Date: Mon, 10 Apr 2000 21:40:04 -0700 (PDT) From: Bruce Evans <bde@zeta.org.au> To: freebsd-bugs@FreeBSD.org Subject: Re: i386/17914: float-to-double core dump on 3.4R Message-ID: <200004110440.VAA48579@freefall.freebsd.org>
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The following reply was made to PR i386/17914; it has been noted by GNATS.
From: Bruce Evans <bde@zeta.org.au>
To: aa8vb@ipass.net
Cc: FreeBSD-gnats-submit@FreeBSD.ORG
Subject: Re: i386/17914: float-to-double core dump on 3.4R
Date: Tue, 11 Apr 2000 14:39:21 +1000 (EST)
On Mon, 10 Apr 2000 aa8vb@ipass.net wrote:
> >Description:
>
> #include <float.h>
>
> main()
> {
> float f = FLT_MAX;
> double d;
> f = f * 2;
> d = f;
> }
>
> Delete the "d=f" line and it doesn't core. Put it in and it does
> (floating-point exception).
>
> From this it appears there may be a bug in the float-to-double
> promotion when f is Inf (or, at least I'd expect that f is Inf).
No. "f * 2" gives undefined behaviour since it overflows. For FreeBSD-3.4
on i386's, the actual behaviour is to leave the operands on the FP stack
and cause a SIGFPE on the next FP instruction after the one that overflowed.
> Note that the core dump does not occur when the Inf is generated,
> but only when it is promoted to a double.
Inf is not generated. The operands are left on the FP stack for the SIGFPE
handler to fix up. This behaviour is easy to see using gdb ("display/i $pc",
"stepi" and "info float").
Bruce
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