On Wed, Aug 12, 2020 at 1:52 AM Tijl Coosemans <t...@freebsd.org> wrote:
> On Tue, 11 Aug 2020 15:56:35 +0400 Gleb Popov <arr...@freebsd.org> wrote:
> > On Sun, Aug 9, 2020 at 7:43 PM Konstantin Belousov <kostik...@gmail.com>
> > wrote:
> >
> > > I do not believe there were any change in the toolchain between p2 and
> p7,
> > > this is more likely indicates some fluctuation in the build. The only
> > > change that could be even remotely declared as possibly related is
> > > EN-20:10.build r360473.
> > >
> >
> > Right, I was using a wrong set of port's OPTIONS that hide the problem.
> >
> > Indeed you need to look at the .o files that reference _8 symbol. I
> would
> > > closely look at the compilation command used for them, for start.
> > >
> >
> > After digging it a bit I found that the following command
> >
> > cc -x c
> >
> /wrkdirs/usr/ports/lang/ghc/work/ghc-8.10.1/libraries/ghc-prim/cbits/atomic.c
> > -o /tmp/ghc_1.s -fno-PIC -Wimplicit -S \
> > -include
> >
> /wrkdirs/usr/ports/lang/ghc/work/ghc-8.10.1/includes/dist-install/build/ghcversion.h
> > -I/usr/local/include \
> > -I/wrkdirs/usr/ports/lang/ghc/work/ghc-8.10.1/libraries/base/include \
> >
> -I/wrkdirs/usr/ports/lang/ghc/work/ghc-8.10.1/libraries/base/dist-install/build/include
> > \
> >
> -I/wrkdirs/usr/ports/lang/ghc/work/ghc-8.10.1/libraries/base/dist-install/build/dist-install/build/include
> > \
> >
> -I/wrkdirs/usr/ports/lang/ghc/work/ghc-8.10.1/libraries/integer-gmp/include
> > \
> >
> -I/wrkdirs/usr/ports/lang/ghc/work/ghc-8.10.1/libraries/integer-gmp/dist-install/build/include
> > \
> >
> -I/wrkdirs/usr/ports/lang/ghc/work/ghc-8.10.1/libraries/integer-gmp/dist-install/build/dist-install/build/include
> > \
> > -I/wrkdirs/usr/ports/lang/ghc/work/ghc-8.10.1/rts/dist/build
> > -I/wrkdirs/usr/ports/lang/ghc/work/ghc-8.10.1/includes \
> >
> -I/wrkdirs/usr/ports/lang/ghc/work/ghc-8.10.1/includes/dist-derivedconstants/header
> > \
> >
> -I/wrkdirs/usr/ports/lang/ghc/work/ghc-8.10.1/includes/dist-install/build \
> > -march=i686 -U__i686
> >
> > produce an assembly file with
> >
> > calll __atomic_load_8
> >
> > instruction.
> >
> > The value of -march flag seems to be ignored.
> >
> > Interestingly, previous version of GHC calls C compiler in the following
> > way:
> >
> > cc -U__i686 '-march=i686' -fno-stack-protector -DTABLES_NEXT_TO_CODE
> > '-march=i686' -x c
> >
> /wrkdirs/usr/ports/lang/ghc/work/ghc-8.10.1/libraries/ghc-prim/cbits/atomic.c
> > -o /tmp/ghc_1.s -Wimplicit -S \
> > -include
> >
> /wrkdirs/usr/ports/lang/ghc/work/ghc-8.6.5-boot/lib/ghc-8.6.5/include/ghcversion.h
> > \
> > -I/usr/local/include \
> >
> -I/wrkdirs/usr/ports/lang/ghc/work/ghc-8.6.5-boot/lib/ghc-8.6.5/base-4.12.0.0/include
> > \
> > -I/wrkdirs/usr/ports/lang/ghc/work/ghc-8.6.5-boot/lib/ghc-8.6.5/include
> >
> > And this command produces an assembly without calls to __atomic_load_8
> >
> > Any ideas what makes it appear?
>
> Looking at atomic.c one possibility is that in the first command
> HAVE_C11_ATOMICS is defined and not in the second.
>
> Apparently gcc always uses fild/fistp for __atomic_load_8 while clang
> only uses it if the address is 8 byte aligned and generates a function
> call otherwise.
>
Indeed, this looks like a culprit! When compiling using first command line
(the long one) I get following warnings:
/wrkdirs/usr/ports/lang/ghc/work/ghc-8.10.1/libraries/ghc-prim/cbits/atomic.c:369:10:
warning: misaligned atomic operation may incur significant performance
penalty [-Watomic-alignment]
return __atomic_load_n((StgWord64 *) x, __ATOMIC_SEQ_CST);
^
/wrkdirs/usr/ports/lang/ghc/work/ghc-8.10.1/libraries/ghc-prim/cbits/atomic.c:417:3:
warning: misaligned atomic operation may incur significant performance
penalty [-Watomic-alignment]
__atomic_store_n((StgWord64 *) x, (StgWord64) val, __ATOMIC_SEQ_CST);
^
2 warnings generated.
I guess this basically means "I'm emitting a call there". So, what's the
correct fix in this case?
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