On 7/25/21 7:33 PM, Dominique Pellé via Gcc wrote:
Hi
Hello.
I read https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html
but was left wondering: is there a way to annotate a function
to indicate that a return value is likely (or unlikely)?
Interesting idea :) No, we don't support that right now.
For example, let's say we have this function:
// Return OK (=0) in case of success (frequent case)
// or an error code != 0 in case of failure (rare case).
int do_something();
If it's unlikely to fail, I wish I could declare the function like
this (pseudo-code!):
int do_something() __likely_return(OK);
So wherever it's used, the optimizer can optimize branch
prediction and the instruction cache. In other words, lines
like this:
if (do_something() == OK)>
... would implicitly be similar to:
// LIKELY defined as __builtin_expect((x), 1).
if (LIKELY(do_something() == OK))
The advantage of being able to annotate the declaration,
is that we only need to annotate once in the header, and
all uses of the function can benefit from the optimization
without polluting/modifying all code where the function
is called.
I see your point, seems like a good idea. The question is,
how much would it take to implement and what's benefit
of the suggested hints.
Another example: a function that would be unlikely to
return NULL could be declared as:
void *foo() __unlikely_returns(NULL);
Note that modern CPUs have branch predictors and a condition
of 'if (ptr == 0)' can be guessed quite easily.
This last example would be a bit similar to the
__attribute__((malloc)) since I read about it in the doc:
In addition, the GCC predicts that a function with
the attribute returns non-null in most cases.
Of course __attribute__((malloc)) gives other guarantees
(return value cannot alias any other pointer) so it's not
equivalent.
Note we have a special branch probability for malloc:
gcc/predict.def:54
Would attribute __likely_return() and __unlikely_return()
make sense?
Similarly, we have now:
/* Branch to basic block containing call marked by noreturn attribute. */
DEF_PREDICTOR (PRED_NORETURN, "noreturn call", PROB_VERY_LIKELY,
PRED_FLAG_FIRST_MATCH)
Thanks for the ideas,
Martin
Is there already a way to achieve this which I missed in
the doc?
Regards
Dominique
