Date: Wed, 21 Jan 2004 21:01:46 +1100 (EST) From: Bruce Evans <bde@zeta.org.au> To: David Schultz <das@FreeBSD.ORG> Cc: Steve Kargl <sgk@troutmask.apl.washington.edu> Subject: Re: Implementing C99's roundf(), round(), and roundl() Message-ID: <20040121205312.E8174@gamplex.bde.org> In-Reply-To: <20040121091817.GA92143@VARK.homeunix.com> References: <20031129000133.GA30662@troutmask.apl.washington.edu> <20031129163105.GA32651@troutmask.apl.washington.edu> <20031201182219.O4431@gamplex.bde.org> <20031202091936.I8778@gamplex.bde.org> <20040120154127.C2417@gamplex.bde.org> <20040121091817.GA92143@VARK.homeunix.com>
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On Wed, 21 Jan 2004, David Schultz wrote:
> On Tue, Jan 20, 2004, Bruce Evans wrote:
> > I think getting FE_INEXACT from rint() is the usual case (if the
> > implementation actually raises it), so we gain little or negative from
> ...
> If you have something in mind that will work, and you have the
> time to code it up, then by all means present or commit it. You
> probably have a better idea than I do about the best way to deal
> with the exceptions and whatnot. (Thanks again for taking the
> time to look at all of this, by the way.)
I probably won't get to this soon (enough). I should give priority
to other things.
> When FENV_ACCESS is on, C99 requires compiler-optimized code to be
> affected by any operative modes in effect at runtime and set any
> exception flags at runtime as if no optimizations had been
> performed. gcc is flat out broken in this regard, as I think
> you've pointed out in the past. Even with FENV_ACCESS on (though
> likely ignored entirely), the code generated for 'd = 0.0 / 0.0' is:
>
> movl $0, %eax
> movl $2146959360, %edx
> movl %eax, -8(%ebp)
> movl %edx, -4(%ebp)
Unless you have a special version of gcc, it is just ignored:
$ cat z.c
#pragma FENV_ACCESS ON
#pragma FENV_ACCESS is broken
$ cc -c z.c
$ cc -c -Wall z.c
z.c:1: warning: ignoring #pragma FENV_ACCESS ON
z.c:2: warning: ignoring #pragma FENV_ACCESS is
> Given that gcc is so broken with respect to exceptions, the fact
> that the math libraries don't raise the appropriate exceptions is
> not a big concern given that basic arithmetic operations often
> don't raise exceptions either. When gcc is fixed, both of these
> problems will go away simultaneously. Nevertheless, if we
> implement fenv.h now, we (a) are able to clean up libm sooner and
> (b) are a step ahead when gcc supports exceptions better. It
> doesn't seem like we have much to lose by going ahead and
> implementing fenv.h, so at the very least we can use it in our own
> libraries.
I agree.
Bruce
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