16.7 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:

• assumptions: If this condition is false, the rest of the information is invalid.
• noloop_assumptions on RTL, may_be_zero on GIMPLE: If this condition is true, the loop exits in the first iteration.
• infinite: If this condition is true, the loop is infinite. This condition is only available on RTL. On GIMPLE, conditions for finiteness of the loop are included in assumptions.
• niter_expr on RTL, niter on GIMPLE: The expression that gives number of iterations. The number of iterations is defined as the number of executions of the loop latch.

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).

On GIMPLE, below constraint flags affect semantics of some APIs of number of iterations analyzer:

• LOOP_C_INFINITE: If this constraint flag is set, the loop is known to be infinite. APIs like number_of_iterations_exit can return false directly without doing any analysis.
• LOOP_C_FINITE: If this constraint flag is set, the loop is known to be finite, in other words, loop’s number of iterations can be computed with assumptions be true.

Generally, the constraint flags are set/cleared by consumers which are loop optimizers. It’s also the consumers’ responsibility to set/clear constraints correctly. Failing to do that might result in hard to track down bugs in scev/niter consumers. One typical use case is vectorizer: it drives number of iterations analyzer by setting LOOP_C_FINITE and vectorizes possibly infinite loop by versioning loop with analysis result. In return, constraints set by consumers can also help number of iterations analyzer in following optimizers. For example, niter of a loop versioned under assumptions is valid unconditionally.

Other constraints may be added in the future, for example, a constraint indicating that loops’ latch must roll thus may_be_zero would be false unconditionally.