[Bug tree-optimization/18487] Warnings for pure and const functions that are not actually pure or const

[dberlin at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=18487

--- Comment #25 from Daniel Berlin  ---
This seems like a bad idea, and is impossible in general.

The whole point of the attributes is to tell the compiler things are pure/const
in cases it can't already prove.

It can already prove a lot, and doesn't need help in most of the simple
examples being given (in other bugs). 

You are basically going to warn in the cases the compiler can't prove it (IE
sees something it thinks makes the function not pure/const), and those are
*exactly* the cases the attribute exists for - where the compiler doesn't know,
but you do.

[Bug tree-optimization/18487] Warnings for pure and const functions that are not actually pure or const

[dberlin at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=18487

--- Comment #29 from Daniel Berlin  ---
Let me try to explain a different way:
The only functions GCC can warn about are those that don’t need the
attributes in the first place. The way any warning would work is to detect
whether it is pure/const, and then see how the user marked it. So anything
it can properly detect as right or wrong didn’t need an attribute to begin
with - the compiler could already tell if it was pure/const

Rather than tell the user they got it wrong, you might as well tell the
user to remove the attribute because it isn’t necessary and won’t be
necessary.

This is precisely why attributes are meant for when you are sure you know
more than the compiler can tell, and *no other time *. It is a tool for
experts.
Giving a bunch of really contrived examples where users may update things
wrong doesn’t seem like a good motivation to make a warning that can only
possibly have a really high false positive rate.
The same logic applies to a lot of expert-use-only attributes.  It is
assumed you know what you are doing, because the compiler can’t tell you
you are wrong accurately




On Sat, Sep 4, 2021 at 4:40 PM federico.kircheis at gmail dot com <
gcc-bugzi...@gcc.gnu.org> wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=18487
>
> --- Comment #28 from Federico Kircheis  com> ---
> >Edit: sorry, my last comment about what GCC thinks is wrong.
>
> Unless it is going to inline the function call, in that case the
> attributes are
> as-if ignored (at least the case I've tested with GCC 11.2).
>
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