[Bug c/101573] New: uninitialized warning could not appear correclty in -O0 optimisation level

[Bug c/101573] New: uninitialized warning could not appear correclty in -O0 optimisation level

[shen980630 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101573

            Bug ID: 101573
           Summary: uninitialized warning could not appear correclty in
                    -O0 optimisation level
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: shen980630 at gmail dot com
  Target Milestone: ---

Hi there,

with the code example below, -O0 -Wunitialized could not display the
uninitialized warning correctly while -O1 -Wunitialized could.

==============
int main() {
    int a;
    for(; a < 5; ++a)
        ;
    return 0;
}
==============

> gcc-12 -O0 -Wuninitialized test.c

produces no warning

> gcc-12 -O1 -Wuninitialized test.c

produces:
test.c: In function 'main':
test.c:3:13: warning: 'a' is used uninitialized [-Wuninitialized]
    3 |     for(; a < 5; ++a)
      |

For comparison, clang with -O0 could correctly display this warning.
> clang -O0 -Wuninitialized test.c

produces:

test.c:3:11: warning: variable 'a' is uninitialized when used here
[-Wuninitialized]
    for(; a < 5; ++a)
          ^
test.c:2:10: note: initialize the variable 'a' to silence this warning
    int a;
         ^
          = 0
1 warning generated.

This behaviour happens in all the versions from gcc-7 to gcc-12 and the latest
version we checked is gcc (GCC) 12.0.0 20210520 (experimental), it also happens
with higher optimisation levels, -O2, -O3, -Os.

Thanks.

[Bug tree-optimization/101573] uninitialized warning could not appear correclty in -O0 optimisation level

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101573

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |msebor at gcc dot gnu.org
          Component|c                           |tree-optimization
             Blocks|                            |24639
           Keywords|                            |diagnostic
     Ever confirmed|0                           |1
           Severity|normal                      |enhancement
   Last reconfirmed|                            |2021-07-22
             Status|UNCONFIRMED                 |ASSIGNED

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
-O0 doesn't run the flow-sensitive uninit part, so that's expected.  What we
could improve is walk PHIs and look at fallthru edges PHI uses.

I have a patch doing that.


Referenced Bugs:

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

[Bug tree-optimization/101573] uninitialized warning could not appear correclty in -O0 optimisation level

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101573

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Created attachment 51195
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51195&action=edit
patch

patch in testing

[Bug tree-optimization/101573] uninitialized warning could not appear correclty in -O0 optimisation level

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101573

--- Comment #3 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:66030d68a7edfc9504a50469598e0707b8f787ce

commit r12-2525-g66030d68a7edfc9504a50469598e0707b8f787ce
Author: Richard Biener <rguenther@suse.de>
Date:   Thu Jul 22 12:26:16 2021 +0200

    tree-optimization/101573 - improve uninit warning at -O0

    We can improve uninit warnings from the early pass by looking
    at PHI arguments on fallthru edges that are uninitialized and
    have uses that are before a possible loop exit.  This catches
    some cases earlier that we'd only warn in a more confusing
    way after early inlining as seen by testcase adjustments.

    It introduces

    FAIL: gcc.dg/uninit-23.c (test for excess errors)

    where we additionally warn

    gcc.dg/uninit-23.c:21:13: warning: 't4' is used uninitialized
[-Wuninitialized]

    which I think is OK even if it's not obvious that the new
    warning is an improvement when you look at the obvious source.

    Somehow for all cases I never get the `'foo' was declared here`
    notes, I didn't dig why that happens but it's odd.

    2021-07-22  Richard Biener  <rguenther@suse.de>

            PR tree-optimization/101573
            * tree-ssa-uninit.c (warn_uninit_phi_uses): New function
            looking at uninitialized PHI arg defs in some constrained cases.
            (warn_uninitialized_vars): Call it.
            (execute_early_warn_uninitialized): Calculate dominators.

            * gcc.dg/uninit-pr101573.c: New testcase.
            * gcc.dg/uninit-15-O0.c: Adjust.
            * gcc.dg/uninit-15.c: Likewise.
            * gcc.dg/uninit-23.c: Likewise.
            * c-c++-common/uninit-17.c: Likewise.

[Bug tree-optimization/101573] uninitialized warning could not appear correclty in -O0 optimisation level

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101573

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |12.0
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
Fixed for GCC 12.

[Bug tree-optimization/101573] uninitialized warning could not appear correclty in -O0 optimisation level

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101573

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |phresnel at gmail dot com

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
*** Bug 43361 has been marked as a duplicate of this bug. ***