Date: Wed, 13 Mar 2019 09:40:39 -0700 From: Steve Kargl <sgk@troutmask.apl.washington.edu> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-toolchain@freebsd.org, freebsd-current@freebsd.org Subject: Re: Optimization bug with floating-point? Message-ID: <20190313164039.GA35340@troutmask.apl.washington.edu> In-Reply-To: <e290b68f-7a1d-2456-4a0c-9f7dfd303f55@FreeBSD.org> References: <20190313024506.GA31746@troutmask.apl.washington.edu> <20190313151635.GA34757@troutmask.apl.washington.edu> <e290b68f-7a1d-2456-4a0c-9f7dfd303f55@FreeBSD.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On Wed, Mar 13, 2019 at 09:32:57AM -0700, John Baldwin wrote:
> On 3/13/19 8:16 AM, Steve Kargl wrote:
> > On Tue, Mar 12, 2019 at 07:45:41PM -0700, Steve Kargl wrote:
> >>
> >> gcc8 --version
> >> gcc8 (FreeBSD Ports Collection) 8.3.0
> >>
> >> gcc8 -fno-builtin -o z a.c -lm && ./z
> >> gcc8 -O -fno-builtin -o z a.c -lm && ./z
> >> gcc8 -O2 -fno-builtin -o z a.c -lm && ./z
> >> gcc8 -O3 -fno-builtin -o z a.c -lm && ./z
> >>
> >> Max ULP: 2.297073
> >> Count: 0 (# of ULP that exceed 21)
> >>
> >
> > clang agrees with gcc8 if one changes ...
> >
> >> int
> >> main(void)
> >> {
> >> double re, im, u, ur, ui;
> >> float complex f;
> >> float x, y;
> >
> > this line to "volatile float x, y".
>
> So it seems to be a regression in clang 7 vs clang 6?
>
/usr/local/bin/clang60 has the same problem.
% /usr/local/bin/clang60 -o z -O2 a.c -lm && ./z
Maximum ULP: 23.061242
# of ULP > 21: 39
Adding volatile as in the above "fixes" the problem.
AFAICT, this a i386/387 code generation problem. Perhaps,
an alignment issue?
--
Steve
20170425 https://www.youtube.com/watch?v=VWUpyCsUKR4
20161221 https://www.youtube.com/watch?v=IbCHE-hONow
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20190313164039.GA35340>