[Bug c/99340] New: [10/11 Regression] -Werror=maybe-uninitialized warning with -fPIE, but not -fPIC

[Bug c/99340] New: [10/11 Regression] -Werror=maybe-uninitialized warning with -fPIE, but not -fPIC

[doko at debian dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99340

            Bug ID: 99340
           Summary: [10/11 Regression] -Werror=maybe-uninitialized warning
                    with -fPIE, but not -fPIC
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: doko at debian dot org
  Target Milestone: ---

seen building the gsequencer package.
the warning was not emitted with gcc-10 branch 20210110, but is emitted with
20210228, also emitted with trunk 20210220.

$ cat ags_midi_buffer_util.i
long ags_midi_buffer_util_seek_message_delta_time;
int ags_midi_buffer_util_seek_message_i;
int ags_midi_buffer_util_get_varlength() { return 0; }
char ags_midi_buffer_util_seek_message() {
  long current_delta_time;
  for (; ags_midi_buffer_util_seek_message_i;
       ags_midi_buffer_util_seek_message_i++) {
    ags_midi_buffer_util_get_varlength(&current_delta_time);
    ags_midi_buffer_util_seek_message_delta_time = current_delta_time;
  }
  return 0;
}

$ gcc -std=gnu99 -Wall -Werror=maybe-uninitialized -fstack-protector-strong
-fPIE -O1 -c ags_midi_buffer_util.i
ags_midi_buffer_util.i: In function 'ags_midi_buffer_util_seek_message':
ags_midi_buffer_util.i:5:8: error: 'current_delta_time' may be used
uninitialized in this function [-Werror=maybe-uninitialized]
    5 |   long current_delta_time;
      |        ^~~~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors

[Bug c/99340] [10/11 Regression] -Werror=maybe-uninitialized warning with -fPIE, but not -fPIC

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99340

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |marxin at gcc dot gnu.org
   Last reconfirmed|                            |2021-03-02
             Status|UNCONFIRMED                 |WAITING
     Ever confirmed|0                           |1

--- Comment #1 from Martin Liška <marxin at gcc dot gnu.org> ---
I see the warning (with -fPIE) for all releases I have (4.8.0+).
But yes, -fPIC does not report the warning.

[Bug c/99340] -Werror=maybe-uninitialized warning with -fPIE, but not -fPIC

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99340

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
PIC allows interposing ags_midi_buffer_util_get_varlength and thus possibly
initializing the argument.  PIE does not allow this so we see it is not
initialized.

I suppose the change on the branch is for some unreduced testcase where
different optimization might trigger the new warning (correctly I think).

[Bug c/99340] -Werror=maybe-uninitialized warning with -fPIE, but not -fPIC

[doko at debian dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99340

--- Comment #3 from Matthias Klose <doko at debian dot org> ---
Created attachment 50284
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50284&action=edit
preprocessed source

original test case before reducing

gcc -std=gnu99 -Werror=uninitialized -Werror=maybe-uninitialized -Wall
-Wno-unused-variable -Wno-unused-but-set-variable -Wformat -Wno-pointer-sign
-Werror=format-security -fstack-protector-strong -fPIC -O2 -c
ags_midi_buffer_util_orig.i

[Bug c/99340] -Werror=maybe-uninitialized warning with -fPIE, but not -fPIC

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99340

--- Comment #4 from Martin Liška <marxin at gcc dot gnu.org> ---
(In reply to Matthias Klose from comment #3)
> Created attachment 50284 [details]
> preprocessed source
> 
> original test case before reducing
> 
> gcc -std=gnu99 -Werror=uninitialized -Werror=maybe-uninitialized -Wall
> -Wno-unused-variable -Wno-unused-but-set-variable -Wformat -Wno-pointer-sign
> -Werror=format-security -fstack-protector-strong -fPIC -O2 -c
> ags_midi_buffer_util_orig.i

Using this with -fPIE reports the warning only on gcc-10 branch. All other is
fine.

[Bug c/99340] -Werror=maybe-uninitialized warning with -fPIE, but not -fPIC

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99340

Martin Liška <marxin at gcc dot gnu.org> changed:

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

--- Comment #5 from Martin Liška <marxin at gcc dot gnu.org> ---
Started on gcc-10 branch with g:eddcb627ccfbd97e025cf366cc3f3bad76211785.
Anyway, the warning contains quite some false positives..

[Bug c/99340] -Werror=maybe-uninitialized warning with -fPIE, but not -fPIC

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99340

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 9 warns as well.  I think this was a false negative which is now fixed.

Note GCC 10.1.0 and GCC 10.2.0 warn for me as well, so something must have
regressed this between 10.2.0 and g:eddcb627ccfbd97e025cf366

I'm inclined to mark as INVALID.

[Bug c/99340] -Werror=maybe-uninitialized warning with -fPIE, but not -fPIC

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99340

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |INVALID
                 CC|                            |msebor at gcc dot gnu.org
           Keywords|                            |diagnostic
             Blocks|                            |24639

--- Comment #7 from Martin Sebor <msebor at gcc dot gnu.org> ---
The IL with -fPIC shows a call to ags_midi_buffer_util_get_varlength() while
the IL with -fPIE has the call expanded inline, which is what less the warning
see current_delta_time is not initialized.   With that, resolving as invalid
per c6.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24639
[Bug 24639] [meta-bug] bug to track all Wuninitialized issues