malloc.go

     1  // Copyright 2025 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package gc
     6  
     7  import "internal/goarch"
     8  
     9  const (
    10  	ptrBits = 8 * goarch.PtrSize
    11  
    12  	// A malloc header is functionally a single type pointer, but
    13  	// we need to use 8 here to ensure 8-byte alignment of allocations
    14  	// on 32-bit platforms. It's wasteful, but a lot of code relies on
    15  	// 8-byte alignment for 8-byte atomics.
    16  	MallocHeaderSize = 8
    17  
    18  	// The minimum object size that has a malloc header, exclusive.
    19  	//
    20  	// The size of this value controls overheads from the malloc header.
    21  	// The minimum size is bound by writeHeapBitsSmall, which assumes that the
    22  	// pointer bitmap for objects of a size smaller than this doesn't cross
    23  	// more than one pointer-word boundary. This sets an upper-bound on this
    24  	// value at the number of bits in a uintptr, multiplied by the pointer
    25  	// size in bytes.
    26  	//
    27  	// We choose a value here that has a natural cutover point in terms of memory
    28  	// overheads. This value just happens to be the maximum possible value this
    29  	// can be.
    30  	//
    31  	// A span with heap bits in it will have 128 bytes of heap bits on 64-bit
    32  	// platforms, and 256 bytes of heap bits on 32-bit platforms. The first size
    33  	// class where malloc headers match this overhead for 64-bit platforms is
    34  	// 512 bytes (8 KiB / 512 bytes * 8 bytes-per-header = 128 bytes of overhead).
    35  	// On 32-bit platforms, this same point is the 256 byte size class
    36  	// (8 KiB / 256 bytes * 8 bytes-per-header = 256 bytes of overhead).
    37  	//
    38  	// Guaranteed to be exactly at a size class boundary. The reason this value is
    39  	// an exclusive minimum is subtle. Suppose we're allocating a 504-byte object
    40  	// and its rounded up to 512 bytes for the size class. If minSizeForMallocHeader
    41  	// is 512 and an inclusive minimum, then a comparison against minSizeForMallocHeader
    42  	// by the two values would produce different results. In other words, the comparison
    43  	// would not be invariant to size-class rounding. Eschewing this property means a
    44  	// more complex check or possibly storing additional state to determine whether a
    45  	// span has malloc headers.
    46  	MinSizeForMallocHeader = goarch.PtrSize * ptrBits
    47  )
    48
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