[Bug tree-optimization/15438] [3.5 Regression] miscompilation with attribute (__malloc__)

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

------- Additional Comments From steven at gcc dot gnu dot org  2004-05-17 07:42 -------
This is an alias bug.  Just before dom2, the tree looks like this: 
 
main () 
{ 
  int * p; 
  int T.0; 
 
  # BLOCK 0 
  # PRED: ENTRY [100.0%]  (fallthru) 
  #   a_5 = VDEF <a_2>; 
  a = 0; 
  #   VUSE <a_5>; 
  foo (); 
  #   VUSE <a_5>; 
  T.0_3 = a; 
  if (T.0_3 == 0) goto <L0>; else goto <L1>; 
  # SUCC: 2 [53.5%]  (false) 1 [46.5%]  (true) 
 
  # BLOCK 1 
  # PRED: 0 [46.5%]  (true) 
<L0>:; 
  abort (); 
  # SUCC: 
 
  # BLOCK 2 
  # PRED: 0 [53.5%]  (false) 
<L1>:; 
  return 0; 
  # SUCC: EXIT [100.0%] 
 
} 
 
Notice that the call to foo() does not clobber a, so we have: 
 
  a = 0; 
  #   VUSE <a_5>; 
  foo (); 
  #   VUSE <a_5>; 
  T.0_3 = a; 
 
and dom2 thinks "T.0_3 = 0" is a valid constant propagation. 
 
The documentation for attribute malloc says: 
 
`malloc' 
     The `malloc' attribute is used to tell the compiler that a function 
     may be treated as if it were the malloc function.  The compiler 
     assumes that calls to malloc result in a pointers that cannot 
     alias anything.  This will often improve optimization. 
 
so while the returned pointer cannot alias anything, it doesn't mean 
that the function call may not call clobber global variables.  So in 
this case, 'a' may be clobbered by the call to foo(), and there should 
be a VDEF for a at the function call. 
 
 

-- 
           What    |Removed                     |Added
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                 CC|                            |dnovillo at redhat dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15438