[Bug ipa/109914] --suggest-attribute=pure misdiagnoses static functions

["eggert at cs dot ucla.edu" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109914

Paul Eggert <eggert at cs dot ucla.edu> changed:

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                 CC|                            |eggert at cs dot ucla.edu

--- Comment #4 from Paul Eggert <eggert at cs dot ucla.edu> ---
(In reply to Jan Hubicka from comment #2)
> The reason why gcc warns is that it is unable to prove that the function is
> always finite.

I don't see why finiteness matters. If a pure function returns the first time
it's called, and if there's no change to visible state after that, it should
return the second time with the same value. And if the pure function didn't
return the first time evaluation won't even get to the second time. So common
subexpression elimination (which is the point of 'pure') will work even if a
pure function does not return.

C23 has standardized this stuff with the [[reproducible]] attribute, and as far
as I can see (the wording is admittedly murky) C23 does not impose a finiteness
constraint on reproducible functions. If my reading of C23 is correct, GCC
should not impose finiteness constraints on reproducible functions when it gets
around to implementing [[reproducible]], and if [[reproducible]] and
__attribute__((pure)) are supposed to be the same thing then GCC should drop
the  finiteness constraint on pure functions as well.


I agree with Bruno's main point that none of this stuff should matter for
static functions. --suggest-attribute=* warnings are useless chatter for static
functions.


(I ran into this GCC bug when building recent versions of the TZDB code, which
is why I found this bug report.)