[Bug middle-end/26840] New: escaping global variables cause 'definition follows use' error.

[geoffk at gcc dot gnu dot org]

The following testcase:

extern int f1 (void **);
extern void f2 (void *);

struct s
{
  unsigned char field1;
  int field2;
};

static inline struct s *
get_globals (void)
{
  struct s * r;
  void * rr;

  if (f1 (&rr))
    return 0;
  r = rr;
  if (! r)
    {
      extern struct s t;
      r = &t;
    }
  r->field1 = 1;
  return r;
}

void
atexit_common (const void *dso)
{
  struct s *g = get_globals ();

  if (! g)
    return;
  if (g->field1)
    {
      g->field2 = 0;
      f2 (g);
    }
  else
    f2 (g);
}

when compiled with

./xgcc -B./ -O2 crt3.c -S

ICEs with:

crt3.c: In function 'atexit_common':
crt3.c:30: error: definition in block 7 follows the use
for SSA_NAME: rr_11 in statement:
#   rr_23 = V_MAY_DEF <rr_11>;
#   SFT.1_24 = V_MAY_DEF <SFT.1_22>;
#   SFT.2_25 = V_MAY_DEF <SFT.2_26>;
f2 (g_1);

It appears that the problem is that phicprop1 makes a fairly harmless
modification to this:

#   rr_11 = V_MAY_DEF <rr_16>;
#   SFT.1_22 = V_MAY_DEF <SFT.1_17>;
g_2->field2 = 0

but since it's modified it, it calls update_operands, which turns it into:

#   SFT.1_22 = V_MAY_DEF <SFT.1_17>;
g_1->field2 = 0

deleting the rr_11 set.  That of course leaves no rr_11 sets at all, and
produces the error.

Now, in this particular case I believe the change is correct, in that 'rr'
really can't be modified by this instruction, but then of course you shouldn't
have had an rr_11 at all.  The 036t.forwprop1 dump file says

  #   SFT.1_22 = V_MAY_DEF <SFT.1_17>;
  g_5->field2 = 0;

then 037t.copyprop1 adds rr_11 and says

  #   rr_11 = V_MAY_DEF <rr_16>;
  #   SFT.1_22 = V_MAY_DEF <SFT.1_17>;
  g_2->field2 = 0;

so it's possible that this is the original problem.


-- 
           Summary: escaping global variables cause 'definition follows use'
                    error.
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: geoffk at gcc dot gnu dot org
 GCC build triplet: powerpc-apple-darwin
  GCC host triplet: powerpc-apple-darwin
GCC target triplet: powerpc-apple-darwin


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26840