allocator/building_blocks/affix_allocator.d

///
module std.experimental.allocator.building_blocks.affix_allocator;

/**

Allocator that adds some extra data before (of type $(D Prefix)) and/or after
(of type $(D Suffix)) any allocation made with its parent allocator. This is
useful for uses where additional allocation-related information is needed, such
as mutexes, reference counts, or walls for debugging memory corruption errors.

If $(D Prefix) is not $(D void), $(D Allocator) must guarantee an alignment at
least as large as $(D Prefix.alignof).

Suffixes are slower to get at because of alignment rounding, so prefixes should
be preferred. However, small prefixes blunt the alignment so if a large
alignment with a small affix is needed, suffixes should be chosen.

The following methods are defined if $(D Allocator) defines them, and forward to it: $(D deallocateAll), $(D empty), $(D owns).
 */
struct AffixAllocator(Allocator, Prefix, Suffix = void)
{
    import std.algorithm.comparison : min;
    import std.conv : emplace;
    import std.experimental.allocator : IAllocator, theAllocator;
    import std.experimental.allocator.common : stateSize, forwardToMember,
        roundUpToMultipleOf, alignedAt, alignDownTo, roundUpToMultipleOf,
        hasStaticallyKnownAlignment;
    import std.math : isPowerOf2;
    import std.traits : hasMember;
    import std.typecons : Ternary;

    static if (hasStaticallyKnownAlignment!Allocator)
    {
        static assert(
                !stateSize!Prefix || Allocator.alignment >= Prefix.alignof,
                "AffixAllocator does not work with allocators offering a smaller"
                ~ " alignment than the prefix alignment.");
    }
    static assert(alignment % Suffix.alignof == 0,
        "This restriction could be relaxed in the future.");

    /**
    If $(D Prefix) is $(D void), the alignment is that of the parent. Otherwise, the alignment is the same as the $(D Prefix)'s alignment.
    */
    static if (hasStaticallyKnownAlignment!Allocator)
    {
        enum uint alignment = isPowerOf2(stateSize!Prefix)
            ? min(stateSize!Prefix, Allocator.alignment)
            : (stateSize!Prefix ? Prefix.alignof : Allocator.alignment);
    }
    else static if (is(Prefix == void))
    {
        enum uint alignment = platformAlignment;
    }
    else
    {
        enum uint alignment = Prefix.alignof;
    }

    /**
    If the parent allocator $(D Allocator) is stateful, an instance of it is
    stored as a member. Otherwise, $(D AffixAllocator) uses
    `Allocator.instance`. In either case, the name $(D _parent) is uniformly
    used for accessing the parent allocator.
    */
    static if (stateSize!Allocator)
    {
        Allocator _parent;
        static if (is(Allocator == IAllocator))
        {
            Allocator parent()
            {
                if (_parent is null) _parent = theAllocator;
                assert(alignment <= _parent.alignment);
                return _parent;
            }
        }
        else
        {
            alias parent = _parent;
        }
    }
    else
    {
        alias parent = Allocator.instance;
    }

    private template Impl()
    {

        size_t goodAllocSize(size_t s)
        {
            import std.experimental.allocator.common : goodAllocSize;
            auto a = actualAllocationSize(s);
            return roundUpToMultipleOf(parent.goodAllocSize(a)
                    - stateSize!Prefix - stateSize!Suffix,
                this.alignment);
        }

        private size_t actualAllocationSize(size_t s) const
        {
            assert(s > 0);
            static if (!stateSize!Suffix)
            {
                return s + stateSize!Prefix;
            }
            else
            {
                return
                    roundUpToMultipleOf(s + stateSize!Prefix, Suffix.alignof)
                    + stateSize!Suffix;
            }
        }

        private void[] actualAllocation(void[] b) const
        {
            assert(b !is null);
            return (b.ptr - stateSize!Prefix)
                [0 .. actualAllocationSize(b.length)];
        }

        void[] allocate(size_t bytes)
        {
            if (!bytes) return null;
            auto result = parent.allocate(actualAllocationSize(bytes));
            if (result is null) return null;
            static if (stateSize!Prefix)
            {
                assert(result.ptr.alignedAt(Prefix.alignof));
                emplace!Prefix(cast(Prefix*) result.ptr);
            }
            static if (stateSize!Suffix)
            {
                auto suffixP = result.ptr + result.length - Suffix.sizeof;
                assert(suffixP.alignedAt(Suffix.alignof));
                emplace!Suffix(cast(Suffix*)(suffixP));
            }
            return result[stateSize!Prefix .. stateSize!Prefix + bytes];
        }

        static if (hasMember!(Allocator, "allocateAll"))
        void[] allocateAll()
        {
            auto result = parent.allocateAll();
            if (result is null) return null;
            if (result.length < actualAllocationSize(1))
            {
                deallocate(result);
                return null;
            }
            static if (stateSize!Prefix)
            {
                assert(result.length > stateSize!Prefix);
                emplace!Prefix(cast(Prefix*) result.ptr);
                result = result[stateSize!Prefix .. $];
            }
            static if (stateSize!Suffix)
            {
                assert(result.length > stateSize!Suffix);
                // Ehm, find a properly aligned place for the suffix
                auto p = (result.ptr + result.length - stateSize!Suffix)
                    .alignDownTo(Suffix.alignof);
                assert(p > result.ptr);
                emplace!Suffix(cast(Suffix*) p);
                result = result[0 .. p - result.ptr];
            }
            return result;
        }

        static if (hasMember!(Allocator, "owns"))
        Ternary owns(void[] b)
        {
            if (b is null) return Ternary.no;
            return parent.owns(actualAllocation(b));
        }

        static if (hasMember!(Allocator, "resolveInternalPointer"))
        Ternary resolveInternalPointer(const void* p, ref void[] result)
        {
            void[] p1;
            Ternary r = parent.resolveInternalPointer(p, p1);
            if (r != Ternary.yes || p1 is null)
                return r;
            p1 = p1[stateSize!Prefix .. $];
            auto p2 = (p1.ptr + p1.length - stateSize!Suffix)
                      .alignDownTo(Suffix.alignof);
            result = p1[0 .. p2 - p1.ptr];
            return Ternary.yes;
        }

        static if (!stateSize!Suffix && hasMember!(Allocator, "expand"))
        bool expand(ref void[] b, size_t delta)
        {
            if (!b.ptr) return delta == 0;
            auto t = actualAllocation(b);
            const result = parent.expand(t, delta);
            if (!result) return false;
            b = b.ptr[0 .. b.length + delta];
            return true;
        }

        static if (hasMember!(Allocator, "reallocate"))
        bool reallocate(ref void[] b, size_t s)
        {
            if (b is null)
            {
                b = allocate(s);
                return b.length == s;
            }
            auto t = actualAllocation(b);
            const result = parent.reallocate(t, actualAllocationSize(s));
            if (!result) return false; // no harm done
            b = t.ptr[stateSize!Prefix .. stateSize!Prefix + s];
            return true;
        }

        static if (hasMember!(Allocator, "deallocate"))
        bool deallocate(void[] b)
        {
            if (!b.ptr) return true;
            return parent.deallocate(actualAllocation(b));
        }

        /* The following methods are defined if $(D ParentAllocator) defines
        them, and forward to it: $(D deallocateAll), $(D empty).*/
        mixin(forwardToMember("parent",
            "deallocateAll", "empty"));

        // Computes suffix type given buffer type
        private template Payload2Affix(Payload, Affix)
        {
            static if (is(Payload[] : void[]))
                alias Payload2Affix = Affix;
            else static if (is(Payload[] : shared(void)[]))
                alias Payload2Affix = shared Affix;
            else static if (is(Payload[] : immutable(void)[]))
                alias Payload2Affix = shared Affix;
            else static if (is(Payload[] : const(shared(void))[]))
                alias Payload2Affix = shared Affix;
            else static if (is(Payload[] : const(void)[]))
                alias Payload2Affix = const Affix;
            else
                static assert(0, "Internal error for type " ~ Payload.stringof);
        }

        // Extra functions
        static if (stateSize!Prefix)
        {
            static auto ref prefix(T)(T[] b)
            {
                assert(b.ptr && b.ptr.alignedAt(Prefix.alignof));
                return (cast(Payload2Affix!(T, Prefix)*) b.ptr)[-1];
            }
        }
        static if (stateSize!Suffix)
            auto ref suffix(T)(T[] b)
            {
                assert(b.ptr);
                auto p = b.ptr - stateSize!Prefix
                    + actualAllocationSize(b.length);
                assert(p && p.alignedAt(Suffix.alignof));
                return (cast(Payload2Affix!(T, Suffix)*) p)[-1];
            }
    }

    version (StdDdoc)
    {
        /**
        Standard allocator methods. Each is defined if and only if the parent
        allocator defines the homonym method (except for $(D goodAllocSize),
        which may use the global default). Also, the methods will be $(D
        shared) if the parent allocator defines them as such.
        */
        size_t goodAllocSize(size_t);
        /// Ditto
        void[] allocate(size_t);
        /// Ditto
        Ternary owns(void[]);
        /// Ditto
        bool expand(ref void[] b, size_t delta);
        /// Ditto
        bool reallocate(ref void[] b, size_t s);
        /// Ditto
        bool deallocate(void[] b);
        /// Ditto
        bool deallocateAll();
        /// Ditto
        Ternary empty();

        /**
        The `instance` singleton is defined if and only if the parent allocator
        has no state and defines its own `it` object.
        */
        static AffixAllocator instance;

        /**
        Affix access functions offering references to the affixes of a
        block `b` previously allocated with this allocator. `b` may not be null.
        They are defined if and only if the corresponding affix is not `void`.

        The qualifiers of the affix are not always the same as the qualifiers
        of the argument. This is because the affixes are not part of the data
        itself, but instead are just $(I associated) with the data and known
        to the allocator. The table below documents the type of `preffix(b)` and
        `affix(b)` depending on the type of `b`.

        $(BOOKTABLE Result of `prefix`/`suffix` depending on argument (`U` is
        any unqualified type, `Affix` is `Prefix` or `Suffix`),
            $(TR $(TH Argument$(NBSP)Type) $(TH Return) $(TH Comments))

            $(TR $(TD `shared(U)[]`) $(TD `ref shared Affix`)
            $(TD Data is shared across threads and the affix follows suit.))

            $(TR $(TD `immutable(U)[]`) $(TD `ref shared Affix`)
            $(TD Although the data is immutable, the allocator "knows" the
            underlying memory is mutable, so `immutable` is elided for the affix
            which is independent from the data itself. However, the result is
            `shared` because `immutable` is implicitly shareable so multiple
            threads may access and manipulate the affix for the same data.))

            $(TR $(TD `const(shared(U))[]`) $(TD `ref shared Affix`)
            $(TD The data is always shareable across threads. Even if the data
            is `const`, the affix is modifiable by the same reasoning as for
            `immutable`.))

            $(TR $(TD `const(U)[]`) $(TD `ref const Affix`)
            $(TD The input may have originated from `U[]` or `immutable(U)[]`,
            so it may be actually shared or not. Returning an unqualified affix
            may result in race conditions, whereas returning a `shared` affix
            may result in inadvertent sharing of mutable thread-local data
            across multiple threads. So the returned type is conservatively
            `ref const`.))

            $(TR $(TD `U[]`) $(TD `ref Affix`)
            $(TD Unqualified data has unqualified affixes.))
        )

        Precondition: `b !is null` and `b` must have been allocated with
        this allocator.
        */
        static ref auto prefix(T)(T[] b);
        /// Ditto
        ref auto suffix(T)(T[] b);
    }
    else static if (is(typeof(Allocator.instance) == shared))
    {
        static shared AffixAllocator instance;
        shared { mixin Impl!(); }
    }
    else
    {
        mixin Impl!();
        static if (stateSize!Allocator == 0)
            static __gshared AffixAllocator instance;
    }
}

///
@system unittest
{
    import std.experimental.allocator.mallocator : Mallocator;
    // One word before and after each allocation.
    alias A = AffixAllocator!(Mallocator, size_t, size_t);
    auto b = A.instance.allocate(11);
    A.instance.prefix(b) = 0xCAFE_BABE;
    A.instance.suffix(b) = 0xDEAD_BEEF;
    assert(A.instance.prefix(b) == 0xCAFE_BABE
        && A.instance.suffix(b) == 0xDEAD_BEEF);
}

@system unittest
{
    import std.experimental.allocator.gc_allocator : GCAllocator;
    import std.experimental.allocator : theAllocator, IAllocator;

    // One word before and after each allocation.
    auto A = AffixAllocator!(IAllocator, size_t, size_t)(theAllocator);
    auto a = A.allocate(11);
    A.prefix(a) = 0xCAFE_BABE;
    A.suffix(a) = 0xDEAD_BEEF;
    assert(A.prefix(a) == 0xCAFE_BABE
        && A.suffix(a) == 0xDEAD_BEEF);

    // One word before and after each allocation.
    auto B = AffixAllocator!(IAllocator, size_t, size_t)();
    auto b = B.allocate(11);
    B.prefix(b) = 0xCAFE_BABE;
    B.suffix(b) = 0xDEAD_BEEF;
    assert(B.prefix(b) == 0xCAFE_BABE
        && B.suffix(b) == 0xDEAD_BEEF);
}

@system unittest
{
    import std.experimental.allocator.building_blocks.bitmapped_block
        : BitmappedBlock;
    import std.experimental.allocator.common : testAllocator;
    testAllocator!({
        auto a = AffixAllocator!(BitmappedBlock!128, ulong, ulong)
            (BitmappedBlock!128(new ubyte[128 * 4096]));
        return a;
    });
}

@system unittest
{
    import std.experimental.allocator.mallocator : Mallocator;
    alias A = AffixAllocator!(Mallocator, size_t);
    auto b = A.instance.allocate(10);
    A.instance.prefix(b) = 10;
    assert(A.instance.prefix(b) == 10);

    import std.experimental.allocator.building_blocks.null_allocator
        : NullAllocator;
    alias B = AffixAllocator!(NullAllocator, size_t);
    b = B.instance.allocate(100);
    assert(b is null);
}

@system unittest
{
    import std.experimental.allocator;
    import std.experimental.allocator.gc_allocator;
    import std.typecons : Ternary;
    alias MyAllocator = AffixAllocator!(GCAllocator, uint);
    auto a = MyAllocator.instance.makeArray!(shared int)(100);
    static assert(is(typeof(&MyAllocator.instance.prefix(a)) == shared(uint)*));
    auto b = MyAllocator.instance.makeArray!(shared const int)(100);
    static assert(is(typeof(&MyAllocator.instance.prefix(b)) == shared(uint)*));
    auto c = MyAllocator.instance.makeArray!(immutable int)(100);
    static assert(is(typeof(&MyAllocator.instance.prefix(c)) == shared(uint)*));
    auto d = MyAllocator.instance.makeArray!(int)(100);
    static assert(is(typeof(&MyAllocator.instance.prefix(d)) == uint*));
    auto e = MyAllocator.instance.makeArray!(const int)(100);
    static assert(is(typeof(&MyAllocator.instance.prefix(e)) == const(uint)*));

    void[] p;
    assert(MyAllocator.instance.resolveInternalPointer(null, p) == Ternary.no);
    Ternary r = MyAllocator.instance.resolveInternalPointer(d.ptr, p);
    assert(p.ptr is d.ptr && p.length >= d.length);
}