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67 lines
1.6 KiB
C
67 lines
1.6 KiB
C
// 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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#include "malloc.h"
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// A PageMap maps page numbers to void* pointers.
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// The AMD64 has 64-bit addresses and 4096-byte pages, so
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// the page numbers are 52 bits. We use a four-level radix tree,
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// with 13 bits for each level. This requires 32 kB per level or
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// 128 kB for a table with one entry. Moving to three levels of 18 bits
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// would require 3 MB for a table with one entry, which seems too expensive.
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// This is easy to change.
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// It may be that a balanced tree would be better anyway.
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// Return the entry for page number pn in m.
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void*
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pmlookup(PageMap *m, uintptr pn)
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{
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int32 i, x;
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void **v;
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v = &m->level0[0];
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for(i=0; i<PMLevels; i++) {
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// Pick off top PMLevelBits bits as index and shift up.
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x = (pn >> (PMBits - PMLevelBits)) & PMLevelMask;
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pn <<= PMLevelBits;
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// Walk down using index.
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v = v[x];
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if(v == nil)
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return nil;
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}
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return v;
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}
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// Set the entry for page number pn in m to s.
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// Return the old value.
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void*
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pminsert(PageMap *m, uintptr pn, void *value)
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{
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int32 i, x;
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void **v, **l;
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l = nil; // shut up 6c
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v = &m->level0[0];
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for(i=0; i<PMLevels; i++) {
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// Pick off top PMLevelBits bits as index and shift up.
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x = (pn >> (PMBits - PMLevelBits)) & PMLevelMask;
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pn <<= PMLevelBits;
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// Walk down using index, but remember location of pointer.
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l = &v[x];
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v = *l;
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// Allocate new level if needed.
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if(v == nil && i < PMLevels-1) {
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v = trivalloc(PMLevelSize * sizeof v[0]);
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*l = v;
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}
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}
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// Record new value and return old.
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*l = value;
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return v;
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}
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