[Bug middle-end/42899] New: DSE fails: no-op modification of global data pessimized code-size

["aldot at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

John Regehr points out this size-regression WRT 3.4:

Sometimes a function modifies globals but even so has no net effect:

http://embed.cs.utah.edu/embarrassing/jan_10/harvest/source/8A/8AB0B238.shtml
http://embed.cs.utah.edu/embarrassing/jan_10/harvest/source/14/14157FE8.shtml

Somehow gcc3 sees these but everyone else including gcc4 fails.

$ cat 8AB0B238.c
struct frame_info;
struct frame_info;
struct frame_info;
struct frame_info;
struct frame_info;
struct frame_info;
struct frame_info;
struct frame_info;
struct frame_info;
void tui_registers_changed_hook (void);
extern struct frame_info *deprecated_selected_frame;
int tui_refreshing_registers = 0;
void
tui_registers_changed_hook (void)
{
  struct frame_info *fi;

  fi = deprecated_selected_frame;
  if (fi)
    {
      if (tui_refreshing_registers == 0)
        {
          tui_refreshing_registers = 1;
          tui_refreshing_registers = 0;
        }
    }
  return;
}

$ for i in 3.4 4.4 4.5.orig-HEAD;do gcc-$i -fomit-frame-pointer -Os -c -o
8AB0B238.o-gcc-$i 8AB0B238.c;done
$ size 8AB0B238.o*
   text    data     bss     dec     hex filename
      1       0       4       5       5 8AB0B238.o-gcc-3.4
     29       0       4      33      21 8AB0B238.o-gcc-4.4
     29       0       4      33      21 8AB0B238.o-gcc-4.5.orig-HEAD


-- 
           Summary: DSE fails: no-op modification of global data pessimized
                    code-size
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: aldot at gcc dot gnu dot org
OtherBugsDependingO 16996
             nThis:


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42899