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 _at__at_ -35,11 +35,13 _at__at_ 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 _at__at_ -49,13 +51,13 _at__at_ 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) _at__at_ -75,7 +77,7 _at__at_ 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 _at__at_ -114,7 +116,7 _at__at_ # 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. -- SteveReceived on Wed Mar 13 2019 - 17:08:12 UTC
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