Re: [Valgrind-users] Possible bug in valgrind-1.9.4

[Dominic M. <dma...@ai...>]

On Fri, 4 Apr 2003, Julian Seward wrote:
> 
> Hi.  Yes, 1.9.4 does have an obscure bug in the handling of floating
> point conditional code sometimes.  I've fixed it in cvs and I hope to
> get it out to the world by shipping 1.9.5 at the weekend, or soon
> thereafter.  In the meantime I attach a patch with the fix -- easy,
> you just need to delete two lines of code.  
>
> It would be good if you could confirm that it works.

Yep, that fixes it.  Thanks!

> Thanks for chasing down a small example, even though I didn't use it
> -- you've no idea how much that helps.  Reproducing problems that people
> report is the #1 problem we have in debugging V; once we reproduce a
> problem, tracking it down is simple.

I understand all too well.  :)

Regards,
Dominic

> J
> 
> Index: coregrind/vg_from_ucode.c
> ===================================================================
> RCS file: /cvsroot/valgrind/valgrind/coregrind/vg_from_ucode.c,v
> retrieving revision 1.41
> retrieving revision 1.42
> diff -u -3 -p -r1.41 -r1.42
> --- coregrind/vg_from_ucode.c   26 Mar 2003 21:08:00 -0000      1.41
> +++ coregrind/vg_from_ucode.c   26 Mar 2003 23:43:57 -0000      1.42
> @@ -3412,8 +3412,6 @@ static void emitUInstr ( UCodeBlock* cb,
>        case FPU:
>           vg_assert(u->tag1 == Lit16);
>           vg_assert(u->tag2 == NoValue);
> -         if (anyFlagUse ( u ))
> -            emit_get_eflags();
>          if (!(*fplive)) {
>             emit_get_fpu_state();
>             *fplive = True;
> 
> 
> On Friday 04 April 2003 6:57 pm, Dominic Mazzoni wrote:
> > Hi,
> >
> > I've been using valgrind for about six months, and it's been wonderful to
> > have.  I was spoiled having access to purify on Solaris machines for a
> > while, and missed having something similar on Linux.  Many thanks to
> > Julian Seward and everyone else who contributed to its development.
> >
> > I've included a very small program that generates different output when
> > it's run through valgrind.  I noticed the error while I was debugging a
> > function to quickly check if an array of single-precision floats has any
> > NaNs in it - it turns out that doing a bitwise test is 2-3x faster than
> > calling the finite() function.
> >
> > Anyway, the error only occurs if you compile it with the options:
> >
> >   gcc -O2 -mcpu=pentiumpro -march=pentiumpro
> >
> > However, the exact same error occurs whether I compile the program with
> > gcc 2.96 (RedHat 7.3's version) or gcc 3.2.
> >
> > The correct output of the program is "0.000000".  When run under valgrind
> > 1.9.4, it outputs "1.000000".
> >
> > I hope this helps!  It's easy enough for me to work around, but I can only
> > guess that this is probably the symptom of a bug that could manifest
> > itself in other ways.
> >
> > - Dominic
> >
> > #include <stdio.h>
> >
> > int main(int argc, char **argv)
> > {
> >    union {
> >       float a[2];
> >       int b[2];
> >    } u;
> >
> >    u.a[0] = 0.0 / 0.0;
> >    u.a[1] = ((*u.b & 0x7FC00000) != 0x7FC00000);
> >    printf("%f\n", u.a[1]);
> >
> >    return 0;
> > }
> >
> >
> >
> >
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>