Re: [PATCH 0/3] Allow LLD 13.0.0 and improve compatibility with gold and clang

["H.J. Lu via Libc-alpha" <libc-alpha@sourceware.org>]

On Wed, Jul 28, 2021 at 2:52 PM Fangrui Song <maskray@google.com> wrote:
>
> On 2021-07-28, H.J. Lu wrote:
> >On Sun, Jul 25, 2021 at 8:58 PM Fangrui Song via Libc-alpha
> ><libc-alpha@sourceware.org> wrote:
> >>
> >> The patches allow LLD 13.0.0 to build glibc. LLD's compatibility with
> >> GNU ld is generally better than gold's compatibility with GNU ld.
> >>
> >> The first two commits improve gold and clang compatibility as well.
> >> (There is still a long way for clang to build glibc.)
> >>
> >> About `make check` results:
> >>
> >> I can't configure glibc --enable-static-pie with gold, so I use
> >> --disable-static-pie with gold.
> >>
> >> * gold (--disable-static-pie) has 160 FAIL.
> >> * ld.bfd has 152 FAIL.

There should be no unexpected failures on x86 with ld.bfd.   If
it isn't the case for you, you should open a binutils bug.

> >> * ld.lld has 159 FAIL.
> >>
> >> ## I have investigated a few failures.
> >>
> >> The tst-ifunc-isa-*.c failures are not ld.lld's fault.
> >> The lld linked tst-ifunc-isa-* work with LD_BIND_NOW=1.
> >> The tests happen to work with GNU ld because the IRELATIVE for foo_ifunc
> >> is placed after JUMP_SLOT in .repa.plt.  The test needs to call
> >> __x86_get_cpuid_feature_leaf which is defined in a different TU. IMHO
> >> such ifunc does not guaranteed to work.
> >>
> >> For gmon/tst-gmon-gprof*, ld.lld linked tst-gmon-gprof has a f3 line,
> >> which appears more correct to me.  But the test considers it a failure.
> >>
> >> % cat gmon/tst-gmon-gprof.out
> >> --- expected
> >> +++ actual
> >> @@ -1,2 +1,3 @@
> >>  f1 2000
> >>  f2 1000
> >> +f3 1
> >> FAIL
> >>
> >>
> >> For malloc/tst-compathooks-on,
> >>
> >>     malloc/tst-compathooks-on: Symbol `__free_hook' has different size in shared object, consider re-linking
> >>
> >> the root cause is that lld's symbol versioning is different from GNU ld in an unusal case:
> >>
> >>     __asm__ (".symver " "__free_hook" "," "__free_hook" "@" "GLIBC_2.2.5");
> >>
> >> This leaves two symbols __free_hook and __free_hook@GLIBC_2.2.5.
> >> __free_hook is then attached a default version GLIBC_2.2.5.
> >> I think malloc/malloc-debug.c uses a fragile versioned symbol here.
> >> If the inline asm uses @@ the failure should go away.
> >>
> >>
> >> In summary, I think the failed tests touch some dark corners of the toolchain.

Please open a separate bug for each distinct failed test with lld.

Thanks.

> >> These things do not really matter for real world applications.
> >>
> >
> >Please open a separate bug for each issue you are trying to fix and reference
> >the bug in your patch.
> >
> >Thanks.
>
> I filed:
>
> https://sourceware.org/bugzilla/show_bug.cgi?id=28151 elf: Some tst-audit* tests need ld --audit and --depaudit which do not work with gold or ld.lld
> https://sourceware.org/bugzilla/show_bug.cgi?id=28152 elf: clang integrated assembler and ld.lld do not support .tls_common
> https://sourceware.org/bugzilla/show_bug.cgi?id=28153 [test] gmon/tst-gmon-gprof* may have a f3 line when built with ld.lld
> https://sourceware.org/bugzilla/show_bug.cgi?id=28154 [test] sysdeps/x86/tst-ifunc-isa-* lazy binding failure with ld.lld
>
> If there is a need to amend patches, I'll attach the BZ numbers.



-- 
H.J.