[Bug c++/95190] New: Documentation for -Wstringop-overflow

[Bug c++/95190] New: Documentation for -Wstringop-overflow

[stayprivate at gmail dot com]

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

            Bug ID: 95190
           Summary: Documentation for -Wstringop-overflow
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stayprivate at gmail dot com
  Target Milestone: ---

I'm compiling code with 10.1 for the first time. I'm getting a warning:

"writing 1 bytes into a region of size 0 -Wstringop-overflow=". Yet
-Wno-stringop-overflow is passed to the compiler.  I tried disabling the
warning with #pragma diagnostic, no luck there.

Then I realized the error came from the link stage. Surely related to lto. I
modified my cmake files to add the -Wno-stringop-overflow to the link command
line and the warning disappear.

I'm surprise by this behavior. I looked through the documentation and could not
find anything that mentionned this. Is this something that is worth improving
in  the documentation or do I just need to get smarter ;-)

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

[msebor at gcc dot gnu.org]

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?

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.)

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?

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

[rguenth at gcc dot gnu.org]

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

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
This is new behavior for warnings in GCC 10 and now how all other optimization
options behave - the option state is fixed per function at compile-time and
carried over to link-time.

Indeed warning options are not mentioned in the docs, let me fix that.

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

[marxin at gcc dot gnu.org]

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

--- Comment #3 from Martin Liška <marxin at gcc dot gnu.org> ---
> "writing 1 bytes into a region of size 0 -Wstringop-overflow=". Yet
> -Wno-stringop-overflow is passed to the compiler.  I tried disabling the
> warning with #pragma diagnostic, no luck there.

I must confirm that '#pragma diagnostic' is completely unsupported by LTO and I
spent some time tweaking into it. Eventually, I bailed out.

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

[stayprivate at gmail dot com]

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?
>
> --
> You are receiving this mail because:
> You reported the bug.

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

[stayprivate at gmail dot com]

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

--- Comment #5 from Mario Charest <stayprivate at gmail dot com> ---
On Tue, May 19, 2020 at 2:35 AM rguenth at gcc dot gnu.org <
gcc-bugzilla@gcc.gnu.org> wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95190
>
> --- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
> This is new behavior for warnings in GCC 10 and now how all other
> optimization
> options behave - the option state is fixed per function at compile-time and
> carried over to link-time.
>
> Indeed warning options are not mentioned in the docs, let me fix that.
>

Great! Thanks, may I suggest adding it the changes and portings web page.
This new behavior may required tweaking of build files.


> --
> You are receiving this mail because:
> You reported the bug.

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

[cvs-commit at gcc dot gnu.org]

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

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Richard Biener <rguenth@gcc.gnu.org>:

https://gcc.gnu.org/g:ab7eca92926fdc1da880120c116a1832fce56a29

commit r11-565-gab7eca92926fdc1da880120c116a1832fce56a29
Author: Richard Biener <rguenther@suse.de>
Date:   Tue May 19 08:36:13 2020 +0200

    lto/95190 - amend -flto docs for diagnostic option handling

    This documents new GCC 10 behavior on diagnostic options and -flto.

    2020-05-22  Richard Biener  <rguenther@suse.de>

            PR lto/95190
            * doc/invoke.texi (flto): Document behavior of diagnostic
            options.

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

[cvs-commit at gcc dot gnu.org]

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

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Richard Biener
<rguenth@gcc.gnu.org>:

https://gcc.gnu.org/g:a24d9b5bbb6a298ff4e55d731d5e436b0da9e38a

commit r10-8169-ga24d9b5bbb6a298ff4e55d731d5e436b0da9e38a
Author: Richard Biener <rguenther@suse.de>
Date:   Tue May 19 08:36:13 2020 +0200

    lto/95190 - amend -flto docs for diagnostic option handling

    This documents new GCC 10 behavior on diagnostic options and -flto.

    2020-05-22  Richard Biener  <rguenther@suse.de>

            PR lto/95190
            * doc/invoke.texi (flto): Document behavior of diagnostic
            options.

    (cherry picked from commit ab7eca92926fdc1da880120c116a1832fce56a29)

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

[egallager at gcc dot gnu.org]

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

Eric Gallager <egallager at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu.org

--- Comment #8 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to CVS Commits from comment #7)
> The releases/gcc-10 branch has been updated by Richard Biener
> <rguenth@gcc.gnu.org>:
> 
> https://gcc.gnu.org/g:a24d9b5bbb6a298ff4e55d731d5e436b0da9e38a
> 
> commit r10-8169-ga24d9b5bbb6a298ff4e55d731d5e436b0da9e38a
> Author: Richard Biener <rguenther@suse.de>
> Date:   Tue May 19 08:36:13 2020 +0200
> 
>     lto/95190 - amend -flto docs for diagnostic option handling
>     
>     This documents new GCC 10 behavior on diagnostic options and -flto.
>     
>     2020-05-22  Richard Biener  <rguenther@suse.de>
>     
>             PR lto/95190
>             * doc/invoke.texi (flto): Document behavior of diagnostic
>             options.
>     
>     (cherry picked from commit ab7eca92926fdc1da880120c116a1832fce56a29)

so... FIXED now?

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

[rguenth at gcc dot gnu.org]

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

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|WAITING                     |RESOLVED

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
I suppose so.