Description
All data types described in
Scalar Data Types and
Vector Data Types) (except bool, half, and void) may be also reinterpreted as another data type of the same size using the as_type operator. 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.
n()
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. 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 as_type
operator to reinterpret data to a type of a different number of bytes.
n()
Notes
While the union is intended to reflect the organization of data in memory, the
as_type construct is intended to
reflect the organization of data in register. The
n()as_type construct
is intended to compile to no instructions on
devices that use a shared register file designed to
operate on both the operand and result types.
n()
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_type
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.
n()
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); |
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