Re: [PATCH] c++/66561 - __builtin_LINE at al. should yield constant expressions

[Jeff Law <law at redhat dot com>]

On 04/26/2016 05:59 PM, Martin Sebor wrote:
> PR c++/66639 asked to declare __func__ , __FUNCTION__ and
> __PRETTY_FUNCTION__ as constexprâ.  With the request fulfilled
> sometime in the 6.0 time frame (possibly as a result of fixing
> c++/70353), the attached patch implements the corresponding
> change suggested in c++/66561 - __builtin_LINE at al. should
> yield constant expression, and updates the manual to reflect
> both.  It has been tested on x86_64 Linux.
>
> Martin
>
> gcc-66561.patch
>
>
> PR c++/66561 - __builtin_LINE at al. should yield constant expressions
> PR c++/66639 - declare __func__ , __FUNCTION__ & __PRETTY_FUNCTION__ constexpr
>
> gcc/testsuite/ChangeLog:
> 2016-04-26  Martin Sebor  <msebor@redhat.com>
>
> 	PR c++/66561
> 	* c-c++-common/builtin_location.c: New test.
> 	* g++.dg/cpp1y/builtin_location.C: New test.
>
> gcc/cp/ChangeLog:
> 2016-04-26  Martin Sebor  <msebor@redhat.com>
>
> 	PR c++/66561
> 	* tree.c (builtin_valid_in_constant_expr_p): Treat BUILT_IN_FILE,
> 	BUILT_IN_FUNCTION, and BUILT_IN_LINE as constant expressions.
>
> gcc/ChangeLog:
> 2016-04-26  Martin Sebor  <msebor@redhat.com>
>
> 	PR c++/66561
> 	* builtins.c (fold_builtin_FILE): New function.
> 	(fold_builtin_FUNCTION, fold_builtin_LINE): New functions.
> 	(fold_builtin_0): Call them.
>
> 	PR c++/66561
> 	* doc/extend.texi (Other Builtins): Update __builtin_FILE,
> 	__builtin_FUNCTION, and __builtin_LINE to reflect they yield
> 	constants.
>
> 	PR c++/66639
> 	* doc/extend.texi (Function Names as Strings): Update __func__,
> 	__FUNCTION__, __PRETTY_FUNCTION__ to reflect they evaluate to
> 	constants.
>
> diff --git a/gcc/builtins.c b/gcc/builtins.c
> index 3d89baf..44c4f63 100644
> --- a/gcc/builtins.c
> +++ b/gcc/builtins.c
> @@ -7927,6 +7927,39 @@ fold_builtin_arith_overflow (location_t loc, enum built_in_function fcode,
>    return build2_loc (loc, COMPOUND_EXPR, boolean_type_node, store, ovfres);
>  }
>
> +/* Fold __builtin_FILE() to a constant string.  */
NIT: When we refer to functions, we don't have the trailing().  So drop 
it from the comment.

> +
> +/* Fold __builtin_FUNCTION() to a constant string.  */
Similarly.

> +
> +/* Fold __builtin_LINE() to an integer constant.  */
Similarly.

So the big question I have is why we have to treat these differently 
than __func__ __FUNCTION__ and __PRETTY_FUNCTION__.

Which leads me to:

constexpr_fn_retval which has:

   case DECL_EXPR:
      {
        tree decl = DECL_EXPR_DECL (body);
        if (TREE_CODE (decl) == USING_DECL
            /* Accept __func__, __FUNCTION__, and __PRETTY_FUNCTION__.  */
            || DECL_ARTIFICIAL (decl))
          return NULL_TREE;
        return error_mark_node;
      }

Is the distinction here that the _builtin_XXX are essentially functions, 
yet the others are _DECLs?  Thus we have to fold the builtin function so 
that it is considered a constant?


jeff