
	}

protected:
	void OutOfMemory(uint64 Size, uint32 Alignment)
	{
		// this is expected not to return
		FPlatformMemory::OnOutOfMemory(Size, Alignment);
	}
};

#endif

#include "HAL/MallocSnmalloc.h"
#include "Math/UnrealMathUtility.h"
#include "HAL/UnrealMemory.h"

#if PLATFORM_WINDOWS

#define SNMALLOC_CHECK_CLIENT
#define SNMALLOC_USE_CXX17
#define SNMALLOC_TRACING
#define SNMALLOC_CHECK_LOADS

THIRD_PARTY_INCLUDES_START
#include "Windows/PreWindowsApi.h"
#pragma warning(push)
#pragma warning(disable:4582)
#pragma warning(disable:4701)
#pragma warning(disable:4703)
#include <snmalloc.h>
#pragma warning(pop)
#include "Windows/PostWindowsApi.h"
THIRD_PARTY_INCLUDES_END

void* FMallocSnmalloc::TryMalloc(SIZE_T Size, uint32 Alignment)
{
	if (!Size)
	{
		return nullptr;
	}

#if !UE_BUILD_SHIPPING
	uint64 LocalMaxSingleAlloc = MaxSingleAlloc.Load(EMemoryOrder::Relaxed);
	if (LocalMaxSingleAlloc != 0 && Size > LocalMaxSingleAlloc)
	{
		return nullptr;
	}
#endif

	if (Alignment != DEFAULT_ALIGNMENT)
	{
		check(Alignment <= snmalloc::natural_alignment(Size));
	}

	return snmalloc::ThreadAlloc::get().alloc(Size);
}

void* FMallocSnmalloc::Malloc(SIZE_T Size, uint32 Alignment)
{
	void* NewPtr = TryMalloc(Size, Alignment);

	if (NewPtr == nullptr && Size)
	{
		OutOfMemory(Size, Alignment);
	}

	return NewPtr;
}

void* FMallocSnmalloc::TryRealloc(void* Ptr, SIZE_T NewSize, uint32 Alignment)
{
	if (!Ptr)
	{
		return TryMalloc(NewSize, Alignment);
	}

	if (!NewSize)
	{
		Free(Ptr);
		return nullptr;
	}

#if !UE_BUILD_SHIPPING
	uint64 LocalMaxSingleAlloc = MaxSingleAlloc.Load(EMemoryOrder::Relaxed);
	if (LocalMaxSingleAlloc != 0 && NewSize > LocalMaxSingleAlloc)
	{
		return nullptr;
	}
#endif

	auto& Alloc = snmalloc::ThreadAlloc::get();
	SIZE_T AllocatedSize = Alloc.alloc_size(Ptr);

	// Keep the current allocation if the given size is in the same size class.
	if (AllocatedSize == snmalloc::round_size(NewSize))
	{
		return Ptr;
	}

	if (Alignment != DEFAULT_ALIGNMENT)
	{
		check(Alignment <= snmalloc::natural_alignment(NewSize));
	}

	void* NewPtr = Alloc.alloc(NewSize);

	if (NewPtr)
	{
		FMemory::Memcpy(NewPtr, Ptr, AllocatedSize);
		Alloc.dealloc(Ptr);
	}

	return NewPtr;
}

void* FMallocSnmalloc::Realloc(void* Ptr, SIZE_T NewSize, uint32 Alignment)
{
	void* NewPtr = TryRealloc(Ptr, NewSize, Alignment);

	if (NewPtr == nullptr && NewSize)
	{
		OutOfMemory(NewSize, Alignment);
	}

	return NewPtr;
}

void FMallocSnmalloc::Free(void* Ptr)
{
	if (!Ptr)
	{
		return;
	}

	snmalloc::ThreadAlloc::get().dealloc(Ptr);
}

bool FMallocSnmalloc::GetAllocationSize(void* Original, SIZE_T &SizeOut)
{
	SizeOut = snmalloc::ThreadAlloc::get().alloc_size(Original);
	return true;
}

void FMallocSnmalloc::Trim(bool bTrimThreadCaches)
{
	snmalloc::cleanup_unused<snmalloc::Globals>();
}

SIZE_T FMallocSnmalloc::QuantizeSize(SIZE_T Count, uint32 Alignment)
{
	return snmalloc::round_size(Count);
}

#endif

I've tried the latest main branch, the main branch before the big "Buddy" refactor, and the release from 2021. I'm running the latest Windows 10 on a Ryzen 5950X and building with the latest MSVC, though Unreal Engine is only compiled in C++17 mode. I also enabled all the debug features I found. Unfortunately, the result is always the same - it crashes during some random allocation very early in startup - before any additional threads are started even.

This is the output of the tracing.

Run init_impl
Making an allocator.
rsize 16
slab size 16384
Alloc chunk: 000003F768A00000 (16384)
Run init_impl
Attach cache to 000003F768C28000
init(): [email protected]
Using C++ destructor clean up
Alloc chunk: 000003F768A40000 (262144)
size 262144 pow2 size 18
rsize 1024
slab size 16384
Alloc chunk: 000003F768A04000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A08000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A0C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A10000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A14000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A18000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A1C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A20000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A24000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A28000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A2C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A30000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A34000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A38000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A3C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A80000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A84000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A88000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A8C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A90000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A94000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A98000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768A9C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AA0000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AA4000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AA8000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AAC000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AB0000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AB4000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AB8000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768ABC000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AC0000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AC4000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AC8000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768ACC000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AD0000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AD4000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AD8000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768ADC000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AE0000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AE4000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AE8000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AEC000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AF0000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AF4000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AF8000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768AFC000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B00000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B04000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B08000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B0C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B10000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B14000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B18000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B1C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B20000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B24000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B28000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B2C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B30000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B34000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B38000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B3C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B40000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B44000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B48000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B4C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B50000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B54000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B58000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B5C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B60000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B64000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B68000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B6C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B70000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B74000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B78000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B7C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B80000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B84000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B88000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B8C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B90000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B94000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B98000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768B9C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BA0000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BA4000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BA8000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BAC000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BB0000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BB4000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BB8000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BBC000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BC0000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BC4000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BC8000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BCC000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BD0000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BD4000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BD8000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BDC000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BE0000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BE4000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BE8000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BEC000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BF0000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BF4000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BF8000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768BFC000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768000000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768004000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768008000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F76800C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768010000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768014000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768018000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F76801C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768020000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768024000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768028000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F76802C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768030000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768034000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768038000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F76803C000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768040000 (16384)
rsize 1024
slab size 16384
Alloc chunk: 000003F768044000 (16384)
rsize 64
slab size 16384
Alloc chunk: 000003F768048000 (16384)
rsize 512
slab size 16384
Alloc chunk: 000003F76804C000 (16384)
rsize 32
slab size 16384
Alloc chunk: 000003F768050000 (16384)
rsize 128
slab size 16384
Alloc chunk: 000003F768054000 (16384)
rsize 768
slab size 16384
Alloc chunk: 000003F768058000 (16384)
Alloc chunk: 000003F768080000 (524288)
size 524280 pow2 size 19
rsize 48
slab size 16384
Alloc chunk: 000003F76805C000 (16384)
rsize 384
slab size 16384
Alloc chunk: 000003F768060000 (16384)
rsize 256
slab size 16384
Alloc chunk: 000003F768064000 (16384)
rsize 96
slab size 16384
Alloc chunk: 000003F768068000 (16384)
rsize 80
slab size 16384
Alloc chunk: 000003F76806C000 (16384)
rsize 640
slab size 16384
Alloc chunk: 000003F768070000 (16384)
rsize 160
slab size 16384
Alloc chunk: 000003F768074000 (16384)
rsize 192
slab size 16384
Alloc chunk: 000003F768078000 (16384)
rsize 112
slab size 16384
Alloc chunk: 000003F76807C000 (16384)
rsize 320
slab size 16384
Alloc chunk: 000003F768100000 (16384)
rsize 224
slab size 16384
Alloc chunk: 000003F768104000 (16384)
rsize 5120
slab size 131072
Alloc chunk: 000003F768120000 (131072)
rsize 448
slab size 16384
Alloc chunk: 000003F768108000 (16384)
rsize 896
slab size 16384
Alloc chunk: 000003F76810C000 (16384)
Pal range alloc: 0x3f767e70000 (0x1000000)
Pagemap.Set 0x3f768a00000
Pagemap.Set 0x3f768a40000
Pagemap.Set 0x3f768a44000
Pagemap.Set 0x3f768a48000
Pagemap.Set 0x3f768a4c000
Pagemap.Set 0x3f768a50000
Pagemap.Set 0x3f768a54000
Pagemap.Set 0x3f768a58000
Pagemap.Set 0x3f768a5c000
Pagemap.Set 0x3f768a60000
Pagemap.Set 0x3f768a64000
Pagemap.Set 0x3f768a68000
Pagemap.Set 0x3f768a6c000
Pagemap.Set 0x3f768a70000
Pagemap.Set 0x3f768a74000
Pagemap.Set 0x3f768a78000
Pagemap.Set 0x3f768a7c000
Pagemap.Set 0x3f768a04000
Pagemap.Set 0x3f768a08000
Pagemap.Set 0x3f768a0c000
Pagemap.Set 0x3f768a10000
Pagemap.Set 0x3f768a14000
Pagemap.Set 0x3f768a18000
Pagemap.Set 0x3f768a1c000
Pagemap.Set 0x3f768a20000
Pagemap.Set 0x3f768a24000
Pagemap.Set 0x3f768a28000
Pagemap.Set 0x3f768a2c000
Pagemap.Set 0x3f768a30000
Pagemap.Set 0x3f768a34000
Pagemap.Set 0x3f768a38000
Pagemap.Set 0x3f768a3c000
Pagemap.Set 0x3f768a80000
Pagemap.Set 0x3f768a84000
Pagemap.Set 0x3f768a88000
Pagemap.Set 0x3f768a8c000
Pagemap.Set 0x3f768a90000
Pagemap.Set 0x3f768a94000
Pagemap.Set 0x3f768a98000
Pagemap.Set 0x3f768a9c000
Pagemap.Set 0x3f768aa0000
Pagemap.Set 0x3f768aa4000
Pagemap.Set 0x3f768aa8000
Pagemap.Set 0x3f768aac000
Pagemap.Set 0x3f768ab0000
Pagemap.Set 0x3f768ab4000
Pagemap.Set 0x3f768ab8000
Pagemap.Set 0x3f768abc000
Pagemap.Set 0x3f768ac0000
Pagemap.Set 0x3f768ac4000
Pagemap.Set 0x3f768ac8000
Pagemap.Set 0x3f768acc000
Pagemap.Set 0x3f768ad0000
Pagemap.Set 0x3f768ad4000
Pagemap.Set 0x3f768ad8000
Pagemap.Set 0x3f768adc000
Pagemap.Set 0x3f768ae0000
Pagemap.Set 0x3f768ae4000
Pagemap.Set 0x3f768ae8000
Pagemap.Set 0x3f768aec000
Pagemap.Set 0x3f768af0000
Pagemap.Set 0x3f768af4000
Pagemap.Set 0x3f768af8000
Pagemap.Set 0x3f768afc000
Pagemap.Set 0x3f768b00000
Pagemap.Set 0x3f768b04000
Pagemap.Set 0x3f768b08000
Pagemap.Set 0x3f768b0c000
Pagemap.Set 0x3f768b10000
Pagemap.Set 0x3f768b14000
Pagemap.Set 0x3f768b18000
Pagemap.Set 0x3f768b1c000
Pagemap.Set 0x3f768b20000
Pagemap.Set 0x3f768b24000
Pagemap.Set 0x3f768b28000
Pagemap.Set 0x3f768b2c000
Pagemap.Set 0x3f768b30000
Pagemap.Set 0x3f768b34000
Pagemap.Set 0x3f768b38000
Pagemap.Set 0x3f768b3c000
Pagemap.Set 0x3f768b40000
Pagemap.Set 0x3f768b44000
Pagemap.Set 0x3f768b48000
Pagemap.Set 0x3f768b4c000
Pagemap.Set 0x3f768b50000
Pagemap.Set 0x3f768b54000
Pagemap.Set 0x3f768b58000
Pagemap.Set 0x3f768b5c000
Pagemap.Set 0x3f768b60000
Pagemap.Set 0x3f768b64000
Pagemap.Set 0x3f768b68000
Pagemap.Set 0x3f768b6c000
Pagemap.Set 0x3f768b70000
Pagemap.Set 0x3f768b74000
Pagemap.Set 0x3f768b78000
Pagemap.Set 0x3f768b7c000
Pagemap.Set 0x3f768b80000
Pagemap.Set 0x3f768b84000
Pagemap.Set 0x3f768b88000
Pagemap.Set 0x3f768b8c000
Pagemap.Set 0x3f768b90000
Pagemap.Set 0x3f768b94000
Pagemap.Set 0x3f768b98000
Pagemap.Set 0x3f768b9c000
Pagemap.Set 0x3f768ba0000
Pagemap.Set 0x3f768ba4000
Pagemap.Set 0x3f768ba8000
Pagemap.Set 0x3f768bac000
Pagemap.Set 0x3f768bb0000
Pagemap.Set 0x3f768bb4000
Pagemap.Set 0x3f768bb8000
Pagemap.Set 0x3f768bbc000
Pagemap.Set 0x3f768bc0000
Pagemap.Set 0x3f768bc4000
Pagemap.Set 0x3f768bc8000
Pagemap.Set 0x3f768bcc000
Pagemap.Set 0x3f768bd0000
Pagemap.Set 0x3f768bd4000
Pagemap.Set 0x3f768bd8000
Pagemap.Set 0x3f768bdc000
Pagemap.Set 0x3f768be0000
Pagemap.Set 0x3f768be4000
Pagemap.Set 0x3f768be8000
Pagemap.Set 0x3f768bec000
Pagemap.Set 0x3f768bf0000
Pagemap.Set 0x3f768bf4000
Pagemap.Set 0x3f768bf8000
Pagemap.Set 0x3f768bfc000
Pagemap.Set 0x3f768000000
Pagemap.Set 0x3f768004000
Pagemap.Set 0x3f768008000
Pagemap.Set 0x3f76800c000
Pagemap.Set 0x3f768010000
Pagemap.Set 0x3f768014000
Pagemap.Set 0x3f768018000
Pagemap.Set 0x3f76801c000
Pagemap.Set 0x3f768020000
Pagemap.Set 0x3f768024000
Pagemap.Set 0x3f768028000
Pagemap.Set 0x3f76802c000
Pagemap.Set 0x3f768030000
Pagemap.Set 0x3f768034000
Pagemap.Set 0x3f768038000
Pagemap.Set 0x3f76803c000
Pagemap.Set 0x3f768040000
Pagemap.Set 0x3f768044000
Pagemap.Set 0x3f768048000
Pagemap.Set 0x3f76804c000
Pagemap.Set 0x3f768050000
Pagemap.Set 0x3f768054000
Pagemap.Set 0x3f768058000
Pagemap.Set 0x3f768080000
Pagemap.Set 0x3f768084000
Pagemap.Set 0x3f768088000
Pagemap.Set 0x3f76808c000
Pagemap.Set 0x3f768090000
Pagemap.Set 0x3f768094000
Pagemap.Set 0x3f768098000
Pagemap.Set 0x3f76809c000
Pagemap.Set 0x3f7680a0000
Pagemap.Set 0x3f7680a4000
Pagemap.Set 0x3f7680a8000
Pagemap.Set 0x3f7680ac000
Pagemap.Set 0x3f7680b0000
Pagemap.Set 0x3f7680b4000
Pagemap.Set 0x3f7680b8000
Pagemap.Set 0x3f7680bc000
Pagemap.Set 0x3f7680c0000
Pagemap.Set 0x3f7680c4000
Pagemap.Set 0x3f7680c8000
Pagemap.Set 0x3f7680cc000
Pagemap.Set 0x3f7680d0000
Pagemap.Set 0x3f7680d4000
Pagemap.Set 0x3f7680d8000
Pagemap.Set 0x3f7680dc000
Pagemap.Set 0x3f7680e0000
Pagemap.Set 0x3f7680e4000
Pagemap.Set 0x3f7680e8000
Pagemap.Set 0x3f7680ec000
Pagemap.Set 0x3f7680f0000
Pagemap.Set 0x3f7680f4000
Pagemap.Set 0x3f7680f8000
Pagemap.Set 0x3f7680fc000
Pagemap.Set 0x3f76805c000
Pagemap.Set 0x3f768060000
Pagemap.Set 0x3f768064000
Pagemap.Set 0x3f768068000
Pagemap.Set 0x3f76806c000
Pagemap.Set 0x3f768070000
Pagemap.Set 0x3f768074000
Pagemap.Set 0x3f768078000
Pagemap.Set 0x3f76807c000
Pagemap.Set 0x3f768100000
Pagemap.Set 0x3f768104000
Pagemap.Set 0x3f768120000
Pagemap.Set 0x3f768124000
Pagemap.Set 0x3f768128000
Pagemap.Set 0x3f76812c000
Pagemap.Set 0x3f768130000
Pagemap.Set 0x3f768134000
Pagemap.Set 0x3f768138000
Pagemap.Set 0x3f76813c000
Pagemap.Set 0x3f768108000
Pagemap.Set 0x3f76810c000
Pagemap.Set 0x3f768110000
Pagemap.Set 0x3f768114000
Pagemap.Set 0x3f768118000
Pagemap.Set 0x3f76811c000
Pagemap.Set 0x3f768140000
Pagemap.Set 0x3f768144000
Pagemap.Set 0x3f768150000
Pagemap.Set 0x3f768154000
Pagemap.Set 0x3f768158000
Pagemap.Set 0x3f76815c000
Pagemap.Set 0x3f768160000
Pagemap.Set 0x3f768164000
Pagemap.Set 0x3f768168000
Pagemap.Set 0x3f76816c000
Pagemap.Set 0x3f768170000
Pagemap.Set 0x3f768174000
Pagemap.Set 0x3f768178000
Pagemap.Set 0x3f76817c000
Pagemap.Set 0x3f768148000
Pagemap.Set 0x3f768180000
Pagemap.Set 0x3f768184000
Pagemap.Set 0x3f768188000
Pagemap.Set 0x3f76818c000
Pagemap.Set 0x3f768190000
Pagemap.Set 0x3f768194000
Pagemap.Set 0x3f768198000
Pagemap.Set 0x3f76819c000
Pagemap.Set 0x3f76814c000
Pagemap.Set 0x3f7681c0000
Pagemap.Set 0x3f7681c4000
Pagemap.Set 0x3f7681c8000
Pagemap.Set 0x3f7681cc000
Pagemap.Set 0x3f7681d0000
Pagemap.Set 0x3f7681d4000
Pagemap.Set 0x3f7681d8000
Pagemap.Set 0x3f7681dc000
Pagemap.Set 0x3f7681e0000
Pagemap.Set 0x3f7681e4000
Pagemap.Set 0x3f7681e8000
Pagemap.Set 0x3f7681ec000
Pagemap.Set 0x3f7681f0000
Pagemap.Set 0x3f7681f4000
Pagemap.Set 0x3f7681f8000
Pagemap.Set 0x3f7681fc000
Pagemap.Set 0x3f7681a0000
Pagemap.Set 0x3f7681a4000
rsize 1536
slab size 32768
Alloc chunk: 000003F768110000 (32768)
rsize 1792
slab size 32768
Alloc chunk: 000003F768118000 (32768)
Slab is woken up
rsize 1280
slab size 32768
Alloc chunk: 000003F768140000 (32768)
rsize 3072
slab size 65536
Alloc chunk: 000003F768150000 (65536)
Slab is woken up
Slab is woken up
Slab is woken up
rsize 2560
slab size 65536
Alloc chunk: 000003F768160000 (65536)
Slab is woken up
Slab is woken up
rsize 4096
slab size 65536
Alloc chunk: 000003F768170000 (65536)
rsize 512
slab size 16384
Alloc chunk: 000003F768148000 (16384)
Slab is woken up
Slab is woken up
Slab is woken up
Slab is woken up
rsize 6144
slab size 131072
Alloc chunk: 000003F768180000 (131072)
Slab is woken up
Slab is woken up
Slab is woken up
rsize 96
slab size 16384
Alloc chunk: 000003F76814C000 (16384)
Slab is woken up
Slab is woken up
rsize 10240
slab size 262144
Alloc chunk: 000003F7681C0000 (262144)
Slab is woken up
Slab is woken up
Slab is woken up
rsize 8192
slab size 131072
Alloc chunk: 000003F7681A0000 (131072)
rsize 384
slab size 16384
Alloc chunk: 000003F768200000 (16384)
Pagemap.Set 0x3f7681a8000
Pagemap.Set 0x3f7681ac000
Pagemap.Set 0x3f7681b0000
Pagemap.Set 0x3f7681b4000
Pagemap.Set 0x3f7681b8000
Pagemap.Set 0x3f7681bc000
Pagemap.Set 0x3f768200000
Heap corruption - free list corrupted!
Exception: Exception 0xc0000409 encountered at address 0x7ffd37a2286e

Any ideas what might be going wrong here? Or suggestions on how to proceed debugging it?

This pr tracks the work of adding a static linking target for rust support.

My suggestion is that the merging should be considered after the discussions in #109 and https://github.com/rust-lang/rust/pull/68381 are finished.

Old libc versions (e.g. glibc < 2.25) do not provide a getentropy function.

As discussed in #328 I added a CMake compile check and exposed a flag LIBC_HAS_GETENTROPY and put the getentropy into #ifdefs, while also disabling the Entropy PAL feature.

Keeping this as a draft for now, as I didnt check on every platform. On a linux system with libc 2.17 it compiles fine with this change, where it previously broke.

Should I verify with some test that the PAL is actually providing some good entropy?

I'm trying different memory allocators for a key value store project. On the left is the rss usage with snmalloc(using rust crate) and on the right is jemalloc(using tikv-jemalloc crate which provides binding for 5.2.1). I wonder when snmalloc will release memory back to the os?

Hello, Would you mind applying these fixes to make the codebase compile with Clang 10 please? Many thanks!

diff --git a/src/ds/aba.h b/src/ds/aba.h
index 21950ff..60dcc2f 100644
--- a/src/ds/aba.h
+++ b/src/ds/aba.h
@@ -121,6 +121,7 @@ namespace snmalloc
       }
 
       Cmp(const Cmp&) = delete;
+      Cmp(Cmp&&) = default;
     };
 
     // This method is used in Verona
diff --git a/src/ds/helpers.h b/src/ds/helpers.h
index b479011..8f5b83c 100644
--- a/src/ds/helpers.h
+++ b/src/ds/helpers.h
@@ -14,7 +14,7 @@ namespace snmalloc
   class Singleton
   {
     inline static std::atomic_flag flag;
-    inline static std::atomic<bool> initialised = false;
+    inline static std::atomic<bool> initialised{};
     inline static Object obj;
 
   public:
diff --git a/src/ds/mpscq.h b/src/ds/mpscq.h
index d5d5161..a397354 100644
--- a/src/ds/mpscq.h
+++ b/src/ds/mpscq.h
@@ -14,7 +14,7 @@ namespace snmalloc
       std::is_same<decltype(T::next), std::atomic<T*>>::value,
       "T->next must be a std::atomic<T*>");
 
-    std::atomic<T*> back = nullptr;
+    std::atomic<T*> back{};
     T* front = nullptr;
 
   public:
@@ -72,10 +72,10 @@ namespace snmalloc
         SNMALLOC_ASSERT(front);
         std::atomic_thread_fence(std::memory_order_acquire);
         invariant();
-        return std::pair(first, true);
+        return std::pair<T*, bool>(first, true);
       }
 
-      return std::pair(nullptr, false);
+      return std::pair<T*, bool>(nullptr, false);
     }
   };
 } // namespace snmalloc
diff --git a/src/mem/alloc.h b/src/mem/alloc.h
index c58015b..4f8aed8 100644
--- a/src/mem/alloc.h
+++ b/src/mem/alloc.h
@@ -240,6 +240,9 @@ namespace snmalloc
     template<size_t size>
     void dealloc(void* p)
     {
+      if (p == nullptr)
+        return;
+
 #ifdef USE_MALLOC
       UNUSED(size);
       return free(p);
@@ -280,6 +283,9 @@ namespace snmalloc
      */
     SNMALLOC_FAST_PATH void dealloc(void* p, size_t size)
     {
+      if (p == nullptr)
+        return;
+
 #ifdef USE_MALLOC
       UNUSED(size);
       return free(p);
@@ -302,6 +308,9 @@ namespace snmalloc
 
     SNMALLOC_SLOW_PATH void dealloc_sized_slow(void* p, size_t size)
     {
+      if (p == nullptr)
+        return;
+
       if (size == 0)
         return dealloc(p, 1);
 
@@ -325,6 +334,9 @@ namespace snmalloc
      */
     SNMALLOC_FAST_PATH void dealloc(void* p)
     {
+      if (p == nullptr)
+        return;
+
 #ifdef USE_MALLOC
       return free(p);
 #else
diff --git a/src/mem/pagemap.h b/src/mem/pagemap.h
index fc3da85..e658cd3 100644
--- a/src/mem/pagemap.h
+++ b/src/mem/pagemap.h
@@ -188,7 +188,7 @@ namespace snmalloc
       {
         PagemapEntry* value = get_node<create_addr>(e, result);
         if (unlikely(!result))
-          return std::pair(nullptr, 0);
+          return std::pair<Leaf*, size_t>(nullptr, 0);
 
         shift -= BITS_PER_INDEX_LEVEL;
         ix = (static_cast<size_t>(addr) >> shift) & ENTRIES_MASK;
@@ -208,11 +208,11 @@ namespace snmalloc
       Leaf* leaf = reinterpret_cast<Leaf*>(get_node<create_addr>(e, result));
 
       if (unlikely(!result))
-        return std::pair(nullptr, 0);
+        return std::pair<Leaf*, size_t>(nullptr, 0);
 
       shift -= BITS_FOR_LEAF;
       ix = (static_cast<size_t>(addr) >> shift) & LEAF_MASK;
-      return std::pair(leaf, ix);
+      return std::pair<Leaf*, size_t>(leaf, ix);
     }
 
     template<bool create_addr>
diff --git a/src/mem/pooled.h b/src/mem/pooled.h
index 71f8d91..f044a77 100644
--- a/src/mem/pooled.h
+++ b/src/mem/pooled.h
@@ -14,7 +14,7 @@ namespace snmalloc
     friend class MPMCStack;
 
     /// Used by the pool for chaining together entries when not in use.
-    std::atomic<T*> next = nullptr;
+    std::atomic<T*> next{};
     /// Used by the pool to keep the list of all entries ever created.
     T* list_next;
     std::atomic_flag in_use = ATOMIC_FLAG_INIT;
diff --git a/src/mem/remoteallocator.h b/src/mem/remoteallocator.h
index 4c1f906..6d8ae55 100644
--- a/src/mem/remoteallocator.h
+++ b/src/mem/remoteallocator.h
@@ -19,7 +19,7 @@ namespace snmalloc
     union
     {
       Remote* non_atomic_next;
-      std::atomic<Remote*> next = nullptr;
+      std::atomic<Remote*> next{};
     };
 
     alloc_id_t allocator_id;
diff --git a/src/mem/slowalloc.h b/src/mem/slowalloc.h
index f2fcba1..0ab1169 100644
--- a/src/mem/slowalloc.h
+++ b/src/mem/slowalloc.h
@@ -63,6 +63,6 @@ namespace snmalloc
    */
   inline SlowAllocator get_slow_allocator()
   {
-    return SlowAllocator{};
+    return {};
   }
 } // namespace snmalloc
diff --git a/src/pal/pal_consts.h b/src/pal/pal_consts.h
index 538f90d..fe9a38d 100644
--- a/src/pal/pal_consts.h
+++ b/src/pal/pal_consts.h
@@ -78,7 +78,7 @@ namespace snmalloc
     /**
      * List of callbacks to notify
      */
-    std::atomic<PalNotificationObject*> callbacks = nullptr;
+    std::atomic<PalNotificationObject*> callbacks{};
 
   public:
     /**
diff --git a/src/pal/pal_windows.h b/src/pal/pal_windows.h
index ba7fcea..8015827 100644
--- a/src/pal/pal_windows.h
+++ b/src/pal/pal_windows.h
@@ -17,7 +17,7 @@
 #  ifdef NTDDI_WIN10_RS5
 #    if (NTDDI_VERSION >= NTDDI_WIN10_RS5) && \
       (WINVER >= _WIN32_WINNT_WIN10) && !defined(USE_SYSTEMATIC_TESTING)
-#      define PLATFORM_HAS_VIRTUALALLOC2
+//#      define PLATFORM_HAS_VIRTUALALLOC2
 #    endif
 #  endif

Hello, I'm seeing a 100% crash in snmalloc's realloc function, in malloc.cc, L92. The crash is caused by the sz being larger than the size initially allocated. We're trying to (re)allocate a large buffer:

llvm-mc.exe!realloc(void * ptr=0x0000023244000000, unsigned __int64 size=0x00000000027fffff)

In malloc.cc, L82, Alloc::alloc_size(ptr); finds sz to be 0x0000000002000000 bytes. However the caller knows the buffer is 0x013fffff. So we end up crashing on rep movsb while copying the buffer to the new location. I can see alloc_size(..) returning at alloc.h, L500, ie. return 1ULL << size; where size is 25.

I haven't investigated any further. I am at git checkout a43773c5b7e1fc291ddd1580fec04a574ef886e1. I'm trying to integrate snmalloc in LLVM to compare it with other modern allocators. Currently half a dozen tests are failing because of this. Any suggestions? I can provide with a repro if needed.

I'm certain there's something wrong with this PR, but I am not certain what, exactly.

I found it confusing that there were two things that called themselves PageMap and kept tripping over the difference when working on #41. So, in order to reduce my tripping rate and in hopes that I might help others do the same, I propose that we rename the alloc.h Pagemap thing a Slabmap. Not great, but at least different.

Having done that, it seemed to make sense to ensure that all accesses from the Allocator to the Pagemap were via the Slabmap. And then it became apparent that everything was static, in fact, and so some additional cleanups were possible.

I suspect there is something I'm not getting; perhaps a different, out-of-tree setting for SNMALLOC_DEFAULT_PAGEMAP breaks this? In any case, let's see what CI thinks about it, since it works for me on my desk...

QEMU 7.1 will begin loongarch support: https://wiki.qemu.org/ChangeLog/7.1#LoongArch.

Currently, everything works but all check variants:

qemu-loongarch64 -strace /home/schrodinger/Downloads/loongarch64-clfs-6.0-cross-tools-gcc_and_clang-full/cross-tools/target/usr/lib64/ld-linux-loongarch-lp64d.so.1 --library-path /home/schrodinger/Downloads/loongarch64-clfs-6.0-cross-tools-gcc_and_clang-full/cross-tools/target/usr/lib64 ./func-thread_alloc_external-check

On twitter @richfelker has said we should consider providing __builtin_dynamic_object_size as a more comprehensive way to provide guarded memcpy like features:

I'd love if there were some clean agreed upon way to get the size as a stronger version of __builtin_dynamic_object_size so this would be usable in all the FORTIFYable interfaces not just memcpy. twitter

and

The right way to do this is the way fortify works. Not putting linkage to malloc in the external memcpy, but providing an enhanced __builtin_dynamic_object_size that can query the allocator for knowledge of size. twitter

I'm raising this issue to build a discussion of what this should mean with snmalloc.

The Clang documentation is here: https://clang.llvm.org/docs/LanguageExtensions.html#evaluating-object-size-dynamically

Here is the LLVM review adding it https://reviews.llvm.org/D56760

It doesn't seem it works from an offset into an object. But I think this is worth experimenting with, as it would allow the compiler to remove some checks, and then what is left to be passed to the snmalloc routine.

I think we could define __builtin_dynamic_object_size as just remaining_bytes from snmalloc.

@davidchisnall thoughts? Is there something sensible that could be experimented with here?

in allocations before returning to user. This is important on CHERI to avoid leaking capabilities and may also reduce the attack surface on other architecutres. This includes: Freelist pointers. RBTree metadata used by smallbuddyallocator.

I think there are two points to improve in current cmpxchg support for 16 byte data structures:

• consider arm lse
• consider using __sync_bool_compare_and_swap to force gcc emit inlined instructions.

#include <atomic>

#if defined(__aarch64__) && defined(__clang__)
#  pragma clang attribute push(__attribute__((target("lse"))),apply_to=function)
#  define PLATFORM_SPECIFIC_OPTIONS_ENDING _Pragma("clang attribute pop")
#elif defined(__aarch64__) && defined(__GNUC__)
#  pragma GCC push_options
#  pragma GCC target("arch=armv8-a+lse")
#  define PLATFORM_SPECIFIC_OPTIONS_ENDING _Pragma("GCC pop_options")
#elif defined(__x86_64__) && defined(__clang__)
#  pragma clang attribute push(__attribute__((target("cx16"))),apply_to=function)
#  define PLATFORM_SPECIFIC_OPTIONS_ENDING _Pragma("clang attribute pop")
#elif defined(__x86_64__) && defined(__GNUC__)
#  pragma GCC push_options
#  pragma GCC target("cx16")
#  define PLATFORM_SPECIFIC_OPTIONS_ENDING _Pragma("GCC pop_options")
#else 
#  define PLATFORM_SPECIFIC_OPTIONS_ENDING
#endif

template<class T>
__attribute__((always_inline)) inline bool cas(std::atomic<T> &src,
                T const& __restrict cmp,
                T const& __restrict with)
{
    auto inline_copy = [](__int128 * dst, const void * __restrict src) {
#if __has_builtin(__builtin_inline_memcpy)
    __builtin_inline_memcpy(dst, src, sizeof(__int128));
#elif __has_builtin(__builtin_memcpy)
    __builtin_memcpy(dst, src, sizeof(__int128));
#else
    ::memcpy(dst, src, sizeof(__int128));
#endif
    };
    __int128 cmp_value;
    __int128 with_value;
    inline_copy(&cmp_value, &cmp);
    inline_copy(&with_value, &with);
    return __sync_bool_compare_and_swap(reinterpret_cast<__int128 *>(&src), cmp_value, with_value);
}

struct A {
    int64_t a, b;
};

bool cas_test(std::atomic<__int128> &src,
                __int128 const& cmp,
                __int128 const& with)
{
    return cas(src, cmp, with);
}

bool cas_test(std::atomic<A> &src,
                A const& cmp,
                A const& with)
{
    return cas(src, cmp, with);
}

PLATFORM_SPECIFIC_OPTIONS_ENDING

bool cas_test2(std::atomic<__int128> &src,
                __int128 & cmp,
                __int128 & with)
{
    return src.compare_exchange_weak(cmp, with);
}

I am not seeing how to use the new hardening features when including snmalloc as a header-only library on Windows. There doesn't seem to be any #define or similar mentioned in the documentation? Is it just enabled by default?