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From: | Paul Eggert |
Subject: | bug#63556: 29.0.90; Use of _Generic breaks Emacs build on GCC <4.9 |
Date: | Thu, 18 May 2023 07:40:47 -0700 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 |
On 5/18/23 04:11, Po Lu wrote:
Would anyone mind if I made configure check for that?
There would be problems in doing that. GCC and Clang both have problems with _Generic (suggested by this original bug report), and some of them have to do with which warnings you've enabled. The configure-time check would run without the warnings whereas the build would run with them.
This is partly why intprops-internal.h has given up on _Generic with GCC and Clang; see its ifdefs. I assumed that these problems didn't apply to their simple use in elogb, but apparently I assumed incorrectly.
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) \
That wouldn't work on unusual platforms that have padding bits. Of course we could simply refuse to port to targets like that; still, I'd be reluctant to do that just for this little issue.
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.
I couldn't measure the slowdown in this case. Until it's measurable I wouldn't worry about it. To some extent I put in that _Generic originally more as documentation than as an actual attempt to speed things up.
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