gcc源代码分析，default_conversion ()函数分析

/* Perform default promotions for C data used in expressions.
   Arrays and functions are converted to pointers;
   enumeral types or short or char, to int.
   In addition, manifest constants symbols are replaced by their values.  */

tree
default_conversion (exp)
     tree exp;
{
  register tree dt = TREE_TYPE (exp);
  register enum tree_code form = TREE_CODE (dt);

......
  if (form == ARRAY_TYPE)
    {
      register tree adr;
      tree restype = TREE_TYPE (dt);
      tree ptrtype;

      if (TREE_CODE (exp) == INDIRECT_REF)
    return convert (TYPE_POINTER_TO (restype),
            TREE_OPERAND (exp, 0));

      if (TREE_CODE (exp) == COMPOUND_EXPR)
    {
      tree op1 = default_conversion (TREE_OPERAND (exp, 1));
      return build (COMPOUND_EXPR, TREE_TYPE (op1),
            TREE_OPERAND (exp, 0), op1);
    }

      if (!lvalue_p (exp)
      && ! (TREE_CODE (exp) == CONSTRUCTOR && TREE_STATIC (exp)))
    {
      error ("invalid use of non-lvalue array");
      return error_mark_node;
    }

      if (TREE_READONLY (exp) || TREE_THIS_VOLATILE (exp))
    restype = c_build_type_variant (restype, TREE_READONLY (exp),
                    TREE_THIS_VOLATILE (exp));

      ptrtype = build_pointer_type (restype);

......

      /* This way is better for a COMPONENT_REF since it can
     simplify the offset for a component.  */
      adr = build_unary_op (ADDR_EXPR, exp, 1);
      return convert (ptrtype, adr);
    }
  return exp;
}

in default_conversion

 <string_cst 84014
    type < array_type 94ef4
        type <integer_type 825bc char permanent QI
            size <integer_cst 82638 literal permanent 1
            align 8 size_unit 8 sep_unit 8 symtab 0
            sep <integer_cst 82608 literal permanent -128 precision 8 min <integer_cst 82608 -128>
            max <integer_cst 82620 literal permanent 127
            pointer_to_this <pointer_type 88a44> chain <integer_type 826a8* long int>
        BLK
        size <integer_cst 94f40 literal 15
        align 8 size_unit 8 sep_unit 8 symtab 0 sep <integer_cst 82638 1>
        domain <integer_type 94ea8 SI
            size <integer_cst 8254c literal permanent 4
            align 32 size_unit 8 sep_unit 32 symtab 0
            sep <integer_cst 84048 literal 0 precision 32 min <integer_cst 84048 0>
            max <integer_cst 84078 literal 14
    static literal "Hello, world!
"
in build_pointer_type
 <integer_type 825bc char permanent QI
    size <integer_cst 82638 type <integer_type 82848* unsigned int> literal permanent 1
    align 8 size_unit 8 sep_unit 8 symtab 0
    sep <integer_cst 82608 type <integer_type 825bc* char> literal permanent -128 precision 8 min <integer_cst 82608 -128>
    max <integer_cst 82620 type <integer_type 825bc* char> literal permanent 127
    pointer_to_this <pointer_type 88a44> chain <integer_type 826a8* long int>
restype is
 <integer_type 825bc char permanent QI
    size <integer_cst 82638 type <integer_type 82848* unsigned int> literal permanent 1
    align 8 size_unit 8 sep_unit 8 symtab 0
    sep <integer_cst 82608 type <integer_type 825bc* char> literal permanent -128 precision 8 min <integer_cst 82608 -128>
    max <integer_cst 82620 type <integer_type 825bc* char> literal permanent 127
    pointer_to_this <pointer_type 88a44> chain <integer_type 826a8* long int>
ptrtype is
 < pointer_type 88a44
    type <integer_type 825bc char permanent QI
        size <integer_cst 82638 literal permanent 1
        align 8 size_unit 8 sep_unit 8 symtab 0
        sep <integer_cst 82608 literal permanent -128 precision 8 min <integer_cst 82608 -128>
        max <integer_cst 82620 literal permanent 127
        pointer_to_this <pointer_type 88a44> chain <integer_type 826a8* long int>
    permanent unsigned SI
    size <integer_cst 8254c type <integer_type 82848* unsigned int> literal permanent 4
    align 32 size_unit 8 sep_unit 32 symtab 0
    chain <array_type 88a90>
before build_unary_op exp is
 <string_cst 84014
    type <array_type 94ef4
        type <integer_type 825bc char permanent QI
            size <integer_cst 82638 literal permanent 1
            align 8 size_unit 8 sep_unit 8 symtab 0
            sep <integer_cst 82608 literal permanent -128 precision 8 min <integer_cst 82608 -128>
            max <integer_cst 82620 literal permanent 127
            pointer_to_this <pointer_type 88a44> chain <integer_type 826a8* long int>
        BLK
        size <integer_cst 94f40 literal 15
        align 8 size_unit 8 sep_unit 8 symtab 0 sep <integer_cst 82638 1>
        domain <integer_type 94ea8 SI
            size <integer_cst 8254c literal permanent 4
            align 32 size_unit 8 sep_unit 32 symtab 0
            sep <integer_cst 84048 literal 0 precision 32 min <integer_cst 84048 0>
            max <integer_cst 84078 literal 14
    static literal "Hello, world!
"
in build_pointer_type
 <array_type 94ef4
    type <integer_type 825bc char permanent QI
        size <integer_cst 82638 literal permanent 1
        align 8 size_unit 8 sep_unit 8 symtab 0
        sep <integer_cst 82608 literal permanent -128 precision 8 min <integer_cst 82608 -128>
        max <integer_cst 82620 literal permanent 127
        pointer_to_this <pointer_type 88a44> chain <integer_type 826a8* long int>
    BLK
    size <integer_cst 94f40 type <integer_type 82848* unsigned int> literal 15
    align 8 size_unit 8 sep_unit 8 symtab 0 sep <integer_cst 82638 1>
    domain <integer_type 94ea8 SI
        size <integer_cst 8254c literal permanent 4
        align 32 size_unit 8 sep_unit 32 symtab 0
        sep <integer_cst 84048 literal 0 precision 32 min <integer_cst 84048 0>
        max <integer_cst 84078 literal 14
after return t
 <pointer_type 94f58 VOID
    align 1 size_unit 1 sep_unit 0 symtab 0
 < addr_expr 840a8
    type <pointer_type 94f58
        type <array_type 94ef4 type <integer_type 825bc* char>
            BLK
            size <integer_cst 94f40 literal 15
            align 8 size_unit 8 sep_unit 8 symtab 0
            sep <integer_cst 82638 literal permanent 1 domain <integer_type 94ea8>
            pointer_to_this <pointer_type 94f58> chain <pointer_type 94f58>
        unsigned SI
        size <integer_cst 8254c literal permanent 4
        align 32 size_unit 8 sep_unit 32 symtab 0
    literal
    arg 0 <string_cst 84014 type <array_type 94ef4>
        static literal "Hello, world!

"

tree
convert (type, expr)
     tree type, expr;
{
  register tree e = expr;
  register enum tree_code code = TREE_CODE (type);

 ...
  if (code == POINTER_TYPE)
    return fold (convert_to_pointer (type, e));

... /*这里会调用convert_to_pointer ()函数*/

static tree
convert_to_pointer (type, expr)
     tree type, expr;
{
  register tree intype = TREE_TYPE (expr);
  register enum tree_code form = TREE_CODE (intype);
 
...

  if (form == POINTER_TYPE)
    return build (NOP_EXPR, type, expr);
/*这里会建立一个NOP_EXPR类型的tree*/

 <nop_expr 840c0
    type < pointer_type 88a44
        type <integer_type 825bc char permanent QI
            size <integer_cst 82638 literal permanent 1
            align 8 size_unit 8 sep_unit 8 symtab 0
            sep <integer_cst 82608 literal permanent -128 precision 8 min <integer_cst 82608 -128>
            max <integer_cst 82620 literal permanent 127
            pointer_to_this <pointer_type 88a44> chain <integer_type 826a8* long int>
        permanent unsigned SI
        size <integer_cst 8254c literal permanent 4
        align 32 size_unit 8 sep_unit 32 symtab 0
        chain <array_type 88a90>
    literal
    arg 0 < addr_expr 840a8
        type <pointer_type 94f58 type <array_type 94ef4>
            unsigned SI size <integer_cst 8254c 4>
            align 32 size_unit 8 sep_unit 32 symtab 0
        literal
        arg 0 <string_cst 84014 type <array_type 94ef4>
            static literal "Hello, world!
"