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Re: Bug in std::floor?
On 2017-11-10 05:58 +0100, Marc Glisse wrote:
> On Fri, 10 Nov 2017, Nil Geisweiller wrote:
> > Indeed! I would really like to have a look at the floor implementation, but
> > it has proven difficult. It calls __builtin_floor, that is no where to be
> > found. It seems to be implemented in gcc/builtins.c but it's too obfuscated
> > for me to understand.
>
> Look at the generated asm? Here, it is a single instruction: roundsd.
> For constant folding, you would want to look at real.c.
> builtins.c essentially delegates the work to each target to specify how
> floor should be expanded, and generates a call to the system's C library
> when that's not specified.
>
I found a machine independent version in glibc and slightly modified it
(expanding some macros and removing optimization barriers). It's attached.
--
Xi Ruoyao <ryxi@stu.xidian.edu.cn>
School of Aerospace Science and Technology, Xidian University
/* Based on glibc sysdeps/ieee754/dbl-64/s_floor.h.
Copyright (C) 2011-2015 Free Software Foundation, Inc.
This file is part of the GNU C Library.
Contributed by Ulrich Drepper <drepper@cygnus.com>, 2011.
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
<http://www.gnu.org/licenses/>. */
/* Based on a version which carries the following copyright: */
/*
* ====================================================
* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
*
* Developed at SunPro, a Sun Microsystems, Inc. business.
* Permission to use, copy, modify, and distribute this
* software is freely granted, provided that this notice
* is preserved.
* ====================================================
*/
#include <stdint.h>
#include <stdio.h>
/*
* floor(x)
* Return x rounded toward -inf to integral value
* Method:
* Bit twiddling.
* Exception:
* Inexact flag raised if x not equal to floor(x).
*/
static const double huge = 1.0e300;
union U
{
double value;
uint64_t word;
};
double
impl_floor (double x)
{
int64_t i0, j0;
union U u;
u.value = x;
i0 = u.word;
j0 = ((i0>>52)&0x7ff)-0x3ff;
if(j0<52) {
if(j0<0) { /* raise inexact if x != 0 */
if(i0>=0) {i0=0;}
else if((i0&0x7fffffffffffffffl)!=0)
{ i0=0xbff0000000000000l;}
} else {
uint64_t i = (0x000fffffffffffffl)>>j0;
if((i0&i)==0) return x; /* x is integral */
if(i0<0) i0 += (0x0010000000000000l)>>j0;
i0 &= (~i);
}
u.word = i0;
x = u.value;
} else if (j0==0x400)
return x+x; /* inf or NaN */
return x;
}
int main()
{
printf("%.10f %.10f\n", impl_floor(4.6*100), impl_floor(1.7*100));
return 0;
}