Re: [PATCH] doc: Describe behaviour of enums with fixed underlying type

[Marek Polacek <polacek@redhat.com>]

On Thu, Apr 27, 2023 at 12:16:34PM +0100, Jonathan Wakely via Gcc-patches wrote:
> C2x adds the ability to give an enumeration type a fixed underlying
> type, as C++ already has. The -fshort-enums option alters the compiler's
> choice of underlying type, but when it's fixed the compiler can't
> choose.
> 
> Similarly for C++ -fstrict-enums has no effect with a fixed underlying
> type, because every value of the underlying type is a valid value of the
> enumeration type.
> 
> This caused confusion recently: https://gcc.gnu.org/PR109532
> 
> OK for trunk?

LGTM.
 
> -- >8 --
> 
> gcc/ChangeLog:
> 
> 	* doc/invoke.texi (Code Gen Options): Note that -fshort-enums
> 	is ignored for a fixed underlying type.
> 	(C++ Dialect Options): Likewise for -fstrict-enums.
> ---
>  gcc/doc/invoke.texi | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 2f40c58b21c..0f91464f8c0 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -3495,6 +3495,8 @@ defined in the C++ standard; basically, a value that can be
>  represented in the minimum number of bits needed to represent all the
>  enumerators).  This assumption may not be valid if the program uses a
>  cast to convert an arbitrary integer value to the enumerated type.
> +This option has no effect for an enumeration type with a fixed underlying
> +type.
>  
>  @opindex fstrong-eval-order
>  @item -fstrong-eval-order
> @@ -18303,6 +18305,8 @@ Use it to conform to a non-default application binary interface.
>  Allocate to an @code{enum} type only as many bytes as it needs for the
>  declared range of possible values.  Specifically, the @code{enum} type
>  is equivalent to the smallest integer type that has enough room.
> +This option has no effect for an enumeration type with a fixed underlying
> +type.
>  
>  @strong{Warning:} the @option{-fshort-enums} switch causes GCC to generate
>  code that is not binary compatible with code generated without that switch.
> -- 
> 2.40.0
> 

Marek