Description
The __attribute__((opencl_unroll_hint))
and __attribute__((opencl_unroll_hint(n)))
attribute qualifiers can be used to specify that a loop
(for, while and do loops)
can be unrolled. This attribute qualifier can be
used to specify full unrolling or partial unrolling
by a specified amount. This is a compiler hint
and the compiler may ignore this directive.
n is the loop unrolling factor and must
be a positive integral compile time constant expression.
An unroll factor of 1 disables unrolling. If
n is not specified, the compiler determines the
unrolling factor for the loop.
NOTE: The __attribute__((opencl_unroll_hint(n)))
attribute qualifier must
appear immediately before the loop to be affected.
Examples
| __attribute__((opencl_unroll_hint(2))) while (*s != 0) *p++ = *s++; |
This tells the compiler to unroll the above while loop by a factor of 2.
| __attribute__((opencl_unroll_hint)) for (int i=0; i<2; i++) { … } |
In the example above, the compiler will determine how much to unroll the loop.
| __attribute__((opencl_unroll_hint(1))) for (int i=0; i<32; i++) { … } |
The above is an example where the loop should not be unrolled.
Below are some examples of invalid usage of
__attribute__((opencl_unroll_hint(n))).
| __attribute__((opencl_unroll_hint(-1))) while (…) { … } |
The above example is an invalid usage of the loop unroll factor as the loop unroll factor is negative.
| __attribute__((opencl_unroll_hint)) if (…) { … } |
The above example is invalid because the unroll attribute qualifier is used on a non-loop construct.
| kernel void my_kernel( … ) { int x; __attribute__((opencl_unroll_hint(x)) for (int i=0; i<x; i++) { … } } |
The above example is invalid because the loop unroll factor is not a compile-time constant expression.
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