From 593b43c422ca7ce5836195bf8e904bbc2434fd77 Mon Sep 17 00:00:00 2001 From: Taylor R Campbell Date: Fri, 12 Jul 2019 16:01:16 +0000 Subject: [PATCH] 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. --- lib/libc/gdtoa/Makefile.inc | 4 ++++ lib/libc/gdtoa/gdtoa_fltrnds.h | 3 ++- lib/libc/gdtoa/gdtoaimp.h | 3 +++ 3 files changed, 9 insertions(+), 1 deletion(-) diff --git a/lib/libc/gdtoa/Makefile.inc b/lib/libc/gdtoa/Makefile.inc index 19268daa9148..d08bc61d2797 100644 --- a/lib/libc/gdtoa/Makefile.inc +++ b/lib/libc/gdtoa/Makefile.inc @@ -4,7 +4,11 @@ .PATH: ${.CURDIR}/gdtoa CPPFLAGS+=-I${.CURDIR}/gdtoa -I${.CURDIR}/locale +.if ${MACHINE_ARCH} == "vax" CPPFLAGS+=-DNO_FENV_H +.else +CPPFLAGS+=-DHonor_FLT_ROUNDS +.endif # machine-dependent directory must provide the following: # arith.h gd_qnan.h diff --git a/lib/libc/gdtoa/gdtoa_fltrnds.h b/lib/libc/gdtoa/gdtoa_fltrnds.h index 33e5f9e5342c..f170de5fd158 100644 --- a/lib/libc/gdtoa/gdtoa_fltrnds.h +++ b/lib/libc/gdtoa/gdtoa_fltrnds.h @@ -1,4 +1,5 @@ - FPI *fpi, fpi1; + CONST FPI *fpi; + FPI fpi1; int Rounding; #ifdef Trust_FLT_ROUNDS /*{{ only define this if FLT_ROUNDS really works! */ Rounding = Flt_Rounds; diff --git a/lib/libc/gdtoa/gdtoaimp.h b/lib/libc/gdtoa/gdtoaimp.h index 32a1bad59be0..034716ddd34b 100644 --- a/lib/libc/gdtoa/gdtoaimp.h +++ b/lib/libc/gdtoa/gdtoaimp.h @@ -200,6 +200,9 @@ THIS SOFTWARE. #include "gd_qnan.h" #ifdef Honor_FLT_ROUNDS #include +__weakref_visible int __libc_fegetround_ref(void) __weak_reference(fegetround); +#define fegetround() \ + (__libc_fegetround_ref ? __libc_fegetround_ref() : FE_TONEAREST) #endif #ifdef DEBUG