3d403aef7f
The primary aim for this refactor is to use a representation for sizeclasses that uniformly covers both large and small. This allows certain operations such as alloc_size and external_pointer to be uniformly implemented. The additional types make clear which kind of sizeclass is in use. This also tidies up the code for sizeclass based divisible by and modulus. It fixes a bug in rust_realloc that didn't correctly determine a realloc was required for large classes.
96 lines
2.8 KiB
C++
96 lines
2.8 KiB
C++
#pragma once
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#include "../ds/bits.h"
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#include "../pal/pal.h"
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namespace snmalloc
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{
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// 0 intermediate bits results in power of 2 small allocs. 1 intermediate
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// bit gives additional sizeclasses at the midpoint between each power of 2.
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// 2 intermediate bits gives 3 intermediate sizeclasses, etc.
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static constexpr size_t INTERMEDIATE_BITS =
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#ifdef USE_INTERMEDIATE_BITS
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USE_INTERMEDIATE_BITS
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#else
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2
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#endif
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;
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enum DecommitStrategy
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{
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/**
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* Never decommit memory.
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*/
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DecommitNone,
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/**
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* Decommit superslabs when they are entirely empty.
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*/
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DecommitSuper,
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/**
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* Decommit superslabs only when we are informed of memory pressure by the
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* OS, do not decommit anything in normal operation.
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*/
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DecommitSuperLazy
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};
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static constexpr DecommitStrategy decommit_strategy =
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#ifdef USE_DECOMMIT_STRATEGY
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USE_DECOMMIT_STRATEGY
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#elif defined(_WIN32) && !defined(OPEN_ENCLAVE)
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DecommitSuperLazy
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#else
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DecommitSuper
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#endif
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;
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// The remaining values are derived, not configurable.
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static constexpr size_t POINTER_BITS =
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bits::next_pow2_bits_const(sizeof(uintptr_t));
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// Used to isolate values on cache lines to prevent false sharing.
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static constexpr size_t CACHELINE_SIZE = 64;
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// Minimum allocation size is space for two pointers.
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static_assert(bits::next_pow2_const(sizeof(void*)) == sizeof(void*));
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static constexpr size_t MIN_ALLOC_SIZE = 2 * sizeof(void*);
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static constexpr size_t MIN_ALLOC_BITS = bits::ctz_const(MIN_ALLOC_SIZE);
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// Minimum slab size.
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static constexpr size_t MIN_CHUNK_BITS = bits::max(
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static_cast<size_t>(14), bits::next_pow2_bits_const(OS_PAGE_SIZE));
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static constexpr size_t MIN_CHUNK_SIZE = bits::one_at_bit(MIN_CHUNK_BITS);
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// Minimum number of objects on a slab
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#ifdef SNMALLOC_CHECK_CLIENT
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static constexpr size_t MIN_OBJECT_COUNT = 13;
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#else
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static constexpr size_t MIN_OBJECT_COUNT = 4;
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#endif
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// Maximum size of an object that uses sizeclasses.
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static constexpr size_t MAX_SMALL_SIZECLASS_BITS = 16;
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static constexpr size_t MAX_SMALL_SIZECLASS_SIZE =
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bits::one_at_bit(MAX_SMALL_SIZECLASS_BITS);
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// Number of slots for remote deallocation.
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static constexpr size_t REMOTE_SLOT_BITS = 8;
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static constexpr size_t REMOTE_SLOTS = 1 << REMOTE_SLOT_BITS;
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static constexpr size_t REMOTE_MASK = REMOTE_SLOTS - 1;
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static_assert(
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INTERMEDIATE_BITS < MIN_ALLOC_BITS,
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"INTERMEDIATE_BITS must be less than MIN_ALLOC_BITS");
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static_assert(
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MIN_ALLOC_SIZE >= (sizeof(void*) * 2),
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"MIN_ALLOC_SIZE must be sufficient for two pointers");
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// Return remote small allocs when the local cache reaches this size.
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static constexpr int64_t REMOTE_CACHE =
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#ifdef USE_REMOTE_CACHE
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USE_REMOTE_CACHE
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#else
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1 << MIN_CHUNK_BITS
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#endif
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;
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} // namespace snmalloc
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