Re: ARM-only [Patch] Allow 4 byte enum size (-fno-short-enums) | Remove hard coded short enum flag from the build scripts?

["Richard Earnshaw (lists)" <Richard dot Earnshaw at arm dot com>]

On 31/08/16 13:44, Schwarz, Konrad wrote:
>> -----Original Message-----
>> From: Richard Earnshaw (lists) [mailto:Richard.Earnshaw@arm.com]
>> Sent: Wednesday, August 31, 2016 2:38 PM
>> To: Schwarz, Konrad (CT RDA ITP SES-DE); newlib@sourceware.org
>> Subject: Re: ARM-only [Patch] Allow 4 byte enum size (-fno-short-enums)
>> | Remove hard coded short enum flag from the build scripts?
> 
> 
>>> But wouldn't it suffice to allow the user to select (or override) the
>>> build attribute to set on the file, e.g., via a compiler command line
>> switch?
>>>
>>
>> Yes, but 1) we have no builds of GCC available *today* that can do that
>> and 2) experience has shown that such 'power user' options generally
>> confuse most users and lead to more problems than they really solve
>> unless they can validate the user's assertions.
>>
>>> Obviously, this requires the compiler to trust the user.
>>>
>>> Conversely, as there is no way in (standard) C to distinguish an ABI
>>> from interfaces internal to the implementation, any scheme to
>>> determine this build attribute automatically cannot be fully
>>> satisfactory, no?
>>>
>>
>> No, this is nothing to do with standard C.  The C standard just thinks
>> about enums at the conceptual level.  It doesn't grubby its hands with
>> nasty details like ABIs or layout in general...  The idea that this
>> might be changed in different translation units is completely off-
>> piste.
> 
> The corollary being that the only way to do this properly is via a compiler
> switch, even it is confusing to non-power users.  (Since when has
> GCC limited itself to the needs of non-power users?)
> 

I think the best way to do this is using the pragmas I previously
mentioned.  This gives far more control (power) than the command-line
switch and has a degree of backward compatibility with some existing
versions of GCC.

#define PUSH_PACK_ENUMS _Pragma("GCC push_options") _Pragma ("GCC
optimize (\"-fshort-enums\")")
#define POP_PACK_ENUMS _Pragma("GCC pop_options")


Now some pre-processor testing for support of the GCC pragmas will allow
us to write

PUSH_PACK_ENUMS
enum {a, b, c} w;
POP_PACK_ENUMS

int x = sizeof (w);

enum {
  d, e, f
} y;

int z = sizeof (y);

Or something like that.