Texas Instruments TMS320C6000 Programmer's Manual page 108

3.4.3.3 Communicating Trip-Count Information to the Compiler
When invoking the compiler, use the following options to communicate trip-
count information to the compiler:
Use the –o3 and –pm compiler options to allow the optimizer to access the
whole program or large parts of it and to characterize the behavior of loop
trip counts.
Use the MUST_ITERATE pragma to help reduce code size by preventing
the generation of a redundant loop or by allowing the compiler (with or
without the –ms option) to software pipeline innermost loops.
You can use the MUST_ITERATE and PROB_ITERATE pragma to convey
many different types of information about the trip count to the compiler.
The MUST_ITERATE pragma can convey that the trip count will always
equal some value.
/* This loop will always execute exactly 30 times */
#pragma MUST_ITERATE (30, 30);
for (j = 0; j < x; j++)
The MUST_ITERATE pragma can convey that the trip count will be great-
er than some minimum value or smaller than some maximum value. The
latter is useful when interrupts need to occur inside of loops and you are
using the -mi<n> option. Refer to section 7.4, Interruptible Loops .
/* This loop will always execute at least 30 times */
#pragma MUST_ITERATE (30);
for (j = 0; j < x; j++)
The MUST_ITERATE pragma can convey that the trip count is always di-
visible by a value.
/* The trip count will execute some multiple of 4 times */
#pragma MUST_ITERATE (,, 4);
for (j = 0; j < x; j++)
This information call all be combined as well into a single C statement:
#pragma MUST_ITERATE (8, 48, 8);
for (j = 0; j < x; j++)
The compiler knows that this loop will execute some multiple of 8 (between 8
and 48) times. This information is useful in providing more information about
unrolling a loop or the ability to perform word accesses on a loop.
Refining C/C++ Code
Optimizing C/C++ Code 3-43