Re: Bug in dlls/oleaut32/tests/vartype.c?

[Alex Villací­s Lasso]

Gerald Pfeifer escribió:
> On Mon, 15 Oct 2007, Gerald Pfeifer wrote:
> > dlls/oleaut32/tests/vartype.c has the following snippet of code:
> >
> > +  f = -1e-400;    /* deliberately cause underflow */
> >   +  hres = pVarBstrFromR4(f, lcid, 0, &bstr);
> >   +  ok(hres == S_OK, "got hres 0x%08lx\n", hres);
> >   +  if (bstr)
> >   +  {
> >   +    todo_wine ok(memcmp(bstr, szZero, sizeof(szZero)) == 0, "negative zero (got %s)\n", wtoascii(bstr));
> >   +  }
> >
> > which was added in
> >
> > revision 1.27
> >   Alex Villacís Lasso <a_villacis(a)palosanto.com>
> >   Test for behavior of negative underflow formatting.
> >
> > This doesn't seem to work as intended, because it is a floating point
> > *constant* which current versions of GCC simply truncate to 0.0.
>
> The following patch tries to address this.
>
> Gerald
>
> ChangeLog:
> Avoid truncation of floating point calculation at compile-time.
> Index: dlls/oleaut32/tests/vartype.c
> ===================================================================
> RCS file: /home/wine/wine/dlls/oleaut32/tests/vartype.c,v
> retrieving revision 1.51
> diff -u -3 -p -r1.51 vartype.c
> --- dlls/oleaut32/tests/vartype.c	20 Aug 2007 12:43:49 -0000	1.51
> +++ dlls/oleaut32/tests/vartype.c	29 Oct 2007 22:40:24 -0000
> @@ -4727,6 +4727,7 @@ static void test_VarBstrFromR4(void)
>    HRESULT hres;
>    BSTR bstr = NULL;
>
> +  const float flarge=1e20; 
>    float f;
>
> CHECKPTR(VarBstrFromR4);
> @@ -4747,7 +4748,7 @@ static void test_VarBstrFromR4(void)
>      }
>    }
>
> -  f = -1e-400;    /* deliberately cause underflow */
> +  f = (-1 / flarge ) / flarge;    /* deliberately cause underflow */
>    hres = pVarBstrFromR4(f, lcid, 0, &bstr);
>    ok(hres == S_OK, "got hres 0x%08x\n", hres);
>    if (bstr)
> Sorry to object, but which version of exactly which compiler optimizes 
> away the negative sign?

Consider the following program:

#include <stdio.h>
#include <stdlib.h>

union x
{
    float f;
    unsigned char b[4];
};

int main(void)
{
    union x u;
   
    u.f = -1e-400;
    printf("Value as float is %f\n", u.f);
    printf("Value as bytes is %02x %02x %02x %02x\n", u.b[0], u.b[1], 
u.b[2], u.b[3]);
   
    return 0;
}

On my machine (i386 architecture, Fedora 7, gcc 4.1.2 20070925 (Red Hat 
4.1.2-27)), the output is as follows:

[alex(a)srv64 cpp]$ gcc -O2 -Wall test_underflow.c -o test_underflow
[alex(a)srv64 cpp]$ ./test_underflow
Value as float is -0.000000
Value as bytes is 00 00 00 80
[alex(a)srv64 cpp]$

What compiler are you using? Which version? What is the output of said 
compiler on the test program?

-- 
perl -e '$x=2.4;print sprintf("%.0f + %.0f = %.0f\n",$x,$x,$x+$x);'