From 0cc0033ef19bd3378445c2b851e53d7255cb1b1e Mon Sep 17 00:00:00 2001 From: Letu Ren Date: Fri, 21 Oct 2022 22:54:50 +0800 Subject: stdlib/strfrom: Add copysign to fix NAN issue on riscv (BZ #29501) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit According to the specification of ISO/IEC TS 18661-1:2014, The strfromd, strfromf, and strfroml functions are equivalent to snprintf(s, n, format, fp) (7.21.6.5), except the format string contains only the character %, an optional precision that does not contain an asterisk *, and one of the conversion specifiers a, A, e, E, f, F, g, or G, which applies to the type (double, float, or long double) indicated by the function suffix (rather than by a length modifier). Use of these functions with any other 20 format string results in undefined behavior. strfromf will convert the arguement with type float to double first. According to the latest version of IEEE754 which is published in 2019, Conversion of a quiet NaN from a narrower format to a wider format in the same radix, and then back to the same narrower format, should not change the quiet NaN payload in any way except to make it canonical. When either an input or result is a NaN, this standard does not interpret the sign of a NaN. However, operations on bit strings—copy, negate, abs, copySign—specify the sign bit of a NaN result, sometimes based upon the sign bit of a NaN operand. The logical predicates totalOrder and isSignMinus are also affected by the sign bit of a NaN operand. For all other operations, this standard does not specify the sign bit of a NaN result, even when there is only one input NaN, or when the NaN is produced from an invalid operation. converting NAN or -NAN with type float to double doesn't need to keep the signbit. As a result, this test case isn't mandatory. The problem is that according to RISC-V ISA manual in chapter 11.3 of riscv-isa-20191213, Except when otherwise stated, if the result of a floating-point operation is NaN, it is the canonical NaN. The canonical NaN has a positive sign and all significand bits clear except the MSB, a.k.a. the quiet bit. For single-precision floating-point, this corresponds to the pattern 0x7fc00000. which means that conversion -NAN from float to double won't keep the signbit. Since glibc ought to be consistent here between types and architectures, this patch adds copysign to fix this problem if the string is NAN. This patch adds two different functions under sysdeps directory to work around the issue. This patch has been tested on x86_64 and riscv64. Resolves: BZ #29501 v2: Change from macros to different inline functions. v3: Add unlikely check to isnan. v4: Fix wrong commit message header. v5: Fix style: add space before parentheses. v6: Add copyright. Signed-off-by: Letu Ren Reviewed-by: Adhemerval Zanella --- sysdeps/riscv/rvd/fix-float-double-convert-nan.h | 37 ++++++++++++++++++++++++ 1 file changed, 37 insertions(+) create mode 100644 sysdeps/riscv/rvd/fix-float-double-convert-nan.h (limited to 'sysdeps/riscv') diff --git a/sysdeps/riscv/rvd/fix-float-double-convert-nan.h b/sysdeps/riscv/rvd/fix-float-double-convert-nan.h new file mode 100644 index 0000000000..cab003f3c1 --- /dev/null +++ b/sysdeps/riscv/rvd/fix-float-double-convert-nan.h @@ -0,0 +1,37 @@ +/* Fix for conversion of float NAN to double. RISC-V version.. + Copyright (C) 2022 Free Software Foundation, Inc. + This file is part of the GNU C Library. + + The GNU C Library is free software; you can redistribute it and/or + modify it under the terms of the GNU Lesser General Public + License as published by the Free Software Foundation; either + version 2.1 of the License, or (at your option) any later version. + + The GNU C Library is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU + Lesser General Public License for more details. + + You should have received a copy of the GNU Lesser General Public + License along with the GNU C Library. If not, see + . */ + +#ifndef FIX_FLOAT_DOUBLE_CONVERT_NAN_H +#define FIX_FLOAT_DOUBLE_CONVERT_NAN_H + +#include + +/* RISC-V rvd instructions do not preserve the signbit of NAN + when converting from float to double. */ +static inline double +keep_sign_conversion (float flt) +{ + if (__glibc_unlikely (isnan (flt))) + { + float x = copysignf (1.f, flt); + return copysign ((double) flt, (double) x); + } + return flt; +} + +#endif -- cgit 1.4.1