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

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.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>