CVS commit: src/lib/libc/gdtoa

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

Module Name:    src
Committed By:   riastradh
Date:           Thu Aug  1 02:06:31 UTC 2019

Modified Files:
        src/lib/libc/gdtoa: Makefile.inc gdtoa_fltrnds.h gdtoaimp.h

Log Message:
Honour the floating-point rounding mode in floating-point formatting.

C99, Sec. 7.19.6.1 `The fprintf function', paragraph 13, p. 281:

   (Recommended practice)

   For e, E, f, F, g, and G conversions, if the number of significant
   decimal digits is at most DECIMAL_DIG, then the result should be
   correctly rounded.  If the number of significant decimal digits is
   more than DECIMAL_DIG but the source value is exactly
   representable with DECIMAL_DIG digits, then the result should be
   an exact representation with trailing zeros.  Otherwise, the
   source value is bounded by two adjacent decimal strings L < U,
   both having DECIMAL_DIG significant idgits; the value of the
   resultant decimal string D should satisfy L <= D <= U, _with the
   extra stipulation that the error should have a correct sign for
   the current rounding direction_.  [emphasis added]

The gdtoa code base already supports respecting the floating-point
rounding mode, as long as we compile it with Honor_FLT_ROUNDS
defined.  However, for this to work, fegetround must be available in
libc, which it is not currently -- the fenv logic is in libm.

Fortunately, we don't have to move all of fenv from libm to libc --
programs that do not link against libm don't have fesetround, so the
rounding mode is always the default (barring asm shenanigans that
bypass the API -- tough).  So use a weak reference to fegetround; by
default, assume FE_TONEAREST if it is not defined.


To generate a diff of this commit:
cvs rdiff -u -r1.10 -r1.11 src/lib/libc/gdtoa/Makefile.inc
cvs rdiff -u -r1.1.1.1 -r1.2 src/lib/libc/gdtoa/gdtoa_fltrnds.h
cvs rdiff -u -r1.14 -r1.15 src/lib/libc/gdtoa/gdtoaimp.h

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.