Interface MemoryLayout

All Superinterfaces:
Constable
All Known Implementing Classes:
GroupLayout, SequenceLayout, ValueLayout

public interface MemoryLayout
extends Constable
A memory layout can be used to describe the contents of a memory segment in a language neutral fashion. There are two leaves in the layout hierarchy, value layouts, which are used to represent values of given size and kind (see ValueLayout) and padding layouts which are used, as the name suggests, to represent a portion of a memory segment whose contents should be ignored, and which are primarily present for alignment reasons (see ofPaddingBits(long)). Some common value layout constants are defined in the MemoryLayouts class.

More complex layouts can be derived from simpler ones: a sequence layout denotes a repetition of one or more element layout (see SequenceLayout); a group layout denotes an aggregation of (typically) heterogeneous member layouts (see GroupLayout).

For instance, consider the following struct declaration in C:


 typedef struct {
     char kind;
     int value;
 } TaggedValues[5];
 
The above declaration can be modelled using a layout object, as follows:

SequenceLayout taggedValues = MemoryLayout.ofSequence(5,
    MemoryLayout.ofStruct(
        MemoryLayout.ofValueBits(8, ByteOrder.NATIVE_ORDER).withName("kind"),
        MemoryLayout.ofPaddingBits(24),
        MemoryLayout.ofValueBits(32, ByteOrder.NATIVE_ORDER).withName("value")
    )
).withName("TaggedValues");
 

All implementations of this interface must be value-based; use of identity-sensitive operations (including reference equality (==), identity hash code, or synchronization) on instances of MemoryLayout may have unpredictable results and should be avoided. The equals method should be used for comparisons.

Non-platform classes should not implement MemoryLayout directly.

Size, alignment and byte order

All layouts have a size; layout size for value and padding layouts is always explicitly denoted; this means that a layout description always has the same size in bits, regardless of the platform in which it is used. For derived layouts, the size is computed as follows:
  • for a finite sequence layout S whose element layout is E and size is L, the size of S is that of E, multiplied by L
  • the size of an unbounded sequence layout is unknown
  • for a group layout G with member layouts M1, M2, ... Mn whose sizes are S1, S2, ... Sn, respectively, the size of G is either S1 + S2 + ... + Sn or max(S1, S2, ... Sn) depending on whether the group is a struct or an union, respectively

Furthermore, all layouts feature a natural alignment which can be inferred as follows:

  • for a padding layout L, the natural alignment is 1, regardless of its size; that is, in the absence of an explicit alignment constraint, a padding layout should not affect the alignment constraint of the group layout it is nested into
  • for a value layout L whose size is N, the natural alignment of L is N
  • for a sequence layout S whose element layout is E, the natural alignment of S is that of E
  • for a group layout G with member layouts M1, M2, ... Mn whose alignments are A1, A2, ... An, respectively, the natural alignment of G is max(A1, A2 ... An)
A layout's natural alignment can be overridden if needed (see withBitAlignment(long)), which can be useful to describe hyper-aligned layouts.

All value layouts have an explicit byte order (see ByteOrder) which is set when the layout is created.

Layout paths

A layout path originates from a root layout (typically a group or a sequence layout) and terminates at a layout nested within the root layout - this is the layout selected by the layout path. Layout paths are typically expressed as a sequence of one or more MemoryLayout.PathElement instances.

Layout paths are for example useful in order to obtain offsets of arbitrarily nested layouts inside another layout (see bitOffset(PathElement...)), to quickly obtain a memory access handle corresponding to the selected layout (see varHandle(Class, PathElement...)), to select an arbitrarily nested layout inside another layout (see select(PathElement...), or to transform a nested layout element inside another layout (see map(UnaryOperator, PathElement...)).

Such layout paths can be constructed programmatically using the methods in this class. For instance, given the taggedValues layout instance constructed as above, we can obtain the offset, in bits, of the member layout named value in the first sequence element, as follows:


long valueOffset = taggedValues.bitOffset(PathElement.sequenceElement(0),
                                          PathElement.groupElement("value")); // yields 32
 
Similarly, we can select the member layout named value, as follows:

MemoryLayout value = taggedValues.select(PathElement.sequenceElement(),
                                         PathElement.groupElement("value"));
 
And, we can also replace the layout named value with another layout, as follows:

MemoryLayout taggedValuesWithHole = taggedValues.map(l -> MemoryLayout.ofPadding(32),
                                            PathElement.sequenceElement(), PathElement.groupElement("value"));
 
That is, the above declaration is identical to the following, more verbose one:

MemoryLayout taggedValuesWithHole = MemoryLayout.ofSequence(5,
    MemoryLayout.ofStruct(
        MemoryLayout.ofValueBits(8, ByteOrder.NATIVE_ORDER).withName("kind").
        MemoryLayout.ofPaddingBits(32),
        MemoryLayout.ofPaddingBits(32)
));
 
Layout paths can feature one or more free dimensions. For instance, a layout path traversing an unspecified sequence element (that is, where one of the path component was obtained with the MemoryLayout.PathElement.sequenceElement() method) features an additional free dimension, which will have to be bound at runtime. This is important when obtaining memory access var handle from layouts, as in the following code:

VarHandle valueHandle = taggedValues.varHandle(int.class,
                                               PathElement.sequenceElement(),
                                               PathElement.groupElement("value"));
 
Since the layout path constructed in the above example features exactly one free dimension (as it doesn't specify which member layout named value should be selected from the enclosing sequence layout), it follows that the memory access var handle valueHandle will feature an additional long access coordinate.

Layout attributes

Layouts can be optionally associated with one or more attributes. A layout attribute forms a name/value pair, where the name is a String and the value is a Constable. The most common form of layout attribute is the layout name (see LAYOUT_NAME), a custom name that can be associated to memory layouts and that can be referred to when constructing layout paths.
API Note:
In the future, if the Java language permits, MemoryLayout may become a sealed interface, which would prohibit subclassing except by explicitly permitted types.
Implementation Requirements:
Implementations of this interface are immutable, thread-safe and value-based.
  • Field Details

  • Method Details

    • describeConstable

      Optional<? extends DynamicConstantDesc<? extends MemoryLayout>> describeConstable()
      Returns an Optional containing the nominal descriptor for this layout, if one can be constructed, or an empty Optional if one cannot be constructed.
      Specified by:
      describeConstable in interface Constable
      Returns:
      An Optional containing the resulting nominal descriptor, or an empty Optional if one cannot be constructed.
    • hasSize

      boolean hasSize()
      Does this layout have a specified size? A layout does not have a specified size if it is (or contains) a sequence layout whose size is unspecified (see SequenceLayout.elementCount()). Value layouts (see ValueLayout) and padding layouts (see ofPaddingBits(long)) always have a specified size, therefore this method always returns true in these cases.
      Returns:
      true, if this layout has a specified size.
    • bitSize

      long bitSize()
      Computes the layout size, in bits.
      Returns:
      the layout size, in bits.
      Throws:
      UnsupportedOperationException - if the layout is, or contains, a sequence layout with unspecified size (see SequenceLayout).
    • byteSize

      default long byteSize()
      Computes the layout size, in bytes.
      Returns:
      the layout size, in bytes.
      Throws:
      UnsupportedOperationException - if the layout is, or contains, a sequence layout with unspecified size (see SequenceLayout), or if bitSize() is not a multiple of 8.
    • name

      Optional<String> name()
      Return the name (if any) associated with this layout.

      This is equivalent to the following code:

      
          attribute(LAYOUT_NAME).map(String.class::cast);
       
      Returns:
      the layout name (if any).
      See Also:
      withName(String)
    • withName

      MemoryLayout withName​(String name)
      Creates a new layout which features the desired layout name.

      This is equivalent to the following code:

      
          withAttribute(LAYOUT_NAME, name);
       
      Parameters:
      name - the layout name.
      Returns:
      a new layout which is the same as this layout, except for the name associated to it.
      See Also:
      name()
    • bitAlignment

      long bitAlignment()
      Returns the alignment constraint associated with this layout, expressed in bits. Layout alignment defines a power of two A which is the bit-wise alignment of the layout. If A <= 8 then A/8 is the number of bytes that must be aligned for any pointer that correctly points to this layout. Thus:
      • A=8 means unaligned (in the usual sense), which is common in packets.
      • A=64 means word aligned (on LP64), A=32 int aligned, A=16 short aligned, etc.
      • A=512 is the most strict alignment required by the x86/SV ABI (for AVX-512 data).
      If no explicit alignment constraint was set on this layout (see withBitAlignment(long)), then this method returns the natural alignment constraint (in bits) associated with this layout.
      Returns:
      the layout alignment constraint, in bits.
    • byteAlignment

      default long byteAlignment()
      Returns the alignment constraint associated with this layout, expressed in bytes. Layout alignment defines a power of two A which is the byte-wise alignment of the layout, where A is the number of bytes that must be aligned for any pointer that correctly points to this layout. Thus:
      • A=1 means unaligned (in the usual sense), which is common in packets.
      • A=8 means word aligned (on LP64), A=4 int aligned, A=2 short aligned, etc.
      • A=64 is the most strict alignment required by the x86/SV ABI (for AVX-512 data).
      If no explicit alignment constraint was set on this layout (see withBitAlignment(long)), then this method returns the natural alignment constraint (in bytes) associated with this layout.
      Returns:
      the layout alignment constraint, in bytes.
      Throws:
      UnsupportedOperationException - if bitAlignment() is not a multiple of 8.
    • withBitAlignment

      MemoryLayout withBitAlignment​(long bitAlignment)
      Creates a new layout which features the desired alignment constraint.
      Parameters:
      bitAlignment - the layout alignment constraint, expressed in bits.
      Returns:
      a new layout which is the same as this layout, except for the alignment constraint associated to it.
      Throws:
      IllegalArgumentException - if bitAlignment is not a power of two, or if it's less than than 8.
    • attribute

      Optional<Constable> attribute​(String name)
      Returns the attribute with the given name (if it exists).
      Parameters:
      name - the attribute name
      Returns:
      the attribute with the given name (if it exists).
    • withAttribute

      MemoryLayout withAttribute​(String name, Constable value)
      Returns a new memory layout which features the same attributes as this layout, plus the newly specified attribute. If this layout already contains an attribute with the same name, the existing attribute value is overwritten in the returned layout.
      Parameters:
      name - the attribute name.
      value - the attribute value.
      Returns:
      a new memory layout which features the same attributes as this layout, plus the newly specified attribute.
    • attributes

      Stream<String> attributes()
      Returns a stream of the attribute names associated with this layout.
      Returns:
      a stream of the attribute names associated with this layout.
    • bitOffset

      default long bitOffset​(MemoryLayout.PathElement... elements)
      Computes the offset, in bits, of the layout selected by a given layout path, where the path is considered rooted in this layout.
      API Note:
      if the layout path has one (or more) free dimensions, the offset is computed as if all the indices corresponding to such dimensions were set to 0.
      Parameters:
      elements - the layout path elements.
      Returns:
      The offset, in bits, of the layout selected by the layout path in elements.
      Throws:
      IllegalArgumentException - if the layout path does not select any layout nested in this layout, or if the layout path contains one or more path elements that select multiple sequence element indices (see MemoryLayout.PathElement.sequenceElement() and MemoryLayout.PathElement.sequenceElement(long, long)).
      UnsupportedOperationException - if one of the layouts traversed by the layout path has unspecified size.
    • byteOffset

      default long byteOffset​(MemoryLayout.PathElement... elements)
      Computes the offset, in bytes, of the layout selected by a given layout path, where the path is considered rooted in this layout.
      API Note:
      if the layout path has one (or more) free dimensions, the offset is computed as if all the indices corresponding to such dimensions were set to 0.
      Parameters:
      elements - the layout path elements.
      Returns:
      The offset, in bytes, of the layout selected by the layout path in elements.
      Throws:
      IllegalArgumentException - if the layout path does not select any layout nested in this layout, or if the layout path contains one or more path elements that select multiple sequence element indices (see MemoryLayout.PathElement.sequenceElement() and MemoryLayout.PathElement.sequenceElement(long, long)).
      UnsupportedOperationException - if one of the layouts traversed by the layout path has unspecified size, or if bitOffset(elements) is not a multiple of 8.
    • varHandle

      default VarHandle varHandle​(Class<?> carrier, MemoryLayout.PathElement... elements)
      Creates a memory access var handle that can be used to dereference memory at the layout selected by a given layout path, where the path is considered rooted in this layout.
      API Note:
      the resulting var handle will feature an additional long access coordinate for every unspecified sequence access component contained in this layout path. Moreover, the resulting var handle features certain access mode restrictions, which are common to all memory access var handles.
      Parameters:
      carrier - the var handle carrier type.
      elements - the layout path elements.
      Returns:
      a var handle which can be used to dereference memory at the (possibly nested) layout selected by the layout path in elements.
      Throws:
      UnsupportedOperationException - if the layout path has one or more elements with incompatible alignment constraints, or if one of the layouts traversed by the layout path has unspecified size.
      IllegalArgumentException - if the carrier does not represent a primitive type, if the carrier is void, boolean, or if the layout path in elements does not select a value layout (see ValueLayout), or if the selected value layout has a size that that does not match that of the specified carrier type.
    • select

      default MemoryLayout select​(MemoryLayout.PathElement... elements)
      Selects the layout from a path rooted in this layout.
      Parameters:
      elements - the layout path elements.
      Returns:
      the layout selected by the layout path in elements.
      Throws:
      IllegalArgumentException - if the layout path does not select any layout nested in this layout, or if the layout path contains one or more path elements that select one or more sequence element indices (see MemoryLayout.PathElement.sequenceElement(long) and MemoryLayout.PathElement.sequenceElement(long, long)).
    • map

      Creates a transformed copy of this layout where a selected layout, from a path rooted in this layout, is replaced with the result of applying the given operation.
      Parameters:
      op - the unary operation to be applied to the selected layout.
      elements - the layout path elements.
      Returns:
      a new layout where the layout selected by the layout path in elements, has been replaced by the result of applying op to the selected layout.
      Throws:
      IllegalArgumentException - if the layout path does not select any layout nested in this layout, or if the layout path contains one or more path elements that select one or more sequence element indices (see MemoryLayout.PathElement.sequenceElement(long) and MemoryLayout.PathElement.sequenceElement(long, long)).
    • isPadding

      boolean isPadding()
      Is this a padding layout (e.g. a layout created from ofPaddingBits(long)) ?
      Returns:
      true, if this layout is a padding layout.
    • equals

      boolean equals​(Object that)
      Compares the specified object with this layout for equality. Returns true if and only if the specified object is also a layout, and it is equal to this layout. Two layouts are considered equal if they are of the same kind, have the same size, name and alignment constraints. Furthermore, depending on the layout kind, additional conditions must be satisfied:
      Overrides:
      equals in class Object
      Parameters:
      that - the object to be compared for equality with this layout.
      Returns:
      true if the specified object is equal to this layout.
      See Also:
      Object.hashCode(), HashMap
    • hashCode

      int hashCode()
      Returns the hash code value for this layout.
      Overrides:
      hashCode in class Object
      Returns:
      the hash code value for this layout.
      See Also:
      Object.equals(java.lang.Object), System.identityHashCode(java.lang.Object)
    • toString

      String toString()
      Returns a string representation of this layout.
      Overrides:
      toString in class Object
      Returns:
      a string representation of this layout.
    • ofPaddingBits

      static MemoryLayout ofPaddingBits​(long size)
      Create a new padding layout with given size.
      Parameters:
      size - the padding size in bits.
      Returns:
      the new selector layout.
      Throws:
      IllegalArgumentException - if size <= 0.
    • ofValueBits

      static ValueLayout ofValueBits​(long size, ByteOrder order)
      Create a value layout of given byte order and size.
      Parameters:
      size - the value layout size.
      order - the value layout's byte order.
      Returns:
      a new value layout.
      Throws:
      IllegalArgumentException - if size <= 0.
    • ofSequence

      static SequenceLayout ofSequence​(long elementCount, MemoryLayout elementLayout)
      Create a new sequence layout with given element layout and element count.
      Parameters:
      elementCount - the sequence element count.
      elementLayout - the sequence element layout.
      Returns:
      the new sequence layout with given element layout and size.
      Throws:
      IllegalArgumentException - if elementCount < 0.
    • ofSequence

      static SequenceLayout ofSequence​(MemoryLayout elementLayout)
      Create a new sequence layout, with unbounded element count and given element layout.
      Parameters:
      elementLayout - the element layout of the sequence layout.
      Returns:
      the new sequence layout with given element layout.
    • ofStruct

      static GroupLayout ofStruct​(MemoryLayout... elements)
      Create a new struct group layout with given member layouts.
      Parameters:
      elements - The member layouts of the struct group layout.
      Returns:
      a new struct group layout with given member layouts.
    • ofUnion

      static GroupLayout ofUnion​(MemoryLayout... elements)
      Create a new union group layout with given member layouts.
      Parameters:
      elements - The member layouts of the union layout.
      Returns:
      a new union group layout with given member layouts.