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From: "eggert at gnu dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/63943] New: -Wmaybe-uninitialized pragma mishandled in inlined function
Date: Tue, 18 Nov 2014 23:01:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63943

            Bug ID: 63943
           Summary: -Wmaybe-uninitialized pragma mishandled in inlined
                    function
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eggert at gnu dot org

Created attachment 34031
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34031&action=edit
Program with -Wmaybe-uninitialized problem

Compile the attached program t.c with "gcc -Wmaybe-uninitialized -O2 -c t.c" on
x86-64 with GCC 4.9.2, and the output will be:

t.c: In function 'main':
t.c:17:3: warning: '*((void *)&v+4)' may be used uninitialized in this function
[-Wmaybe-uninitialized]
   return argc < 1 ? !argv : dereference (v + 1);
   ^

'#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"' is in effect when the
possibly-uninitialized variable is used, but this use is in a function that GCC
inlined, and GCC is incorrectly warning based on the pragma setting in the
calling function, not based on the pragma setting where the dereferencing
actually occurs.

The problem does not occur with GCC 4.8.3 so this appears to be a regression.

The problem also goes away if I add '__attribute__((noinline))' to the
'dereference' function, so this appears to be related to inlining.

I ran into this problem while thinking about enabling -Werror when compiling
glibc; my first cut at this ran into the above problem.