Subject: bug#63556: 29.0.90; Use of _Generic breaks Emacs build on GCC <4.9

From: Po Lu <[email-masked]>
Date: Thu, 18 May 2023 21:21:51 +0800

Eli Zaretskii <[email-masked]> writes:

> Depends on how complex is that, and what problem does it solve.

The problem of using `long long' on systems with less than 64
significant bits in their words.  I don't think that's a particularly
good idea -- once such uses pile up, Emacs will gradually become slower
and slower on such systems.

>> Btw, couldn't the macro be defined to something along the lines of this
>> when the compiler only supports C99?
>> 
>>   #define foo(expression)			  \
>>     (sizeof (expression) == sizeof (signed char)  \
>>      ? code_for_signed_char			  \
>>      : (sizeof (expression) == sizeof (short int) \
>> 	? code_for_short_int			  \
>> 	: (sizeof (expression) == sizeof (int)	  \
>> 	   ? code_for_int			  \
>> 	   ... and so on
>
> AFAIR, sizeof is a compile-time expression, the preprocessor cannot
> evaluate it.

It doesn't have to be evaluated by the preprocessor (and nor is
_Generic.)  It just needs to expand to an expression that does the right
thing.