🚀 go-pugleaf

"Edward Rosten" <u98ejr@ecs.ox.ac.uk> wrote in message
news:clcm-20010907-0010@plethora.net...
> > When checking for possible difficulty with division by zero, if you
don't
> > wish to get into trapping overflow and the like, you should check at
least
> > for small non-zero numbers.  If all your numerators have absolute value
< 4,
> > you could safely do
> > #include <float.h>
> > if(fabs(y)>FLT_MIN)
> >     proceed with division by y
> > else
> >     treat as error
>
> I'm not trying to be pedantic, I'm genuinely curious, but isn't
>
> fabs(y) > FLT_MIN
>
> equivalent to
>
> f != 0
>
> Since 0 can be represented exactly and FLT_MIN is the smallest
> possible float greater than zero?


Depends upon whether or not the compiler has denormal numbers.

C:\tmp>type foo.c
#include <stdio.h>
#include <float.h>
int             main(void)
{
    float           fm = FLT_MIN;
    double          dm = DBL_MIN;

    printf("FLT_MIN     =%g\n", fm);
    printf("FLT_MIN*0.5f=%g\n", fm * 0.5f);

    printf("DBL_MIN     =%g\n", dm);
    printf("DBL_MIN*0.5f=%g\n", dm * 0.5);
    return 0;
}

C:\tmp>cl /W4 /Ox foo.c
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 12.00.8804 for 80x86
Copyright (C) Microsoft Corp 1984-1998. All rights reserved.

foo.c
Microsoft (R) Incremental Linker Version 6.00.8447
Copyright (C) Microsoft Corp 1992-1998. All rights reserved.

/out:foo.exe
foo.obj

C:\tmp>foo
FLT_MIN     =1.17549e-038
FLT_MIN*0.5f=5.87747e-039
DBL_MIN     =2.22507e-308
DBL_MIN*0.5f=1.11254e-308

C:\tmp>icl /W5 /Ox foo.c
Intel(R) C++ Compiler for 32-bit applications, Version 5.0.1   Build 010525Z
Copyright (C) 1985-2001 Intel Corporation.  All rights reserved.

foo.c
Microsoft (R) Incremental Linker Version 6.00.8447
Copyright (C) Microsoft Corp 1992-1998. All rights reserved.

-out:foo.exe
foo.obj

C:\tmp>foo
FLT_MIN     =1.17549e-038
FLT_MIN*0.5f=5.87747e-039
DBL_MIN     =2.22507e-308
DBL_MIN*0.5f=1.11254e-308

$ gcc -O3 -W -Wall -ansi -pedantic foo.c -ofoo.exe

dcorbit@DANNFAST c:/tmp
$ ./foo
FLT_MIN     =1.17549e-38
FLT_MIN*0.5f=5.87747e-39
DBL_MIN     =2.22507e-308
DBL_MIN*0.5f=1.11254e-308

Of course, I'm not terribly sure how the test helps, since
FLT_MAX/FLT_MIN would trigger an FPE, if floating point exceptions were
available and enabled for overflow.
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