Simplify, document, and port floating-point.
The porting part of this patch fixes bugs on non-IEEE platforms with frexp, ldexp, logb. * admin/CPP-DEFINES (HAVE_CBRT, HAVE_LOGB, logb): Remove. * configure.ac (logb, cbrt): Do not check for these functions, as they are not being used. * doc/lispref/numbers.texi (Float Basics, Arithmetic Operations, Math Functions): Document that / and mod (with floating point arguments), along with asin, acos, log, log10, expt and sqrt, return special values instead of signaling exceptions. (Float Basics): Document that logb operates on the absolute value of its argument. (Math Functions): Document that (log ARG BASE) also returns NaN if BASE is negative. Document that (expt X Y) returns NaN if X is a finite negative number and Y a finite non-integer. * etc/NEWS: Document NaNs versus signaling-error change. * src/data.c, src/lisp.h (Qdomain_error, Qsingularity_error, Qunderflow_error): Now static. * src/floatfns.c: Simplify discussion of functions that Emacs doesn't support, by removing commented-out code and briefly listing the C89 functions excluded. The commented-out stuff was confusing maintenance, e.g., we thought we needed cbrt but it was commented out. (logb): Remove decl; no longer needed. (isfinite): New macro, if not already supplied. (isnan): Don't replace any existing macro. (Ffrexp, Fldexp): Define even if !HAVE_COPYSIGN, as frexp and ldexp are present on all C89 platforms. (Ffrexp): Do not special-case zero, as frexp does the right thing for that case. (Flogb): Do not use logb, as it doesn't have the desired meaning on hosts that use non-base-2 floating point. Instead, stick with frexp, which is C89 anyway. Do not pass an infinity or a NaN to frexp, to avoid getting an unspecified result.
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