Date: Thu, 8 Apr 93 10:06:33 +0200
From: kuku AT acds DOT physik DOT rwth-aachen DOT de (Christoph Kukulies)
To: ae1181t AT stnfor DOT ae DOT ge DOT com, djgpp AT sun DOT soe DOT clarkson DOT edu
Subject: Re:  bug in double

>Graeme Gill writes:
>>       I have discovered a bug in the handling of
>> floating point constants that caused a significant error
>> in a program I am trying to compile.
>> 
>> The workaround is to change line in include/values.h below
>> #if defined(i396)
>> 
>> from 
>> 
>> #define MAXDOUBLE       1.79769313486231570e+308
>> 
>> to
>> 
>> #define MAXDOUBLE       1.79769313486231470e+308
>>                                         ^ this digit changes.
>
>I'm at work so I can't test your program on a PC.  For what it's
>worth, I ran your test program on my workstation, and it works fine.
>
>I'm responding to this because I believe that the existing value for
>MAXDOUBLE is "correct" in that it reflects the correct maximum value
>for 64-bit floating point numbers in IEEE format.  Rather than change
>MAXDOUBLE, we really should figure out why this correct value is not
>working properly, and fix that.  (Do you have a 486, a 386+387, or
>386+emulator?  Maybe this is just an emulator bug.)
 
>It would be interesting to look at the actual bit pattern that the
>compiler is generating for these floating point numbers.  Here's a
>program that would print it:
>
 
>#define MAXDOUBLE       1.79769313486231570e+308
>#define ALMOSTMAX       1.79769313486231470e+308
>
>double  dd = MAXDOUBLE;
>double  ddd = ALMOSTMAX;
>
>int
>main()
>{
>   (void) printf("MAXDOUBLE = %.20e\n", MAXDOUBLE);
>   (void) printf("ALMOSTMAX = %.20e\n", ALMOSTMAX);
>   (void) printf("dd        = %.20e = 0x%8.8x 0x%8.8x\n",
>                  dd, *(int *) &dd, *((int *) &dd + 1));
>   (void) printf("ddd       = %.20e = 0x%8.8x 0x%8.8x\n",
>                  ddd, *(int *) &ddd, *((int *) &ddd + 1));
>}
 
 
>On my (big endian) workstation, this prints:
>
>MAXDOUBLE = 1.79769313486231570000e+308
>ALMOSTMAX = 1.79769313486231470000e+308
>dd        = 1.79769313486231570000e+308 = 0x7fefffff 0xffffffff
>ddd       = 1.79769313486231470000e+308 = 0x7fefffff 0xfffffffa
>--
>                                Eric Backus
>                                ericb AT lsid DOT hp DOT com
>                                (206) 335-2495
 
On my 386BSD system I'm getting:
 
Script started on Thu Apr  8 09:56:24 1993
386bsd> gcc eric.c
386bsd> a.out
MAXDOUBLE = 1.79769313486231570000e+308
ALMOSTMAX = 1.79769313486231570000e+308
dd        = NaN = 0x00000002 0x7ff00000
ddd       = 1.79769313486231570000e+308 = 0xffffffff 0x7fefffff
 
386bsd>
Script done on Thu Apr  8 09:57:35 1993
 

--Chris
Christoph P. U. Kukulies
kuku AT acds DOT physik DOT rwth-aachen DOT de
*** Error code 1
 
Stop.