diff mbox series

doc: Describe behaviour of enums with fixed underlying type

Message ID 20230427111634.1728893-1-jwakely@redhat.com
State Accepted
Headers
Series doc: Describe behaviour of enums with fixed underlying type |

Checks

Context Check Description
snail/gcc-patch-check success Github commit url

Commit Message

Jonathan Wakely April 27, 2023, 11:16 a.m. UTC 
  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?

-- >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(+)

Comments

Marek Polacek April 27, 2023, 3:58 p.m. UTC | #1 
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
Jonathan Wakely May 2, 2023, 8:43 p.m. UTC | #2 
On Thu, 27 Apr 2023 at 16:58, Marek Polacek wrote:

> 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.
>

That's an ack C front end reviewer (thanks!), do I need an ack from a
C++/docs/global review too, or can I push?



>
> > -- >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
>
>
diff mbox series

Patch

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.