bug#72145: rare Emacs screwups on x86 due to GCC bug 58416

[Paul Eggert <eggert@cs.ucla.edu>]

On 2024-07-17 20:22, Richard Stallman wrote:
>    > * Rewrite Emacs to never use 'double' (or 'float' or 'long double')
>    > inside a union. This could be painful and hardly seems worthwhile.
> 
> Where does Emacs use those types inside a union?
> Maybe this is not difficult.

I found the bug in src/timefns.c, which uses a union to represent 
timestamp forms (one of which represents an Emacs float). Other uses 
that come to mind are src/lisp.h's struct Lisp_Float, which uses a union 
to save space when representing Lisp floats, and src/lread.c's and 
src/print.c's use of <ieee754.h>'s unions to deal with NaNs when reading 
and printing Lisp floats. Although I have not done an audit I expect 
there are other places too, and I expect it would take some time to 
audit, rewrite and thoroughly test Emacs to not use floating point in 
these places, with runtime performance degraded somewhat as a result.

Although that effort might be worth it if the bug was likely and there 
was no other workaround, the bug is quite rare (we've lived with it for 
decades and I'm the first person to notice it, or at least track it 
down), and with the proposed compiler-flag workaround the remaining 
affected platforms are so obsolescent (decades-old CPUs) that they're 
also rare. I doubt whether it's worth significantly contorting the C 
code (possibly introducing bugs on mainstream platforms) to fix these 
exceedingly rare bugs in obsolescent platforms.