Re: proposed patch: verify: new macro verify_expr; verify_true deprecated

[Eric Blake]

On 06/13/2011 08:10 PM, Paul Eggert wrote:
> It's relatively common to want to verify a property at compile-time
> inside an expression, when using a macro.  For example, in Emacs,
> I might want to do something like this:
> 
>    #define ENCODE_CHAR(charset, c) \
>     (verify_true (sizeof (c) <= sizeof (int)), \
>      [some expression that assumes sizeof (c) <= sizeof (int)])
> 
> Unfortunately this elicits the following diagnostic with GCC 4.6.0:
> 
>   charset.c:1767:14: error: left-hand operand of comma expression has no 
> effect [-Werror=unused-value]

And I agree that adding a cast to void to silence that gcc warning is
undesirable.

> 
> So I am thinking of a new macro verify_expr (R, E), which verifies R
> at compile-time and then yields the value of E.  Then the Emacs macro
> can be written this way:
> 
>    #define ENCODE_CHAR(charset, c) \
>      verify_expr (sizeof (c) <= sizeof (int), \
>                 [some expression that assumes sizeof (c) <= sizeof (int)])

Looks good to me.

> +++ b/tests/test-verify.c
> @@ -39,7 +39,7 @@ verify (1 == 1); verify (1 == 1); /* should be ok */
> 
>  enum
>  {
> -  item = verify_true (1 == 1) * 0 + 17 /* should be ok */
> +  item = verify_expr (1 == 1, 1) * 0 + 17 /* should be ok */
>  };
> 
>  static int
> @@ -55,9 +55,9 @@ function (int n)
>    verify (1 == 1); verify (1 == 1); /* should be ok */
> 
>    if (n)
> -    return ((void) verify_true (1 == 1), verify_true (1 == 1) + 7); /* 
> should be ok */
> +    return ((void) verify_expr (1 == 1, 1), verify_expr (1 == 1, 8)); /* 
> should be ok */

Even though verify_true is deprecated by your patch, should we still be
testing that it compiles?  Particularly since in your patch, you did not
#define verify_true in terms of verify_expr, but instead defined both in
terms of _GL_VERIFY_EXPR.

-- 
Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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