[PATCH] D13825: [ELF2] Don't create RelativeReloc for weak undef symbols

[Hal Finkel via llvm-commits]

----- Original Message -----
> From: "Rafael Espíndola" <rafael.espindola at gmail.com>
> To: reviews+D13825+public+7254df52e865f21a at reviews.llvm.org
> Cc: "Hal Finkel" <hfinkel at anl.gov>, "Rui Ueyama" <ruiu at google.com>, "llvm-commits" <llvm-commits at lists.llvm.org>,
> "Adhemerval Zanella" <adhemerval.zanella at linaro.org>
> Sent: Friday, October 16, 2015 3:52:12 PM
> Subject: Re: [PATCH] D13825: [ELF2] Don't create RelativeReloc for weak undef symbols
> 
> The test is OK, but would the attached patch fix the problem for you?

Yes, this also seems to work. Would you like me to commit this with the test?

 -Hal

> 
> I think it is a better spot to put the test.
> 
> On 16 October 2015 at 16:24, Rafael Espíndola
> <rafael.espindola at gmail.com> wrote:
> > The test I think is correct. Give me a minute to see if there is a
> > better place in the code to put the check.
> >
> > On 16 October 2015 at 15:30, hfinkel at anl.gov <hfinkel at anl.gov>
> > wrote:
> >> hfinkel created this revision.
> >> hfinkel added reviewers: rafael, ruiu.
> >> hfinkel added a subscriber: llvm-commits.
> >> hfinkel added a project: lld.
> >>
> >> When we have a R_PPC64_ADDR64 for a weak undef symbol, which thus
> >> resolves to 0, and we're creating a shared library, we need to
> >> make sure that it stays 0 (because code that conditionally calls
> >> the weak function tests for this). Unfortunately, we were
> >> creating a R_PPC64_RELATIVE for these relocation targets, making
> >> the address of the undefined weak symbol equal to the base
> >> address of the shared library (which is non-zero). It seems like,
> >> in general, we should not be creating RelativeReloc relocs for
> >> undef weak symbols.
> >>
> >>
> >> http://reviews.llvm.org/D13825
> >>
> >> Files:
> >>   ELF/Writer.cpp
> >>   test/elf2/ppc64-weak-undef-call-shared.s
> >>
> >> Index: test/elf2/ppc64-weak-undef-call-shared.s
> >> ===================================================================
> >> --- /dev/null
> >> +++ test/elf2/ppc64-weak-undef-call-shared.s
> >> @@ -0,0 +1,11 @@
> >> +# RUN: llvm-mc -filetype=obj -triple=powerpc64-unknown-linux %s
> >> -o %t.o
> >> +# RUN: ld.lld2 -shared %t.o -o %t.so
> >> +# RUN: llvm-readobj -t -r -dyn-symbols %t.so | FileCheck %s
> >> +# REQUIRES: ppc
> >> +
> >> +.section        ".toc","aw"
> >> +.quad weakfunc
> >> +// CHECK-NOT: R_PPC64_RELATIVE
> >> +
> >> +.weak weakfunc
> >> +
> >> Index: ELF/Writer.cpp
> >> ===================================================================
> >> --- ELF/Writer.cpp
> >> +++ ELF/Writer.cpp
> >> @@ -201,7 +201,8 @@
> >>      }
> >>
> >>      bool CBP = canBePreempted(Body, NeedsGot);
> >> -    if (!CBP && (!Config->Shared || Target->isRelRelative(Type)))
> >> +    if (!CBP && (!Config->Shared || Target->isRelRelative(Type)
> >> ||
> >> +                 (Body && Body->isWeak() &&
> >> Body->isUndefined())))
> >>        continue;
> >>      if (CBP)
> >>        Body->setUsedInDynamicReloc();
> >>
> >>
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory