unroll_and_jam/nounroll_and_jam
Enables or disables
loop unrolling and jamming. These pragmas can only be applied to iterative for
loops.
Syntax
#pragma unroll_and_jam
#pragma unroll_and_jam
(
n
)
#pragma nounroll_and_jam
Arguments
- n
- The unrolling factor representing the number of times to unroll a loop; it must be an integer constant from 0 through 255
Description
The
unroll_and_jam
pragma partially
unrolls one or more loops higher in the nest than the innermost loop and
fuses/jams the resulting loops back together. This transformation allows more
reuses in the loop.
This pragma is not effective on innermost loops. Ensure
that the immediately following loop is not the innermost loop after
compiler-initiated interchanges are completed.
Specifying this pragma is a hint to the compiler that the
unroll and jam sequence is legal and profitable. The compiler enables this
transformation whenever possible.
The
unroll_and_jam
pragma must
precede the
for
statement for each
for
loop it affects. If
n
is specified, the optimizer unrolls the loop
n
times. If
n
is omitted or if
it is outside the allowed range, the optimizer assigns the number of times to
unroll the loop. The compiler generates correct code by comparing
n
and the loop
count.
This pragma is supported only when compiler option
O3
is set. The
unroll_and_jam
pragma overrides any
setting of loop unrolling from the command line.
When unrolling a loop increases register pressure and
code size it may be necessary to prevent unrolling of a nested loop or an
imperfect nested loop. In such cases, use the
nounroll_and_jam
pragma. The
nounroll_and_jam
pragma hints to
the compiler not to unroll a specified loop.
The
unroll_and_jam
and
nounroll_and_jam
pragmas are supported in host code only.
Examples
Use the
unroll_and_jam
pragma:
int a[10][10];
int b[10][10];
int c[10][10];
int d[10][10];
void unroll(int n) {
int i,j,k;
#pragma unroll_and_jam (6)
for (i = 1; i < n; i++) {
#pragma unroll_and_jam (6)
for (j = 1; j < n; j++) {
for (k = 1; k < n; k++){
a[i][j] += b[i][k]*c[k][j];
}
}
}
}