-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, e.g.,

  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; 
} 

When the -Cu compiler option is used, 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; 
} 

The Compiler issues a warning 'Unrolling loop' and transforms the for loop into

j += 1;

i  = 2;

because the loop body is executed only once.