Intel® oneAPI DPC++/C++ Compiler Developer Guide and Reference

Development Reference Guides

Contents

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.
Example: Using 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];
            }
       }
    } 
}

Product and Performance Information

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