Re: [Qemu-devel] [PATCH] softfloat: rename make_float[x80|128]_init to const_float[x80|128]

[Laszlo Ersek]

On 09/19/17 19:39, Eric Blake wrote:
> On 09/19/2017 11:30 AM, Richard Henderson wrote:
>> On 09/18/2017 05:46 AM, Laurent Vivier wrote:
>>> Redefine make_floatx80() and make_float128() as make_float16(),
>>> make_float32() and make_float64() using a variable and not only
>>> a cast.
> 
>>> -#define make_floatx80(exp, mant) ((floatx80) { mant, exp })
>>> -#define make_floatx80_init(exp, mant) { .low = mant, .high = exp }
>>> +#define make_floatx80(exp, mant) __extension__ \
>>> +    ({ floatx80 f80_val = { .low = mant, .high = exp }; f80_val; })
>>> +#define const_floatx80(exp, mant) { .low = mant, .high = exp }
> 
>>
>> I don't like this part -- (type){ init } is a standard C99 compound literal.
>> There's no point using a gcc extension instead.
> 
> The C99 compound literal is not a const initializer in all situations,
> though :(  Here's another thread where we had a similar discussion, but
> there, the solution was to just make the macro behave as an initializer
> (which is C99 compliant, but loses some type safety) instead of relying
> on a gcc extension:
> 
> https://lists.gnu.org/archive/html/qemu-devel/2017-08/msg06566.html
> 
> I suspect you're running into the same issues that Laszlo already helped
> us understand regarding QLit.
> 

Thanks for the CC! I don't have much context, but the patch looks quite
isolated.

I think I agree with Richard here -- I don't think there's any reason to
change the replacement text of make_floatx80.

The patch names make_float64() as an earlier example (already using
__extension__), but I don't understand why make_float64() was written
that way. ... It seems to go back to ancient commit f090c9d4ad58 ("Add
strict checking mode for softfp code.", 2007-11-18). Was C99 support
(esp. compound literals) spotty in gcc back then?

Not having much background, I'd suggest the opposite change -- replace
the statement-expression in make_float64() with a compound literal.

Thanks
Laszlo