[Bug analyzer/94732] New: Analyzer: false positive in MPFR's atan.c

[Bug analyzer/94732] New: Analyzer: false positive in MPFR's atan.c

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94732

            Bug ID: 94732
           Summary: Analyzer: false positive in MPFR's atan.c
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: analyzer
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

Created attachment 48360
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48360&action=edit
testcase

Test with: gcc-10 (Debian 10-20200418-1) 10.0.1 20200418 (experimental) [master
revision 27c171775ab:4c277008be0:c5bac7d127f288fd2f8a1f15c3f30da5903141c6]

When I want to compile GNU MPFR with -fanalyzer, the compilation of atan.c
fails on what appears to be a false positive. I've managed to reduce the
6000-line preprocessed code to code with fewer than 300 lines (attached bug.i
file). More specifically, I've removed
  * blank lines and comments;
  * unused declarations/definitions;
  * code that could have an influence only after the "error";
  * code testing and handling special cases.

I order to see where the issue could come from, I've added 2 lines
  * "((y)->_mpfr_d)[0] = 0;" at the beginning of mpfr_atan_aux;
  * "((tmp2)->_mpfr_d)[0] = 0;" just before the call to mpfr_atan_aux.

Without these 2 lines, "gcc-10 -c -fanalyzer bug.i" gives:

bug.i: In function ‘set_table’:
bug.i:145:9: warning: use of uninitialized value ‘yp’ [CWE-457]
[-Wanalyzer-use-of-uninitialized-value]
  145 |   yp[0] &= ~(((void) 0), sh ==
      |         ^~

where yp is set with

  mp_limb_t *yp = ((y)->_mpfr_d);

So I suppose that the analyzer complains that (y)->_mpfr_d is uninitialized.
This comes from mpfr_atan_aux, and "((y)->_mpfr_d)[0] = 0;" at the beginning of
this function should trigger the same error. If I add this line, I get in a
consistent way:

bug.i: In function ‘mpfr_atan_aux’:
bug.i:154:19: warning: use of uninitialized value ‘<unknown>’ [CWE-457]
[-Wanalyzer-use-of-uninitialized-value]
  154 | ((y)->_mpfr_d)[0] = 0;
      | ~~~~~~~~~~~~~~~~~~^~~

This mpfr_atan_aux function is called at only one place:

              mpfr_atan_aux (tmp2, ukz, twopoweri, n0 - i, tabz);

So I added "((tmp2)->_mpfr_d)[0] = 0;" just before this call. I thought that I
would get an error on this, but I still get an error only on "((y)->_mpfr_d)[0]
= 0;" in mpfr_atan_aux. If I remove this line (just keeping the one before the
call to mpfr_atan_aux), I get the error in set_table only, just like in the
first test.

Now, this appears to be a false positif since (tmp2)->_mpfr_d was initialized
earlier.

I could probably simplify the code even further, focusing on (tmp2)->_mpfr_d
only.

[Bug analyzer/94732] Analyzer: false positive in MPFR's atan.c

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94732

--- Comment #1 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
Here's the corresponding simple testcase:

typedef struct { int *a; } S;
int *f (void);
static void g (S *x)
{
  int *p = x->a;
  p[0] = 0;
}
void h (void)
{
  S x[1];
  x->a = f ();
  g (x);
}

$ gcc-10 -c -fanalyzer bug.i
bug.i: In function ‘g’:
bug.i:6:8: warning: use of uninitialized value ‘p’ [CWE-457]
[-Wanalyzer-use-of-uninitialized-value]
    6 |   p[0] = 0;
      |   ~~~~~^~~
  ‘h’: events 1-2
    |
    |    8 | void h (void)
    |      |      ^
    |      |      |
    |      |      (1) entry to ‘h’
    |......
    |   12 |   g (x);
    |      |   ~~~~~
    |      |   |
    |      |   (2) calling ‘g’ from ‘h’
    |
    +--> ‘g’: events 3-4
           |
           |    3 | static void g (S *x)
           |      |             ^
           |      |             |
           |      |             (3) entry to ‘g’
           |......
           |    6 |   p[0] = 0;
           |      |   ~~~~~~~~   
           |      |        |
           |      |        (4) use of uninitialized value ‘p’ here
           |

[Bug analyzer/94732] Analyzer: false positive in MPFR's atan.c

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94732

--- Comment #2 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:78b9783774bfd3540f38f5b1e3c7fc9f719653d7

commit r10-8012-g78b9783774bfd3540f38f5b1e3c7fc9f719653d7
Author: David Malcolm <dmalcolm@redhat.com>
Date:   Thu Apr 23 21:31:22 2020 -0400

    analyzer: remove -Wanalyzer-use-of-uninitialized-value for GCC 10

    From what I can tell -Wanalyzer-use-of-uninitialized-value has not
    yet found a true diagnostic in real-world code, and seems to be
    particularly susceptible to false positives.  These relate to bugs in
    the region_model code.

    For GCC 10 it seems best to remove this warning, which this patch does.
    Internally it also removes POISON_KIND_UNINIT.

    I'm working on a rewrite of the region_model code for GCC 11 that I
    hope will fix these issues, and allow this warning to be reintroduced.

    gcc/analyzer/ChangeLog:
            PR analyzer/94447
            PR analyzer/94639
            PR analyzer/94732
            PR analyzer/94754
            * analyzer.opt (Wanalyzer-use-of-uninitialized-value): Delete.
            * program-state.cc (selftest::test_program_state_dumping): Update
            expected dump result for removal of "uninit".
            * region-model.cc (poison_kind_to_str): Delete POISON_KIND_UNINIT
            case.
            (root_region::ensure_stack_region): Initialize stack with null
            svalue_id rather than with a typeless POISON_KIND_UNINIT value.
            (root_region::ensure_heap_region): Likewise for the heap.
            (region_model::dump_summary_of_rep_path_vars): Remove
            summarization of uninit values.
            (region_model::validate): Remove check that the stack has a
            POISON_KIND_UNINIT value.
            (poisoned_value_diagnostic::emit): Remove POISON_KIND_UNINIT
            case.
            (poisoned_value_diagnostic::describe_final_event): Likewise.
            (selftest::test_dump): Update expected dump result for removal of
            "uninit".
            (selftest::test_svalue_equality): Remove "uninit" and "freed".
            * region-model.h (enum poison_kind): Remove POISON_KIND_UNINIT.

    gcc/ChangeLog:
            PR analyzer/94447
            PR analyzer/94639
            PR analyzer/94732
            PR analyzer/94754
            * doc/invoke.texi (Static Analyzer Options): Remove
            -Wanalyzer-use-of-uninitialized-value.
            (-Wno-analyzer-use-of-uninitialized-value): Remove item.

    gcc/testsuite/ChangeLog:
            PR analyzer/94447
            PR analyzer/94639
            PR analyzer/94732
            PR analyzer/94754
            * gcc.dg/analyzer/data-model-1.c: Mark "use of uninitialized
            value" warnings as xfail for now.
            * gcc.dg/analyzer/data-model-5b.c: Remove uninitialized warning.
            * gcc.dg/analyzer/pr94099.c: Mark "uninitialized" warning as xfail
            for now.
            * gcc.dg/analyzer/pr94447.c: New test.
            * gcc.dg/analyzer/pr94639.c: New test.
            * gcc.dg/analyzer/pr94732.c: New test.
            * gcc.dg/analyzer/pr94754.c: New test.
            * gcc.dg/analyzer/zlib-6.c: Mark "uninitialized" warning as xfail
            for now.

[Bug analyzer/94732] Analyzer: false positive in MPFR's atan.c

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94732

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

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

--- Comment #3 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
Should be fixed by r10-8012-g78b9783774bfd3540f38f5b1e3c7fc9f719653d7.