casting return values

[Dan Nicolaescu <dann at godzilla dot ics dot uci dot edu>]

Currently return values for C/C++ functions that return types for
which c_promoting_integer_type_p is true are cast to integer 
(see c-decl.c:start_function and cp/decl.c:start_function)

The effect of this can be seen when compiling:

static _Bool bar (_Bool a)
{
  return a;
}

_Bool foo1 (_Bool a)
{
  if (bar(a))
    return 1;
  else
    return 0;
}

The -fdump-tree-generic dump looks like: 

;; Function bar (bar)
bar (a)
{
  return (int)a;  /* see the cast here! */
}


foo1 (a)
{
  int T.11;
  _Bool T.12;
  int T.13;

  T.11 = (int)a; /* this is probably not needed either */
  T.12 = bar (T.11);
  T.13 = (int)T.12;
  if (T.13 != 0)
    {
      return 1;
    }
  else
    {
      return 0;
    }
}

And the .vars dump like:

foo1 (a)
{
<bb 0>:
  /*Note the useless casts and comparisons. */      
  if ((_Bool)(int)((int)a != 0) != 0) goto <L1>; else goto <L2>;
<L1>:;
  return 1;
<L2>:;
  return 0;
}

These casts are also the cause of PR14441. 

The casts introduced by start_functions are required by the ABI. But
start_functions is too early to introduce such casts in the tree-ssa
world.  They are probably not needed before generating RTL.  

The following patch, although it bootstraps with no regressions on
x86-linux and sparc-solaris is probably technically incorrect. 
Where should the deleted code be placed? As a langhook called in
expand_return? Please advice. 


Index: c-decl.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/c-decl.c,v
retrieving revision 1.334.2.94
diff -c -3 -p -c -r1.334.2.94 c-decl.c
*** c-decl.c	12 May 2004 13:33:28 -0000	1.334.2.94
--- c-decl.c	14 May 2004 02:38:48 -0000
*************** start_function (tree declspecs, tree dec
*** 5791,5807 ****
    make_decl_rtl (current_function_decl, NULL);
  
    restype = TREE_TYPE (TREE_TYPE (current_function_decl));
-   /* Promote the value to int before returning it.  */
-   if (c_promoting_integer_type_p (restype))
-     {
-       /* It retains unsignedness if not really getting wider.  */
-       if (TYPE_UNSIGNED (restype)
- 	  && (TYPE_PRECISION (restype)
- 		  == TYPE_PRECISION (integer_type_node)))
- 	restype = unsigned_type_node;
-       else
- 	restype = integer_type_node;
-     }
    DECL_RESULT (current_function_decl)
      = build_decl (RESULT_DECL, NULL_TREE, restype);
  
--- 5791,5796 ----

Index: decl.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/cp/decl.c,v
retrieving revision 1.911.2.72
diff -c -3 -p -c -r1.911.2.72 decl.c
*** decl.c	12 May 2004 13:34:47 -0000	1.911.2.72
--- decl.c	14 May 2004 02:39:34 -0000
*************** start_function (tree declspecs, tree dec
*** 10148,10156 ****
  
    /* Build the return declaration for the function.  */
    restype = TREE_TYPE (fntype);
-   /* Promote the value to int before returning it.  */
-   if (c_promoting_integer_type_p (restype))
-     restype = type_promotes_to (restype);
    if (DECL_RESULT (decl1) == NULL_TREE)
      {
        DECL_RESULT (decl1)
--- 10148,10153 ----

The patch reduces the GC memory consumption for PR8361 by about 1%. 

Thanks.
        --dan