-Cu: Loop Unrolling

OPTIMIZATIONS

Function

-Cu[=i<number>] 

<number>: number of iterations for unrolling, between 0 and 1024

None

None

#pragma LOOP_UNROLL: Force Loop Unrolling

#pragma NO_LOOP_UNROLL: Disable Loop Unrolling

Enables loop unrolling with the following restrictions:

• Only simple for statements are unrolled, for example,

  for (i=0; i<10; i++)

• Initialization and test of the loop counter must be done with a constant.
• Only <, >, <=, >= are permitted in a condition.
• Only ++ or -- are allowed for the loop variable increment or decrement.
• The loop counter must be integral.
• No change of the loop counter is allowed within the loop.
• The loop counter must not be used on the left side of an assignment.
• No address operator (&) is allowed on the loop counter within the loop.
• Only small loops are unrolled:
  • Loops with few statements within the loop.
  • Loops with fewer than 16 increments or decrements of the loop counter. The bound may be changed with the optional argument = i<number>. The -Cu=i20 option unrolls loops with a maximum of 20 iterations.

-Cu int i, j;

j = 0;

for (i=0; i<3; i++) {

  j += i;

}

With the -Cu compiler option given, the Compiler issues an information message 'Unrolling loop' and transforms this loop as shown in the following listing.

j += 1;
j += 2;

i  = 3;

The Compiler also transforms some special loops, i.e., loops with a constant condition or loops with only one pass:

for (i=1; i>3; i++) {
  j += i;

}

The Compiler issues an information message 'Constant condition found, removing loop' and transforms the loop into a simple assignment, because the loop body is never executed:

i=1;

for (i=1; i<2; i++) {
  j += i;

}

Because the loop body is executed only once, the Compiler issues a warning 'Unrolling loop' and transforms the for loop into:

j += 1;

i  = 2;