diff mbox

Define TS 18661-1 CR_DECIMAL_DIG in <float.h>

Message ID alpine.DEB.2.20.1609092112570.29814@digraph.polyomino.org.uk
State New
Headers show

Commit Message

Joseph Myers Sept. 9, 2016, 9:13 p.m. UTC 
TS 18661-1 defines a macro CR_DECIMAL_DIG in <float.h>, for the number
of decimal digits for which conversions between decimal character
strings and (IEEE) binary formats, in both directions, are correctly
rounded.  This patch implements support for this macro in GCC's
<float.h>.

The definition __UINTMAX_MAX__ is the right one for GCC's conversions
of floating constants, since I made those use MPFR to make them
correctly rounding.  The macro also covers standard library functions
such as strtod and printf.  The definition is also correct for current
glibc.  If any targets' libcs support correct rounding in a way that
conforms to TS 18661-1 with a smaller value of CR_DECIMAL_DIG, making
<float.h> reflect that could not be done in isolation without changes
to the interpretation of floating constants as well, since a smaller
CR_DECIMAL_DIG requires double rounding of floating constants (first
to CR_DECIMAL_DIG decimal digits, then to the desired binary format).

Boostrapped with no regressions on x86_64-pc-linux-gnu.  OK to commit?

gcc:
2016-09-09  Joseph Myers  <joseph@codesourcery.com>

	* ginclude/float.h [__STDC_WANT_IEC_60559_BFP_EXT__]
	(CR_DECIMAL_DIG): New macro.

gcc/testsuite:
2016-09-09  Joseph Myers  <joseph@codesourcery.com>

	* gcc.dg/cr-decimal-dig-1.c: New test.

Comments

Joseph Myers Sept. 15, 2016, 12:56 p.m. UTC | #1 
Ping.  This patch 
<https://gcc.gnu.org/ml/gcc-patches/2016-09/msg00546.html> is pending 
review.
Jeff Law Sept. 19, 2016, 8:31 p.m. UTC | #2 
On 09/15/2016 06:56 AM, Joseph Myers wrote:
> Ping.  This patch
> <https://gcc.gnu.org/ml/gcc-patches/2016-09/msg00546.html> is pending
> review.
I suspect you know more about this than anyone else here, so ISTM you're 
the authority on correctness.

Rubber stamp OK. :-)

jeff
diff mbox

Patch

Index: gcc/ginclude/float.h
===================================================================
--- gcc/ginclude/float.h	(revision 240048)
+++ gcc/ginclude/float.h	(working copy)
@@ -185,6 +185,13 @@ 
 
 #endif /* C11 */
 
+#ifdef __STDC_WANT_IEC_60559_BFP_EXT__
+/* Number of decimal digits for which conversions between decimal
+   character strings and binary formats, in both directions, are
+   correctly rounded.  */
+#define CR_DECIMAL_DIG	__UINTMAX_MAX__
+#endif
+
 #ifdef __STDC_WANT_IEC_60559_TYPES_EXT__
 /* Constants for _FloatN and _FloatNx types from TS 18661-3.  See
    comments above for their semantics.  */
Index: gcc/testsuite/gcc.dg/cr-decimal-dig-1.c
===================================================================
--- gcc/testsuite/gcc.dg/cr-decimal-dig-1.c	(nonexistent)
+++ gcc/testsuite/gcc.dg/cr-decimal-dig-1.c	(working copy)
@@ -0,0 +1,14 @@ 
+/* Test TS 18661-1 CR_DECIMAL_DIG.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c11" } */
+
+#define __STDC_WANT_IEC_60559_BFP_EXT__
+#include <float.h>
+
+#ifndef CR_DECIMAL_DIG
+#error "CR_DECIMAL_DIG not defined"
+#endif
+
+#if CR_DECIMAL_DIG < DECIMAL_DIG + 3
+#error "CR_DECIMAL_DIG too small"
+#endif