Re: gcc possibly generating invalid assembler

[Wouter Verhelst <wouter@debian.org>]

On Thu, Apr 20, 2017 at 05:00:35PM -0400, Michael Jeanson wrote:
> On Thu, Apr 13, 2017 at 01:08:59PM +0200, John Paul Adrian Glaubitz wrote:
> > 
> > I have unsuccessfully been trying to build liburcu on Debian/m68k. After
> > adding "m68k" as a detected architecture in configure.ac, the build
> > fails with the assembler generating syntax error messages [1]:
> > 
> > gcc -DHAVE_CONFIG_H -I. -I../.. -I../../urcu   -Wdate-time -D_FORTIFY_SOURCE=2 -I../.. -I../.. -I../../tests/common -g -g -O2
> > -fdebug-prefix-map=/<<PKGBUILDDIR>>=. -fstack-protector-strong -Wformat -Werror=format-security -pthread -c -o test_urcu_defer.o test_urcu_defer.c
> > /tmp/ccXHVXOc.s: Assembler messages:
> > /tmp/ccXHVXOc.s:312: Error: syntax error -- statement `add.l %d1,nr_writes@TLSLE+4(%a0)' ignored
> > /tmp/ccXHVXOc.s:440: Error: syntax error -- statement `add.l %d1,nr_writes@TLSLE+4(%a0)' ignored
> 
> The invalid asm is related to Thread Local Storage and liburcu has a
> compat layer that can be enabled at configure for platforms that don't
> have it.

Last I checked, m68k does have TLS, though. It is a requirement for
recent versions of glibc, and although the m68k port was stuck on it for
a while, recent versions should have it.

In fact, when checking the generated asm code, it shows that the
nr_writes@TLSLE indirection is used in other locations too; but those
don't have the "+4" bit right after that. I suspect that's the problem.

(then again, trying to use gcc5 rather than gcc6 does not fix the issue,
so meh)

> With './configure --disable-compiler-tls' the build succeeds and the
> regression test suite completes without having to disable optimisations.

Well, that's good to know :-)

-- 
< ron> I mean, the main *practical* problem with C++, is there's like a dozen
       people in the world who think they really understand all of its rules,
       and pretty much all of them are just lying to themselves too.
 -- #debian-devel, OFTC, 2016-02-12