attributes-loopUnroll.adoc

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Specifying Attribute For Unrolling Loops

Specifying Attribute For Unrolling Loops.

__attribute__((opencl_unroll_hint))

__attribute__((opencl_unroll_hint(n)))

Description

The attributeopencl_unroll_hint and attributeopencl_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 attributeopencl_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 attributeopencl_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.

Also see

attribute, Type Attributes, Variable Attributes

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