Date: Wed, 13 Mar 2019 10:16:12 -0700 From: John Baldwin <jhb@FreeBSD.org> To: sgk@troutmask.apl.washington.edu Cc: freebsd-toolchain@freebsd.org, freebsd-current@freebsd.org Subject: Re: Optimization bug with floating-point? Message-ID: <e638e0bf-b8d0-9bd1-d034-8acc6eef872c@FreeBSD.org> In-Reply-To: <20190313164039.GA35340@troutmask.apl.washington.edu> References: <20190313024506.GA31746@troutmask.apl.washington.edu> <20190313151635.GA34757@troutmask.apl.washington.edu> <e290b68f-7a1d-2456-4a0c-9f7dfd303f55@FreeBSD.org> <20190313164039.GA35340@troutmask.apl.washington.edu>
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On 3/13/19 9:40 AM, Steve Kargl wrote:
> 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?
Oh, I misread your earlier e-mail to say that clang60 worked.
One issue I'm aware of is that clang does not have any support for the
special arrangement FreeBSD/i386 uses where it uses different precision
for registers vs in-memory for some of the floating point types (GCC has
a special hack that is only used on FreeBSD for this but isn't used on
any other OS's). I wonder if that could be a factor? Volatile probably
forces a round trip between memory which might explain why this is the
case.
I wonder what your test program does on i386 Linux with GCC?
--
John Baldwin
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