[Bug lto/95190] Documentation for -Wstringop-overflow

["stayprivate at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95190

--- Comment #4 from Mario Charest <stayprivate at gmail dot com> ---
On Mon, May 18, 2020 at 1:09 PM msebor at gcc dot gnu.org <
gcc-bugzilla@gcc.gnu.org> wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95190
>
> Martin Sebor <msebor at gcc dot gnu.org> changed:
>
>            What    |Removed                     |Added
>
> ----------------------------------------------------------------------------
>    Last reconfirmed|                            |2020-05-18
>              Status|UNCONFIRMED                 |WAITING
>           Component|c++                         |lto
>      Ever confirmed|0                           |1
>                  CC|                            |marxin at gcc dot gnu.org
> ,
>                    |                            |msebor at gcc dot gnu.org
>            Keywords|                            |documentation
>
> --- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
> Which part do you find surprising?  That the warning is issued during the
> LTO
> stage at all or that -Wno-stringop-overflow can be used during the LTO
> stage to
> suppress it?
>

Mostly the LTO stage. I got bitten because I assumed warning came from the
compiler. If the LTO stage would have handle #pragma diagnostic i would
have never noticed where it came from.


>
> During LTO the same compiler options should be implicitly enabled as during
> ordinary compilation, including warnings.  (This presents some challenges
> when
> the compilation was done with different options for different files.)
>

Most project with cmake using lto will required tweaking.  Typically
warning options are specified with add_compiler_options(). Now every
warning flags must also be passed to the linker, impossible to know which
flag has an effect on the LTO stage or not.


> It also means that all the same warnings should be expected to be
> implicitly
> enabled during LTO that were explicitly enabled during the compilation
> stage.
> I'd expect to see only "late" warnings during LTO, i.e., those that depend
> on
> optimization.  (I understand this doesn't work completely consistently yet
> but
> I believe that's the goal.)
>
> So this effect isn't specific to -Wstringop-overflow, but perhaps it would
> be
> worth mentioning under -flto?
>
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