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0b5fa792e1
This fixes the Windows platform, where the aligned allocation path is not malloc-compatible. It won't have any observable difference on Darwin or Linux, aside from manually allocated memory on Linux now being consistently 16-byte aligned (heap objects will still be 8-byte aligned on Linux). It is unfortunate that we can't guarantee Swift-allocated memory via Unsafe*Pointer is malloc compatible on Windows. It would have been nice for that to be a cross platform guarantee since it's normal to allocate in C and deallocate in Swift or vice-versa. Now we have to tell developers to always use _aligned_malloc/_aligned_free when transitioning between Swift/C if they expect their code to work on Windows. Even though this fix isn't required today on Darwin/Linux, it makes good sense to guarantee that the allocation/deallocation paths are consistent. This is done by specifying a constant that stdlib can use to round up alignment, _swift_MinAllocationAlignment. The runtime asserts that this constant is greater than MALLOC_ALIGN_MASK for all platforms. This way, manually allocated buffers will always use the aligned allocation path. If users specify an alignment less than m round up so users don't need to pass the same alignment to deallocate the buffer). This constant does not need to be ABI. Alternatives are: 1. Require users of Unsafe*Pointer to specify the same alignment during deallocation. This is obviously madness. 2. Introduce new runtime entry points: swift_alignedAlloc/swift_alignedDealloc, introduce corresponding new builtins, and have Unsafe*Pointer always call those. This would make the runtime API a little more obvious but would introduce complexity in other areas of the compiler and it doesn't have any other significant benefit. Less than 16-byte alignment of manually allocated buffers on Linux is a non-goal.
114 lines
4.3 KiB
C++
114 lines
4.3 KiB
C++
//===--- Heap.cpp - Swift Language Heap Logic -----------------------------===//
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//
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// This source file is part of the Swift.org open source project
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//
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// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
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// Licensed under Apache License v2.0 with Runtime Library Exception
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//
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// See https://swift.org/LICENSE.txt for license information
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// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
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//
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//===----------------------------------------------------------------------===//
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//
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// Implementations of the Swift heap
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//
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//===----------------------------------------------------------------------===//
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#include "swift/Runtime/HeapObject.h"
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#include "swift/Runtime/Heap.h"
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#include "Private.h"
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#include "swift/Runtime/Debug.h"
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#include "../SwiftShims/RuntimeShims.h"
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#include <algorithm>
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#include <stdlib.h>
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using namespace swift;
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#if defined(__APPLE__)
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// Apple malloc is always 16-byte aligned.
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# define MALLOC_ALIGN_MASK 15
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#elif defined(__linux__)
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// Linux malloc is 16-byte aligned on 64-bit, and 8-byte aligned on 32-bit.
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# if defined(__LP64)
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# define MALLOC_ALIGN_MASK 15
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# else
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# define MALLOC_ALIGN_MASK 7
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# endif
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#elif defined(_WIN64)
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// Windows malloc is 16-byte aligned on 64-bit and 8-byte aligned on 32-bit.
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# define MALLOC_ALIGN_MASK 15
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#elif defined(_WIN32)
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# define MALLOC_ALIGN_MASK 7
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#else
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// Unknown alignment, but the standard requires alignment suitable for the largest
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// standard types.
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# define MALLOC_ALIGN_MASK std::max(alignof(void *), alignof(double))
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#endif
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// This assert ensures that manually allocated memory always uses the
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// AlignedAlloc path. The stdlib will use "default" alignment for any user
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// requested alignment less than or equal to _swift_MinAllocationAlignment. The
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// runtime must ensure that any alignment > _swift_MinAllocationAlignment also
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// uses the "aligned" deallocation path.
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static_assert(_swift_MinAllocationAlignment > MALLOC_ALIGN_MASK,
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"Swift's default alignment must exceed platform malloc mask.");
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// When alignMask == ~(size_t(0)), allocation uses the "default"
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// _swift_MinAllocationAlignment. This is different than calling swift_slowAlloc
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// with `alignMask == _swift_MinAllocationAlignment - 1` because it forces
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// the use of AlignedAlloc. This allows manually allocated to memory to always
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// be deallocated with AlignedFree without knowledge of its original allocation
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// alignment.
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//
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// For alignMask > (_minAllocationAlignment-1)
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// i.e. alignment == 0 || alignment > _minAllocationAlignment:
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// The runtime must use AlignedAlloc, and the standard library must
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// deallocate using an alignment that meets the same condition.
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//
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// For alignMask <= (_minAllocationAlignment-1)
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// i.e. 0 < alignment <= _minAllocationAlignment:
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// The runtime may use either malloc or AlignedAlloc, and the standard library
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// must deallocate using an identical alignment.
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void *swift::swift_slowAlloc(size_t size, size_t alignMask) {
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void *p;
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// This check also forces "default" alignment to use AlignedAlloc.
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if (alignMask <= MALLOC_ALIGN_MASK) {
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p = malloc(size);
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} else {
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size_t alignment = (alignMask == ~(size_t(0)))
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? _swift_MinAllocationAlignment
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: alignMask + 1;
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p = AlignedAlloc(size, alignment);
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}
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if (!p) swift::crash("Could not allocate memory.");
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return p;
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}
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// Unknown alignment is specified by passing alignMask == ~(size_t(0)), forcing
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// the AlignedFree deallocation path for unknown alignment. The memory
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// deallocated with unknown alignment must have been allocated with either
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// "default" alignment, or alignment > _swift_MinAllocationAlignment, to
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// guarantee that it was allocated with AlignedAlloc.
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//
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// The standard library assumes the following behavior:
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//
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// For alignMask > (_minAllocationAlignment-1)
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// i.e. alignment == 0 || alignment > _minAllocationAlignment:
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// The runtime must use AlignedFree.
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//
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// For alignMask <= (_minAllocationAlignment-1)
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// i.e. 0 < alignment <= _minAllocationAlignment:
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// The runtime may use either `free` or AlignedFree as long as it is
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// consistent with allocation with the same alignment.
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void swift::swift_slowDealloc(void *ptr, size_t bytes, size_t alignMask) {
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if (alignMask <= MALLOC_ALIGN_MASK) {
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free(ptr);
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} else {
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AlignedFree(ptr);
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}
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}
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