[Bug bootstrap/55644] maybe-uninitialized false positive

["matt at use dot net" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55644

Matt Hargett <matt at use dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|bootstrap-lto fails on      |maybe-uninitialized false
                   |current trunk (with and     |positive
                   |without profiledbootstrap)  |

--- Comment #11 from Matt Hargett <matt at use dot net> 2013-03-01 23:34:53 UTC ---
The current false positives emitted:

mhargett@chert:/work/mhargett/gcc-trunk-lto-O3-debug/gcc$
/work/mhargett/gcc-trunk-lto-O3-debug/./prev-gcc/xg++
-B/work/mhargett/gcc-trunk-lto-O3-debug/./prev-gcc/
-B/u/mhargett/x86_64-unknown-linux-gnu/bin/ -nostdinc++
-B/work/mhargett/gcc-trunk-lto-O3-debug/prev-x86_64-unknown-linux-gnu/libstdc++-v3/src/.libs
-B/work/mhargett/gcc-trunk-lto-O3-debug/prev-x86_64-unknown-linux-gnu/libstdc++-v3/libsupc++/.libs
-I/work/mhargett/gcc-trunk-lto-O3-debug/prev-x86_64-unknown-linux-gnu/libstdc++-v3/include/x86_64-unknown-linux-gnu
-I/work/mhargett/gcc-trunk-lto-O3-debug/prev-x86_64-unknown-linux-gnu/libstdc++-v3/include
-I/work/mhargett/gcc-trunk/libstdc++-v3/libsupc++
-L/work/mhargett/gcc-trunk-lto-O3-debug/prev-x86_64-unknown-linux-gnu/libstdc++-v3/src/.libs
-L/work/mhargett/gcc-trunk-lto-O3-debug/prev-x86_64-unknown-linux-gnu/libstdc++-v3/libsupc++/.libs
  -O3 -g -flto=jobserver -flto-partition=none -frandom-seed=1 -gtoggle -DIN_GCC
  -fno-exceptions -fno-rtti -fasynchronous-unwind-tables -W -Wall
-Wno-narrowing -Wwrite-strings -Wcast-qual -Wmissing-format-attribute -pedantic
-Wno-long-long -Wno-variadic-macros -Wno-overlength-strings -Werror -fno-common
 -DHAVE_CONFIG_H -static-libstdc++ -static-libgcc  gcov-dump.o                
libcommon.a ../libcpp/libcpp.a   ../libbacktrace/.libs/libbacktrace.a
../libiberty/libiberty.a ../libdecnumber/libdecnumber.a  -o gcov-dump
../../gcc-trunk/libbacktrace/elf.c: In function 'elf_add':
../../gcc-trunk/libbacktrace/mmapio.c:98:14: error: 'ehdr_view.len' may be used
uninitialized in this function [-Werror=maybe-uninitialized]
   if (munmap (const_cast.v, view->len) < 0)
              ^
../../gcc-trunk/libbacktrace/elf.c:476:25: note: 'ehdr_view.len' was declared
here
   struct backtrace_view ehdr_view;
                         ^
../../gcc-trunk/libbacktrace/mmapio.c:98:14: error: 'ehdr_view.base' may be
used uninitialized in this function [-Werror=maybe-uninitialized]
   if (munmap (const_cast.v, view->len) < 0)
              ^
../../gcc-trunk/libbacktrace/elf.c:476:25: note: 'ehdr_view.base' was declared
here
   struct backtrace_view ehdr_view;
                         ^
../../gcc-trunk/libbacktrace/elf.c:516:10: error: 'ehdr_view.data' may be used
uninitialized in this function [-Werror=maybe-uninitialized]
   memcpy (&ehdr, ehdr_view.data, sizeof ehdr);
          ^
../../gcc-trunk/libbacktrace/elf.c:476:25: note: 'ehdr_view.data' was declared
here
   struct backtrace_view ehdr_view;
                         ^


which is a false positive, because

ehdr_view's fields are correctly initialized via a call from elf_add() to
backtrace_get_view()
  view->data = (char *) map + inpage;
  view->base = map;
  view->len = size;

if the mmap() earlier in the backtrace_get_view() didn't fail, view is
initialized and the backtrace_get_view() returns 1. if that mmap() fails, view
remains uninitialized and backtrace_get_view() returns 0.

elf_add()'s invocation checks the return value of backtrace_get_view(). in the
0 (failure) case, backtrace_release_view() is never invoked:
  if (!backtrace_get_view (state, descriptor, 0, sizeof ehdr, error_callback,
                           data, &ehdr_view))
    goto fail;

since backtrace_release_view() is never called in the case where the fields are
uninitialized, the warning is a false positive. while it occurs here with the
confluence of several optimizations, it looks like the same problem would
happen in a contrived single-file case.

attached are the temp files for the module the generated this false positive.