Attributes of variables.
The keyword __attribute__ allows you to specify special attributes of variables or structure fields. This keyword is followed by an attribute specification inside double parentheses. The aligned, packed, and endian attribute qualifiers are defined below.
This attribute specifies a minimum alignment for the variable or structure field, measured in bytes. For example, the declaration:
| int x __attribute__ ((aligned (16))) = 0; |
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causes the compiler to allocate the global variable x on a 16-byte boundary. The alignment value specified must be a power of two.
You can also specify the alignment of structure fields. For example, to create double-word aligned int pair, you could write:
| struct foo { int x[2] __attribute__ ((aligned (8))); }; |
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This is an alternative to creating a union with a double member that forces the union to be double-word aligned.
As in the preceding examples, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given variable or structure field. Alternatively, you can leave out the alignment factor and just ask the compiler to align a variable or field to the maximum useful alignment for the target machine you are compiling for. For example, you could write:
| short array[3] __attribute__ ((aligned)); |
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Whenever you leave out the alignment factor in an aligned attribute specification, the OpenCL compiler automatically sets the alignment for the declared variable or field to the largest alignment which is ever used for any data type on the target device you are compiling for.
When used on a struct, or struct member, the aligned attribute can only increase the alignment; in order to decrease it, the packed attribute must be specified as well. When used as part of a typedef, the aligned attribute can both increase and decrease alignment, and specifying the packed attribute will generate a warning.
Note that the effectiveness of aligned attributes may be limited by inherent limitations of the OpenCL device and compiler. For some devices, the OpenCL compiler may only be able to arrange for variables to be aligned up to a certain maximum alignment. If the OpenCL compiler is only able to align variables up to a maximum of 8 byte alignment, then specifying aligned(16) in an __attribute__ will still only provide you with 8 byte alignment. See your platform-specific documentation for further information.
The packed attribute specifies that a variable or structure field should have the smallest possible alignment -- one byte for a variable, unless you specify a larger value with the aligned attribute.
Here is a structure in which the field x is packed, so that it immediately follows a:
| struct foo { char a; int x[2] __attribute__ ((packed)); }; |
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An attribute list placed at the beginning of a user-defined type applies to the variable of that type and not the type, while attributes following the type body apply to the type.
For example:
| /* a has alignment of 128 */ __attribute__((aligned(128))) struct A {int i;} a; /* b has alignment of 16 */ __attribute__((aligned(16))) struct B {double d;} __attribute__((aligned(32))) b ; struct A a1; /* a1 has alignment of 4 */ struct B b1; /* b1 has alignment of 32 */ |
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The endian attribute determines the byte ordering of a variable. endiantype can be set to host indicating the variable uses the endianness of the host processor or can be set to device indicating the variable uses the endianness of the device on which the kernel will be executed. The default is device. For example:
| float4 *p __attribute__ ((endian(host))); |
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specifies that data stored in memory pointed to by p will be in the host endian format.
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attribute(3clc), attributes-blocksAndControlFlow(3clc), attributes-types(3clc)
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OpenCL Specification
page 239, section 6.11.3 - Specifying Attributes of Variables