PR 15454: wrong code due to tree constant propagation

[Steven Bosscher]

Hi,

This is a wrong code bug due to tree ssa CCP.  Consider:

void abort ();
int main () {
        int foo;
        int bar (void)
        {         
                int baz = 0;
                if (foo)
                        baz = foo;
                return baz;
        }

        foo = 1;
        if (!bar ())
                abort ();
        return 0;
}

Before CCP the nested function bar looks like this:

bar ()
{
  int baz;
  int foo.1;

<bb 0>:
  foo.1_4 = CHAIN.3_3->foo;
  if (foo.1_4 != 0) goto <L0>; else goto <L1>;

<L0>:;

  # baz_1 = PHI <0(0), foo.1_4(1)>;
<L1>:;
  return baz_1;

}

The SSA_NAME_DEF_STMT for CHAIN.3_3 is an empty statement, so we
treat it as an uninitialized variable.  Now, we apparently assume
ununitialized variables to be UNDEFINED.  So the meet of the
CONSTANT 0 and the UNDEFINED foo.1_4 in the PHI: 
 
  # baz_1 = PHI <0(0), foo.1_4(1)>; 
 
returns CONSTANT for baz_1, which is overly optimistic in this case. 
 
I believe we should assume that variables without an  initializing
statement are VARYING.  That's what the attached patch does.
Another solution would be to introduce some concept of a hidden DEF
but the result would be the same and this patch shouldn't hide any
safe optimization opportunities anyway.

Bootstrapped on x86-64-unknown-linux-gnu.  OK?

Gr.
Steven


	* tree-ssa-ccp.c (get_default_value): Treat SSA names with an
	empty SSA_NAME_DEF_STMT as VARYING.

Index: tree-ssa-ccp.c 
=================================================================== 
RCS file: /cvs/gcc/gcc/gcc/tree-ssa-ccp.c,v 
retrieving revision 2.2 
diff -c -3 -p -r2.2 tree-ssa-ccp.c 
*** tree-ssa-ccp.c      14 May 2004 02:29:23 -0000      2.2 
--- tree-ssa-ccp.c      15 May 2004 11:53:24 -0000 
*************** get_default_value (tree var) 
*** 2106,2112 **** 
        enum tree_code code; 
        tree stmt = SSA_NAME_DEF_STMT (var); 
 
!       if (!IS_EMPTY_STMT (stmt)) 
          { 
          code = TREE_CODE (stmt); 
          if (code != MODIFY_EXPR && code != PHI_NODE) 
--- 2106,2114 ---- 
        enum tree_code code; 
        tree stmt = SSA_NAME_DEF_STMT (var); 
 
!       if (IS_EMPTY_STMT (stmt)) 
!       val.lattice_val = VARYING; 
!       else 
          { 
          code = TREE_CODE (stmt); 
          if (code != MODIFY_EXPR && code != PHI_NODE)