[Bug c/54561] New: incorrect setjmp -Wclobber diagnostics

[eggert at gnu dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54561

             Bug #: 54561
           Summary: incorrect setjmp -Wclobber diagnostics
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: eggert@gnu.org


Created attachment 28180
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28180
generates incorrect diagnostics with gcc -O2 -Wclobbered

If I compile the attached program via the command "gcc -S -Wclobbered
-O2 w.i" (GCC 4.7.1, x86-64, Fedora 17), the output is:

w.i: In function 'png_load_body':
w.i:37:13: warning: variable 'info_ptr' might be clobbered by 'longjmp' or
'vfork' [-Wclobbered]
w.i:38:9: warning: variable 'fp' might be clobbered by 'longjmp' or 'vfork'
[-Wclobbered]

Both warnings are incorrect, because info_ptr and fp are not live
across any possible longjmp.  If longjmp munges these variables,
setjmp must return a nonzero value, and no uses of these variables are
reachable in that case.

Like Bug#48968, I ran into this problem when building Emacs.  This test
case is much shorter, though (I whittled it down), so it should be
easier to debug.