[Bug analyzer/108704] New: Many -Wanalyzer-use-of-uninitialized-value false positives seen in qemu's softfloat.c

[Bug analyzer/108704] New: Many -Wanalyzer-use-of-uninitialized-value false positives seen in qemu's softfloat.c

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108704

            Bug ID: 108704
           Summary: Many -Wanalyzer-use-of-uninitialized-value false
                    positives seen in qemu's softfloat.c
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: analyzer
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: dmalcolm at gcc dot gnu.org
  Target Milestone: ---

Created attachment 54425
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54425&action=edit
Reduced reproducer

Trunk shows many (dozens?) of -Wanalyzer-use-of-uninitialized-value false
positives on qemu's softfloat.c

https://godbolt.org/z/MTe9597Eh shows:

<source>: In function 'test':
<source>:27:9: warning: use of uninitialized value 'fp0' [CWE-457]
[-Wanalyzer-use-of-uninitialized-value]
   27 |   fp0 = floatx80_add(fp0, float32_to_floatx80((0x3F800000)));
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  'test': events 1-3
    |
    |   24 |   floatx80 fp0;
    |      |            ^~~
    |      |            |
    |      |            (1) region created on stack here
    |      |            (2) capacity: 16 bytes
    |......
    |   27 |   fp0 = floatx80_add(fp0, float32_to_floatx80((0x3F800000)));
    |      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |         |
    |      |         (3) use of uninitialized value 'fp0' here
    |
Compiler returned: 0

where fp0 appears to have been initialized on the line above.

[Bug analyzer/108704] [13 Regression] Many -Wanalyzer-use-of-uninitialized-value false positives seen in qemu's softfloat.c

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108704

David Malcolm <dmalcolm at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |13.0
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2023-02-07
            Summary|Many                        |[13 Regression] Many
                   |-Wanalyzer-use-of-uninitial |-Wanalyzer-use-of-uninitial
                   |ized-value false positives  |ized-value false positives
                   |seen in qemu's softfloat.c  |seen in qemu's softfloat.c
     Ever confirmed|0                           |1

--- Comment #1 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
Doesn't seem to affect gcc 12.2 so marking as a regression.

[Bug analyzer/108704] [13 Regression] Many -Wanalyzer-use-of-uninitialized-value false positives seen in qemu's softfloat.c

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108704

--- Comment #2 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
Adding -fno-analyzer-state-purge fixes the false positive, looks like it's
erroneously pruning the value of fp0 immediately after the first assignment.

[Bug analyzer/108704] [13 Regression] Many -Wanalyzer-use-of-uninitialized-value false positives seen in qemu's softfloat.c

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108704

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by David Malcolm <dmalcolm@gcc.gnu.org>:

https://gcc.gnu.org/g:77bb54b1b07add45007c664724b726541d672ef3

commit r13-5745-g77bb54b1b07add45007c664724b726541d672ef3
Author: David Malcolm <dmalcolm@redhat.com>
Date:   Wed Feb 8 13:47:36 2023 -0500

    analyzer: fix overzealous state purging with on-stack structs [PR108704]

    PR analyzer/108704 reports many false positives seen from
    -Wanalyzer-use-of-uninitialized-value on qemu's softfloat.c on code like
    the following:

       struct st s;
       s = foo ();
       s = bar (s); // bogusly reports that s is uninitialized here

    where e.g. "struct st" is "floatx80" in the qemu examples.

    The root cause is overzealous purging of on-stack structs in the code I
    added in r12-7718-gfaacafd2306ad7, where at:

            s = bar (s);

    state_purge_per_decl::process_point_backwards "sees" the assignment to 's'
    and stops processing, effectively treating 's' as unneeded before this
    stmt, not noticing the use of 's' in the argument.

    Fixed thusly.

    The patch greatly reduces the number of
    -Wanalyzer-use-of-uninitialized-value warnings from my integration tests:
      ImageMagick-7.1.0-57:  10 ->  6   (-4)
                  qemu-7.2: 858 -> 87 (-771)
             haproxy-2.7.1:   1 ->  0   (-1)
    All of the above that I've examined appear to be false positives.

    gcc/analyzer/ChangeLog:
            PR analyzer/108704
            * state-purge.cc (state_purge_per_decl::process_point_backwards):
            Don't stop processing the decl if it's fully overwritten by
            this stmt if it's also used by this stmt.

    gcc/testsuite/ChangeLog:
            PR analyzer/108704
            * gcc.dg/analyzer/uninit-7.c: New test.
            * gcc.dg/analyzer/uninit-pr108704.c: New test.

    Signed-off-by: David Malcolm <dmalcolm@redhat.com>

[Bug analyzer/108704] Many -Wanalyzer-use-of-uninitialized-value false positives seen in qemu's softfloat.c

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108704

David Malcolm <dmalcolm at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[13 Regression] Many        |Many
                   |-Wanalyzer-use-of-uninitial |-Wanalyzer-use-of-uninitial
                   |ized-value false positives  |ized-value false positives
                   |seen in qemu's softfloat.c  |seen in qemu's softfloat.c

--- Comment #4 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
Should be fixed on trunk for GCC 13 by the above commit.

I introduced the bug in r12-7718-gfaacafd2306ad7 so in theory it affects GCC 12
as well, but I haven't been able to reproduce it there.  The bug is probably
present there but latent, so I'm keeping this bug open to track backporting the
fix to the gcc 12 branch.

[Bug analyzer/108704] Many -Wanalyzer-use-of-uninitialized-value false positives seen in qemu's softfloat.c

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108704

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by David Malcolm
<dmalcolm@gcc.gnu.org>:

https://gcc.gnu.org/g:5da2126c4df8d83c2b2f9de7bb393ab4f5832840

commit r12-9364-g5da2126c4df8d83c2b2f9de7bb393ab4f5832840
Author: David Malcolm <dmalcolm@redhat.com>
Date:   Wed Mar 29 14:16:48 2023 -0400

    analyzer: fix overzealous state purging with on-stack structs [PR108704]

    PR analyzer/108704 reports many false positives seen from
    -Wanalyzer-use-of-uninitialized-value on qemu's softfloat.c on code like
    the following:

       struct st s;
       s = foo ();
       s = bar (s); // bogusly reports that s is uninitialized here

    where e.g. "struct st" is "floatx80" in the qemu examples.

    The root cause is overzealous purging of on-stack structs in the code I
    added in r12-7718-gfaacafd2306ad7, where at:

            s = bar (s);

    state_purge_per_decl::process_point_backwards "sees" the assignment to 's'
    and stops processing, effectively treating 's' as unneeded before this
    stmt, not noticing the use of 's' in the argument.

    Fixed thusly.

    The patch greatly reduces the number of
    -Wanalyzer-use-of-uninitialized-value warnings from my integration tests:
      ImageMagick-7.1.0-57:  10 ->  6   (-4)
                  qemu-7.2: 858 -> 87 (-771)
             haproxy-2.7.1:   1 ->  0   (-1)
    All of the above that I've examined appear to be false positives.

    Cherrypicked from r13-5745-g77bb54b1b07add.

    gcc/analyzer/ChangeLog:
            PR analyzer/108704
            * state-purge.cc (state_purge_per_decl::process_point_backwards):
            Don't stop processing the decl if it's fully overwritten by
            this stmt if it's also used by this stmt.

    gcc/testsuite/ChangeLog:
            PR analyzer/108704
            * gcc.dg/analyzer/uninit-7.c: New test.
            * gcc.dg/analyzer/uninit-pr108704.c: New test.

    Signed-off-by: David Malcolm <dmalcolm@redhat.com>

[Bug analyzer/108704] Many -Wanalyzer-use-of-uninitialized-value false positives seen in qemu's softfloat.c

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108704

David Malcolm <dmalcolm at gcc dot gnu.org> changed:

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

--- Comment #6 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
Should be fixed on gcc 12 branch by the above (for the eventual gcc 12.3
release); marking as resolved.