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	std.experimental.allocator.building_blocks.bitmapped_block
- structBitmappedBlock(size_t theBlockSize, uint theAlignment = platformAlignment, ParentAllocator = NullAllocator, Flag!"multiblock" f = Yes.multiblock);
- BitmappedBlockimplements a simple heap consisting of one contiguous area of memory organized in blocks, each of size theBlockSize. A block is a unit of allocation. A bitmap serves as bookkeeping data, more precisely one bit per block indicating whether that block is currently allocated or not.Passing NullAllocator as ParentAllocator (the default) means user code manages allocation of the memory block from the outside; in that case- BitmappedBlockmust be constructed with a ubyte[] preallocated block and has no responsibility regarding the lifetime of its support underlying storage. If another allocator type is passed,- BitmappedBlockdefines a destructor that uses the parent allocator to release the memory block. That makes the combination of AllocatorList,- BitmappedBlock, and a back-end allocator such as MmapAllocator a simple and scalable solution for memory allocation. There are advantages to storing bookkeeping data separated from the payload (as opposed to e.g. using AffixAllocator to store metadata together with each allocation). The layout is more compact (overhead is one bit per block), searching for a free block during allocation enjoys better cache locality, and deallocation does not touch memory around the payload being deallocated (which is often cold). Allocation requests are handled on a first-fit basis. Although linear in complexity, allocation is in practice fast because of the compact bookkeeping representation, use of simple and fast bitwise routines, and caching of the first available block position. A known issue with this general approach is fragmentation, partially mitigated by coalescing. Since- BitmappedBlockdoes not need to maintain the allocated size, freeing memory implicitly coalesces free blocks together. Also, tuning blockSize has a considerable impact on both internal and external fragmentation. If the last template parameter is set to No.multiblock, the allocator will only serve allocations which require at most theBlockSize. The- BitmappedBlockhas a specialized implementation for single-block allocations which allows for greater performance, at the cost of not being able to allocate more than one block at a time. The size of each block can be selected either during compilation or at run time. Statically-known block sizes are frequent in practice and yield slightly better performance. To choose a block size statically, pass it as the blockSize parameter as in- BitmappedBlock!(4096). To choose a block size parameter, use- BitmappedBlock!(chooseAtRuntime) and pass the block size to the constructor.Parameters:- theBlockSize - the length of a block, which must be a multiple of theAlignment - theAlignment - alignment of each block - ParentAllocator - allocator from which the - BitmappedBlockwill draw memory. If set to NullAllocator, the storage must be passed via the constructor- f - Yes.multiblock to support allocations spanning across multiple blocks and No.multiblock to support single block allocations. Although limited by single block allocations, No.multiblock will generally provide higher performance. Examples:- // Create a block allocator on top of a 10KB stack region. import std.experimental.allocator.building_blocks.region : InSituRegion; import std.traits : hasMember; InSituRegion!(10_240, 64) r; auto a = BitmappedBlock!(64, 64)(cast(ubyte[])(r.allocateAll())); static assert(hasMember!(InSituRegion!(10_240, 64), "allocateAll")); const b = a.allocate(100); writeln(b.length); // 100 Examples:- import std.experimental.allocator.mallocator : Mallocator; import std.typecons : Flag, Yes; enum blockSize = 64; enum numBlocks = 10; // The 'BitmappedBlock' is implicitly instantiated with Yes.multiblock auto a = BitmappedBlock!(blockSize, 8, Mallocator, Yes.multiblock)(numBlocks * blockSize); // Instantiated with Yes.multiblock, can allocate more than one block at a time void[] buf = a.allocate(2 * blockSize); writeln(buf.length); // 2 * blockSize assert(a.deallocate(buf)); // Can also allocate less than one block buf = a.allocate(blockSize / 2); writeln(buf.length); // blockSize / 2 // Expands inside the same block assert(a.expand(buf, blockSize / 2)); writeln(buf.length); // blockSize // If Yes.multiblock, can expand past the size of a single block assert(a.expand(buf, 3 * blockSize)); writeln(buf.length); // 4 * blockSize assert(a.deallocate(buf)); Examples:- import std.experimental.allocator.mallocator : Mallocator; import std.typecons : Flag, No; enum blockSize = 64; auto a = BitmappedBlock!(blockSize, 8, Mallocator, No.multiblock)(1024 * blockSize); // Since instantiated with No.multiblock, can only allocate at most the block size void[] buf = a.allocate(blockSize + 1); assert(buf is null); buf = a.allocate(blockSize); writeln(buf.length); // blockSize assert(a.deallocate(buf)); // This is also fine, because it's less than the block size buf = a.allocate(blockSize / 2); writeln(buf.length); // blockSize / 2 // Can expand the buffer until its length is at most 64 assert(a.expand(buf, blockSize / 2)); writeln(buf.length); // blockSize // Cannot expand anymore assert(!a.expand(buf, 1)); assert(a.deallocate(buf)); - this(ubyte[]data);
 this(ubyte[]data, uintblockSize);
 this(size_tcapacity);
 this(ParentAllocatorparent, size_tcapacity);
 this(size_tcapacity, uintblockSize);
 this(ParentAllocatorparent, size_tcapacity, uintblockSize);
- Constructs a block allocator given a hunk of memory, or a desired capacity in bytes.- If ParentAllocator is NullAllocator,
        only the constructor taking datais defined and the user is responsible for freeingdataif desired.
- Otherwise, both constructors are defined. The data-based constructor assumes memory has been allocated with the parent allocator. Thecapacity-based constructor uses ParentAllocator to allocate an appropriate contiguous hunk of memory. Regardless of the constructor used, the destructor releases the memory by using ParentAllocator.deallocate.
 
- If ParentAllocator is NullAllocator,
        only the constructor taking 
- aliasblockSize= theBlockSize;
- IfblockSize== chooseAtRuntime, BitmappedBlock offers a read/write propertyblockSize. It must be set before any use of the allocator. Otherwise (i.e. theBlockSize is a legit constant),blockSizeis an alias for theBlockSize. Whether constant or variable, must also be a multiple of alignment. This constraint is asserted statically and dynamically.
- aliasalignment= theAlignment;
- The alignment offered is user-configurable statically through parameter theAlignment, defaulted to platformAlignment.
- ParentAllocatorparent;
- The parent allocator. Depending on whether ParentAllocator holds state or not, this is a member variable or an alias for ParentAllocator.instance.
- pure nothrow @nogc @safe size_tgoodAllocSize(size_tn);
- Returns the actual bytes allocated whennbytes are requested, i.e.n.roundUpToMultipleOf(blockSize).
- pure nothrow @nogc @trusted Ternaryowns(const void[]b) const;
- Returns Ternary.yes ifbbelongs to the BitmappedBlock object, Ternary.no otherwise. Never returns Ternary.unkown. (This method is somewhat tolerant in that accepts an interior slice.)
- pure nothrow @nogc @trusted boolexpand(ref void[]b, immutable size_tdelta);
- Expands in place a buffer previously allocated by BitmappedBlock. If instantiated with No.multiblock, the expansion fails if the new length exceeds theBlockSize.
- nothrow @nogc booldeallocate(void[]b);
- Deallocates a block previously allocated with this allocator.
- pure nothrow @nogc @trusted void[]allocate(const size_ts);
- Allocatessbytes of memory and returns it, or null if memory could not be allocated.The following information might be of help with choosing the appropriate block size. Actual allocation occurs in sizes multiple of the block size. Allocating one block is the fastest because only one 0 bit needs to be found in the metadata. Allocating 2 through 64 blocks is the next cheapest because it affects a maximum of two ulong in the metadata. Allocations greater than 64 blocks require a multiword search through the metadata. If instantiated with No.multiblock, it performs a search for the first zero bit in the bitmap and sets it.
- @trusted void[]allocateFresh(const size_ts);
- Allocates s bytes of memory and returns it, or null if memory could not be allocated.allocateFreshbehaves just like allocate, the only difference being that this always returns unused(fresh) memory. Although there may still be available space in the BitmappedBlock,allocateFreshcould still return null, because all the available blocks have been previously deallocated.
- void[]allocateAll();
- If the BitmappedBlock object is empty (has no active allocation), allocates all memory within and returns a slice to it. Otherwise, returns null (i.e. no attempt is made to allocate the largest available block).
- pure nothrow @nogc @safe Ternaryempty();
- Returns Ternary.yes if no memory is currently allocated with this allocator, otherwise Ternary.no. This method never returns Ternary.unknown.
- pure nothrow @nogc booldeallocateAll();
- Forcibly deallocates all memory allocated by this allocator, making it available for further allocations. Does not return memory to ParentAllocator.
- @system boolalignedReallocate(ref void[]b, size_tnewSize, uinta);
- Reallocates a block previously allocated with alignedAllocate. Contractions do not occur in place.
- @system boolreallocate(ref void[]b, size_tnewSize);
- Reallocates a previously-allocated block. Contractions occur in place.
- void[]alignedAllocate(size_tn, uinta);
- Allocates a block with specified alignmenta. The alignment must be a power of 2. Ifa<= alignment, function forwards to allocate. Otherwise, it attempts to overallocate and then adjust the result for proper alignment. In the worst case the slack memory is around two blocks.
 
- structSharedBitmappedBlock(size_t theBlockSize, uint theAlignment = platformAlignment, ParentAllocator = NullAllocator, Flag!"multiblock" f = Yes.multiblock);
- The threadsafe version of the BitmappedBlock. The semantics of theSharedBitmappedBlockare identical to the regular BitmappedBlock.Parameters:theBlockSize the length of a block, which must be a multiple of theAlignment theAlignment alignment of each block ParentAllocator allocator from which the BitmappedBlock will draw memory. If set to NullAllocator, the storage must be passed via the constructor f Yes.multiblock to support allocations spanning across multiple blocks and No.multiblock to support single block allocations. Although limited by single block allocations, No.multiblock will generally provide higher performance. Examples:import std.experimental.allocator.mallocator : Mallocator; import std.experimental.allocator.common : platformAlignment; import std.typecons : Flag, Yes, No; // Create 'numThreads' threads, each allocating in parallel a chunk of memory static void testAlloc(Allocator)(ref Allocator a, size_t allocSize) { import core.thread : ThreadGroup; import std.algorithm.sorting : sort; import core.internal.spinlock : SpinLock; SpinLock lock = SpinLock(SpinLock.Contention.brief); enum numThreads = 10; void[][numThreads] buf; size_t count = 0; // Each threads allocates 'allocSize' void fun() { void[] b = a.allocate(allocSize); writeln(b.length); // allocSize lock.lock(); scope(exit) lock.unlock(); buf[count] = b; count++; } auto tg = new ThreadGroup; foreach (i; 0 .. numThreads) { tg.create(&fun); } tg.joinAll(); // Sorting the allocations made by each thread, we expect the buffers to be // adjacent inside the SharedBitmappedBlock sort!((a, b) => a.ptr < b.ptr)(buf[0 .. numThreads]); foreach (i; 0 .. numThreads - 1) { assert(buf[i].ptr + a.goodAllocSize(buf[i].length) <= buf[i + 1].ptr); } // Deallocate everything foreach (i; 0 .. numThreads) { assert(a.deallocate(buf[i])); } } enum blockSize = 64; auto alloc1 = SharedBitmappedBlock!(blockSize, platformAlignment, Mallocator, Yes.multiblock)(1024 * 1024); auto alloc2 = SharedBitmappedBlock!(blockSize, platformAlignment, Mallocator, No.multiblock)(1024 * 1024); testAlloc(alloc1, 2 * blockSize); testAlloc(alloc2, blockSize); - this(ubyte[]data);
 this(ubyte[]data, uintblockSize);
 this(size_tcapacity);
 this(ParentAllocatorparent, size_tcapacity);
 this(size_tcapacity, uintblockSize);
 this(ParentAllocatorparent, size_tcapacity, uintblockSize);
- Constructs a block allocator given a hunk of memory, or a desired capacity in bytes.- If ParentAllocator is NullAllocator,
        only the constructor taking datais defined and the user is responsible for freeingdataif desired.
- Otherwise, both constructors are defined. The data-based constructor assumes memory has been allocated with the parent allocator. Thecapacity-based constructor uses ParentAllocator to allocate an appropriate contiguous hunk of memory. Regardless of the constructor used, the destructor releases the memory by using ParentAllocator.deallocate.
 
- If ParentAllocator is NullAllocator,
        only the constructor taking 
- aliasblockSize= theBlockSize;
- IfblockSize== chooseAtRuntime, SharedBitmappedBlock offers a read/write propertyblockSize. It must be set before any use of the allocator. Otherwise (i.e. theBlockSize is a legit constant),blockSizeis an alias for theBlockSize. Whether constant or variable, must also be a multiple of alignment. This constraint is asserted statically and dynamically.
- aliasalignment= theAlignment;
- The alignment offered is user-configurable statically through parameter theAlignment, defaulted to platformAlignment.
- ParentAllocatorparent;
- The parent allocator. Depending on whether ParentAllocator holds state or not, this is a member variable or an alias for ParentAllocator.instance.
- pure nothrow @nogc @safe size_tgoodAllocSize(size_tn);
- Returns the actual bytes allocated whennbytes are requested, i.e.n.roundUpToMultipleOf(blockSize).
- pure nothrow @nogc @trusted Ternaryowns(const void[]b) const;
- Returns Ternary.yes ifbbelongs to the SharedBitmappedBlock object, Ternary.no otherwise. Never returns Ternary.unkown. (This method is somewhat tolerant in that accepts an interior slice.)
- boolexpand(ref void[]b, immutable size_tdelta);
- Expands in place a buffer previously allocated by SharedBitmappedBlock. Expansion fails if the new length exceeds the block size.
- nothrow @nogc booldeallocate(void[]b);
- Deallocates the given bufferb, by atomically setting the corresponding bit to 0.bmust be valid, and cannot contain multiple adjacent blocks.
- nothrow @nogc @trusted void[]allocate(const size_ts);
- Allocatessbytes of memory and returns it, or null if memory could not be allocated.The SharedBitmappedBlock cannot allocate more than the given block size. Allocations are satisfied by searching the first unset bit in the bitmap, and atomically setting it. In rare memory pressure scenarios, the allocation could fail.
- @trusted void[]allocateFresh(const size_ts);
- Allocates s bytes of memory and returns it, or null if memory could not be allocated.allocateFreshbehaves just like allocate, the only difference being that this always returns unused(fresh) memory. Although there may still be available space in the SharedBitmappedBlock,allocateFreshcould still return null, because all the available blocks have been previously deallocated.
- void[]allocateAll();
- If the SharedBitmappedBlock object is empty (has no active allocation), allocates all memory within and returns a slice to it. Otherwise, returns null (i.e. no attempt is made to allocate the largest available block).
- nothrow @nogc @safe Ternaryempty();
- Returns Ternary.yes if no memory is currently allocated with this allocator, otherwise Ternary.no. This method never returns Ternary.unknown.
- nothrow @nogc booldeallocateAll();
- Forcibly deallocates all memory allocated by this allocator, making it available for further allocations. Does not return memory to ParentAllocator.
- @system boolalignedReallocate(ref void[]b, size_tnewSize, uinta);
- Reallocates a block previously allocated with alignedAllocate. Contractions do not occur in place.
- @system boolreallocate(ref void[]b, size_tnewSize);
- Reallocates a previously-allocated block. Contractions occur in place.
- void[]alignedAllocate(size_tn, uinta);
- Allocates a block with specified alignmenta. The alignment must be a power of 2. Ifa<= alignment, function forwards to allocate. Otherwise, it attempts to overallocate and then adjust the result for proper alignment. In the worst case the slack memory is around two blocks.
 
- structBitmappedBlockWithInternalPointers(size_t theBlockSize, uint theAlignment = platformAlignment, ParentAllocator = NullAllocator);
- A BitmappedBlock with additional structure for supporting resolveInternalPointer. To that end,BitmappedBlockWithInternalPointersadds a bitmap (one bit per block) that marks object starts. The bitmap itself has variable size and is allocated together with regular allocations.The time complexity of resolveInternalPointer is Ο(k), where k is the size of the object within which the internal pointer is looked up.- this(ubyte[]data);
 this(size_tcapacity);
 this(ParentAllocatorparent, size_tcapacity);
- Constructors accepting desired capacity or a preallocated buffer, similar in semantics to those of BitmappedBlock.
- aliasalignment= theAlignment;
 pure nothrow @nogc @safe size_tgoodAllocSize(size_tn);
 void[]allocate(size_tbytes);
 void[]allocateAll();
 boolexpand(ref void[]b, size_tbytes);
 booldeallocate(void[]b);
 nothrow @nogc @safe TernaryresolveInternalPointer(const void*p, ref void[]result);
 Ternaryempty();
- Allocator primitives.
 
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Ddoc on Mon Mar 31 10:28:19 2025