GNU Compiler Collection (GCC) Internals: Number of iterations |
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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.
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