Date: Wed, 13 Mar 2019 11:08:06 -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: <20190313180806.GA35586@troutmask.apl.washington.edu> In-Reply-To: <e638e0bf-b8d0-9bd1-d034-8acc6eef872c@FreeBSD.org> 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> <e638e0bf-b8d0-9bd1-d034-8acc6eef872c@FreeBSD.org>
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On Wed, Mar 13, 2019 at 10:16:12AM -0700, John Baldwin wrote:
> 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?
I don't have an i386 Linux environment. I tried comparing the
assembly generated with and without volatile, but it proves
difficult as register numbers are changed between the 2 listings
so almost all lines mismatch
If I move ranged(), rangef(), dp_csinh(), and ulpfd() into b.c
so a.c only contains main(), add appropriate prototypes to a.c,
and comment out the printf() statements, I still see the problem.
Judging from the diff, there is a difference in the spills and
loads in 2 places.
% diff -uw without_volatile with_volatile
--- without_volatile 2019-03-13 10:51:33.244226000 -0700
+++ with_volatile 2019-03-13 10:51:54.088095000 -0700
@@ -35,11 +35,13 @@
movl %esi, 68(%esp) # 4-byte Spill
calll rangef
fadds .LCPI0_0
- fstpl 24(%esp) # 8-byte Folded Spill
+ fstps 28(%esp)
calll rangef
fadds .LCPI0_1
- fstl 100(%esp) # 8-byte Folded Spill
- fldl 24(%esp) # 8-byte Folded Reload
+ fstps 24(%esp)
+ flds 28(%esp)
+ flds 24(%esp)
+ fxch %st(1)
fstps 48(%esp)
fstps 52(%esp)
movl 48(%esp), %eax
@@ -49,13 +51,13 @@
calll csinhf
movl %eax, %esi
movl %edx, %edi
+ flds 28(%esp)
+ flds 24(%esp)
leal 72(%esp), %eax
movl %eax, 20(%esp)
leal 80(%esp), %eax
movl %eax, 16(%esp)
- fldl 100(%esp) # 8-byte Folded Reload
fstpl 8(%esp)
- fldl 24(%esp) # 8-byte Folded Reload
fstpl (%esp)
calll dp_csinh
movl %esi, 40(%esp)
@@ -75,7 +77,7 @@
fnstsw %ax
# kill: def $ah killed $ah killed $ax
sahf
- fstl 24(%esp) # 8-byte Folded Spill
+ fstl 100(%esp) # 8-byte Folded Spill
ja .LBB0_3
# %bb.2: # %for.body
# in Loop: Header=BB0_1 Depth=1
@@ -114,7 +116,7 @@
# in Loop: Header=BB0_1 Depth=1
fstp %st(2)
fldl 92(%esp) # 8-byte Folded Reload
- fldl 24(%esp) # 8-byte Folded Reload
+ fldl 100(%esp) # 8-byte Folded Reload
fucomp %st(1)
fnstsw %ax
# kill: def $ah killed $ah killed $ax
Adding ieeefp.h to a.c and fpsetprec(FP_PE) in main()
produces a massive diff, but still wrong results if
volatile is not use.
Clang appears to be broken for FP on i386/387.
--
Steve
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