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

Developer Guide and Reference

Contents

unroll/nounroll

Tells the compiler to unroll or not to unroll a counted loop.

Syntax

#pragma unroll
#pragma unroll
(
n
)
#pragma nounroll
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
[n]
 pragma tells the compiler how many times to unroll a counted loop.
The
unroll
 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.
This pragma is supported only when option
O3
 is set. The
unroll
 pragma overrides any setting of loop unrolling from the command line.
The pragma can be applied for the innermost loop nest as well as for the outer loop nest. If applied to outer loop nests, the current implementation supports complete outer loop unrolling. The loops inside the loop nest are either not unrolled at all or completely unrolled. The compiler generates correct code by comparing
n
 and the loop count.
When unrolling a loop increases register pressure and code size it may be necessary to prevent unrolling of a loop. In such cases, use the
nounroll
 pragma. The
nounroll
 pragma instructs the compiler not to unroll a specified loop.
Examples
Example: Using the
unroll
 pragma for innermost loop unrolling 
void unroll(int a[], int b[], int c[], int d[]) {
  #pragma unroll(4)
  for (int i = 1; i < 100; i++) {
    b[i] = a[i] + 1;
    d[i] = c[i] + 1;
  } 
}
Example: Using the
unroll
 pragma for outer loop unrolling 
int m = 0; 
int dir[4]= {1,2,3,4}; 
int data[10]; 
#pragma unroll (4)  // outer loop unrolling
  for (int i = 0; i < 4; i++) {
     for (int j = dir[i]; data[j]==N ; j+=dir[i])
        m++;
  }
When you place the
unroll
 pragma before the first
for
 loop, it causes the compiler to unroll the outer loop completely. If an
unroll
 pragma is placed before the inner
for
 loop as well as before the outer
for
 loop, the compiler ignores the inner
for
 loop
unroll
 pragma. If the
unroll
 pragma is placed only for the innermost loop, the compiler unrolls the innermost loop according to some factor.

Product and Performance Information

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