NAME¶
Reinterpreting_Types_Using_as_type()_and_as_typen()
DESCRIPTION¶
All data types described in
scalarDataTypes(3clc) and
vectorDataTypes(3clc) (except
bool,
half (unless the
cl_khr_fp16(3clc) extension is supported), and
void) may be also
reinterpreted as another data type of the same size using the
as_type()
operator for scalar data types and the
as_typen()
operator for vector data types. When the operand and result type contain the
same number of elements, the bits in the operand shall be returned directly
without modification as the new type. The usual type promotion for function
arguments shall not be performed.
For example, as_float(0x3f800000) returns 1.0f, which is the value that the bit
pattern 0x3f800000 has if viewed as an IEEE-754 single precision value.
When the operand and result type contain a different number of elements, the
result shall be implementation-defined except if the operand is a 4-component
vector and the result is a 3-component vector. In this case, the bits in the
operand shall be returned directly without modification as the new type. That
is, a conforming implementation shall explicitly define a behavior, but two
conforming implementations need not have the same behavior when the number of
elements in the result and operand types does not match. The implementation
may define the result to contain all, some or none of the original bits in
whatever order it chooses. It is an error to use the
as_type() or
as_type n() operators to reinterpret data to a
type of a different number of bytes.
NOTES¶
While the union is intended to reflect the organization of data in memory, the
as_type() and
as_typen() constructs are
intended to reflect the organization of data in register. The
as_type()
and
as_typen() constructs are intended to compile
to no instructions on devices that use a shared register file designed to
operate on both the operand and result types.
Note that while differences in memory organization are expected to largely be
limited to those arising from endianness, the register based representation
may also differ due to size of the element in register. (For example, an
architecture may load a char into a 32-bit register, or a char vector into a
SIMD vector register with fixed 32-bit element size.)
If the element count does not match, then the implementation should pick a data
representation that most closely matches what would happen if an appropriate
result type operator was applied to a register containing data of the source
type. If the number of elements matches, then the
as_typen () should faithfully reproduce the
behavior expected from a similar data type reinterpretation using
memory/unions. So, for example if an implementation stores all single
precision data as double in register, it should implement as_int( float ) by
first downconverting the double to single precision and then (if necessary)
moving the single precision bits to a register suitable for operating on
integer data.
If data stored in different address spaces do not have the same endianness, then
the "dominant endianness" of the device should prevail.
Example¶
float f = 1.0f; uint u = as_uint(f); // Legal. Contains: 0x3f800000
float4 f = (float4)(1.0f, 2.0f, 3.0f, 4.0f); // Legal. Contains: (int4) //
(0x3f800000, 0x40000000, 0x40400000, 0x40800000) int4 i = as_int4(f);
float4 f, g; int4 is_less = f < g; // Legal. f[i] = f[i] < g[i] ?
f[i] : 0.0f f = as_float4(as_int4(f) & is_less); int i; // Legal.
Result is implementation-defined. short2 j = as_short2(i); int4 i; //
Legal. Result is implementation-defined. short8 j = as_short8(i); float4
f; //Error. result and operand have different size double4 g =
as_double4(f); // Only if double is supported. float4 f; // Legal. g.xyz
will have same values as f.xyz. g.w is undefined float3 g =
as_float3(f); |
SPECIFICATION¶
OpenCL Specification[1]
SEE ALSO¶
scalarDataTypes(3clc),
vectorDataTypes(3clc)
AUTHORS¶
The Khronos Group
COPYRIGHT¶
Copyright © 2007-2011 The Khronos Group Inc.
Permission is hereby granted, free of charge, to any person obtaining a copy of
this software and/or associated documentation files (the
"Materials"), to deal in the Materials without restriction,
including without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Materials, and to permit
persons to whom the Materials are furnished to do so, subject to the condition
that this copyright notice and permission notice shall be included in all
copies or substantial portions of the Materials.
NOTES¶
- 1.
- OpenCL Specification
page 212, section 6.2.4.2 - Reinterpreting Types Using
as_typen()