Date: Thu, 26 Jun 2008 15:49:18 GMT From: Satoshi Kimura <satosi.kimura@nifty>, com@FreeBSD.org To: freebsd-gnats-submit@FreeBSD.org Subject: i386/125011: precision of constants for long double Message-ID: <200806261549.m5QFnIcM058517@www.freebsd.org> Resent-Message-ID: <200806261550.m5QFo1EQ057544@freefall.freebsd.org>
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>Number: 125011
>Category: i386
>Synopsis: precision of constants for long double
>Confidential: no
>Severity: non-critical
>Priority: medium
>Responsible: freebsd-i386
>State: open
>Quarter:
>Keywords:
>Date-Required:
>Class: sw-bug
>Submitter-Id: current-users
>Arrival-Date: Thu Jun 26 15:50:01 UTC 2008
>Closed-Date:
>Last-Modified:
>Originator: Satoshi Kimura
>Release: 6.2
>Organization:
private use
>Environment:
FreeBSD 6.2-RELEASE #0: Fri Jan 12 11:05:30 UTC 2007
root@dessler.cse.buffalo.edu:/usr/obj/usr/src/sys/SMP
>Description:
1. problem
On rather new environment of FreeBSD (ex. 6.2, 7.0),
standard gcc (i.e /usr/bin/gcc) makes strange code
concerning "long double". The precision of all constants
of "long double" are rounded to that of "double". In addition,
if we add -m128bit-long-double option to gcc, this behavior
did not appear.
Historically, precision of constants of "long double" were
as follows:
FreeBSD gcc 96bit 128bit
2.2.6 2.2.2.1 NR -
3.4 2.7.2.3 NR -
4.9 2.95.4 NR -
3.0.4 NR NR
5.3 3.2.3 NR NR
3.3.5 R R <- (1)
3.4.2 NR NR <- (2)
6.2 3.4.6 R NR <- (3)
6.3 3.4.6 R NR
4.2.4 R NR
7.0 4.2.1 R NR
Meanings of this table are as follows:
FreeBSD : version of FreeBSD (RELEASE)
gcc : version of gcc
96bit : 96bit long double
128bit : 128bit long double (compiled with -m128bit-long-double option)
NR : not rounded, i.e keep precision of long double.
R : rounded to precision of double.
2. question
On above table, modification from (2) to (3) was made by
discussions at
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17778
The reason of this midification was as follows:
> On FreeBSD/i386, FPU precision is set to 53 bits to avoid problems with extra
> precision and double rounding for doubles. gcc understands this and evaluates
> FP constants in 53-bit precision, even for long doubles.
He compared (1) and (2) on above table, and concluded to
resume the condition of (1). But, I cannot find the reason
that 128bit "long double" did not modified. ((3) on above table.)
The most impotant question is that his understanding above is
correct or not ? If false, this modification is to be avoided.
Or, if true, 128bit "long double" is also to be modified.
>How-To-Repeat:
1. run sample code on
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17778
or
2. compile this source s.c
int main(void)
{
double c;
long double lc;
c = 3.14159265358979323846;
lc = 3.14159265358979323846L;
}
% gcc -m96bit-long-double -S s.c -o s1.s
% gcc -m128bit-long-double -S s.c -o s2.s
and compare two assembler source, so wo can see the difference as follows:
24c24
< movl $560513024, %eax
---
> movl $560513589, %eax
>Fix:
>Release-Note:
>Audit-Trail:
>Unformatted:
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