14.7 Number of iterations analysis

Both on GIMPLE and on RTL, there are functions available to determine the number of iterations of a loop, with a similar interface. The number of iterations of a loop in GCC is defined as the number of executions of the loop latch. In many cases, it is not possible to determine the number of iterations unconditionally – the determined number is correct only if some assumptions are satisfied. The analysis tries to verify these conditions using the information contained in the program; if it fails, the conditions are returned together with the result. The following information and conditions are provided by the analysis:

Both on GIMPLE and on RTL, it necessary for the induction variable analysis framework to be initialized (SCEV on GIMPLE, loop-iv on RTL). On GIMPLE, the results are stored to struct tree_niter_desc structure. Number of iterations before the loop is exited through a given exit can be determined using number_of_iterations_exit function. On RTL, the results are returned in struct niter_desc structure. The corresponding function is named check_simple_exit. There are also functions that pass through all the exits of a loop and try to find one with easy to determine number of iterations – find_loop_niter on GIMPLE and find_simple_exit on RTL. Finally, there are functions that provide the same information, but additionally cache it, so that repeated calls to number of iterations are not so costly – number_of_latch_executions on GIMPLE and get_simple_loop_desc on RTL.

Note that some of these functions may behave slightly differently than others – some of them return only the expression for the number of iterations, and fail if there are some assumptions. The function number_of_latch_executions works only for single-exit loops. The function number_of_cond_exit_executions can be used to determine number of executions of the exit condition of a single-exit loop (i.e., the number_of_latch_executions increased by one).