// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Fixed-size object allocator. Returned memory is not zeroed.
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//
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// See malloc.go for overview.
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package runtime
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import "unsafe"
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// FixAlloc is a simple free-list allocator for fixed size objects.
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// Malloc uses a FixAlloc wrapped around sysAlloc to manage its
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// mcache and mspan objects.
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//
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// Memory returned by fixalloc.alloc is zeroed by default, but the
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// caller may take responsibility for zeroing allocations by setting
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// the zero flag to false. This is only safe if the memory never
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// contains heap pointers.
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//
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// The caller is responsible for locking around FixAlloc calls.
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// Callers can keep state in the object but the first word is
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// smashed by freeing and reallocating.
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//
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// Consider marking fixalloc'd types go:notinheap.
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type fixalloc struct {
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size uintptr
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first func(arg, p unsafe.Pointer) // called first time p is returned
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arg unsafe.Pointer
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list *mlink
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chunk uintptr // use uintptr instead of unsafe.Pointer to avoid write barriers
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nchunk uint32
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inuse uintptr // in-use bytes now
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stat *uint64
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zero bool // zero allocations
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}
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// A generic linked list of blocks. (Typically the block is bigger than sizeof(MLink).)
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// Since assignments to mlink.next will result in a write barrier being performed
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// this cannot be used by some of the internal GC structures. For example when
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// the sweeper is placing an unmarked object on the free list it does not want the
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// write barrier to be called since that could result in the object being reachable.
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//
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//go:notinheap
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type mlink struct {
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next *mlink
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}
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// Initialize f to allocate objects of the given size,
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// using the allocator to obtain chunks of memory.
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func (f *fixalloc) init(size uintptr, first func(arg, p unsafe.Pointer), arg unsafe.Pointer, stat *uint64) {
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f.size = size
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f.first = first
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f.arg = arg
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f.list = nil
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f.chunk = 0
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f.nchunk = 0
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f.inuse = 0
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f.stat = stat
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f.zero = true
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}
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func (f *fixalloc) alloc() unsafe.Pointer {
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if f.size == 0 {
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print("runtime: use of FixAlloc_Alloc before FixAlloc_Init\n")
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throw("runtime: internal error")
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}
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if f.list != nil {
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v := unsafe.Pointer(f.list)
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f.list = f.list.next
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f.inuse += f.size
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if f.zero {
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memclrNoHeapPointers(v, f.size)
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}
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return v
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}
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if uintptr(f.nchunk) < f.size {
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f.chunk = uintptr(persistentalloc(_FixAllocChunk, 0, f.stat))
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f.nchunk = _FixAllocChunk
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}
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v := unsafe.Pointer(f.chunk)
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if f.first != nil {
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f.first(f.arg, v)
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}
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f.chunk = f.chunk + f.size
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f.nchunk -= uint32(f.size)
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f.inuse += f.size
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return v
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}
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func (f *fixalloc) free(p unsafe.Pointer) {
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f.inuse -= f.size
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v := (*mlink)(p)
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v.next = f.list
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f.list = v
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}
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