libgcc _BitInt helper documentation [PR102989]

On Mon, Aug 21, 2023 at 05:32:04PM +0000, Joseph Myers wrote:
> I think the libgcc functions (i.e. those exported by libgcc, to which
> references are generated by the compiler) need documenting in libgcc.texi.
> Internal functions or macros in the libgcc patch need appropriate comments
> specifying their semantics; especially FP_TO_BITINT and FP_FROM_BITINT
> which have a lot of arguments and no comments saying what the semantics of
> the macros and their arguments are supposed to me.

Here is an incremental patch which does that.

2023-09-06  Jakub Jelinek  <jakub@redhat.com>

	PR c/102989
gcc/
	* doc/libgcc.texi (Bit-precise integer arithmetic functions):
	Document general rules for _BitInt support library functions
	and document __mulbitint3 and __divmodbitint4.
	(Conversion functions): Document __fix{s,d,x,t}fbitint,
	__floatbitint{s,d,x,t,h,b}f, __bid_fix{s,d,t}dbitint and
	__bid_floatbitint{s,d,t}d.
libgcc/
	* libgcc2.c (bitint_negate): Add function comment.
	* soft-fp/bitint.h (bitint_negate): Add function comment.
	(FP_TO_BITINT, FP_FROM_BITINT): Add comment explaining the macros.