[PATCH] D65755: [ELF][PPC] Don't relax ifunc toc-indirect accesses to toc-relative

[Fangrui Song via Phabricator via llvm-commits]

MaskRay created this revision.
MaskRay added reviewers: luporl, ruiu, sfertile.
Herald added subscribers: llvm-commits, jsji, kbarton, arichardson, nemanjai, emaste.
Herald added a reviewer: espindola.
Herald added a project: LLVM.

Fixes PR42759.

  // If ifunc is taken address in -fPIC code, it may have a toc entry
  .section .toc,"aw", at progbits
    .quad ifunc
  
  // ifunc may be defined as STT_GNU_IFUNC in another object file
  .type ifunc, %gnu_indirect_function

If ifunc is non-preemptable (e.g. when linking an executable), the toc
entry will be relocated by R_PPC64_IRELATIVE.

R_*_IRELATIVE is used to represent the symbolic value of a
non-preemptable ifunc in a writable location. It has an unknown value at
link time, so we cannot apply toc-indirect to toc-relative relaxation.


Repository:
  rLLD LLVM Linker

https://reviews.llvm.org/D65755

Files:
  ELF/Arch/PPC64.cpp
  test/ELF/ppc64-toc-relax-ifunc.s


Index: test/ELF/ppc64-toc-relax-ifunc.s
===================================================================
--- /dev/null
+++ test/ELF/ppc64-toc-relax-ifunc.s
@@ -0,0 +1,19 @@
+# REQUIRES: ppc
+
+# RUN: llvm-mc -filetype=obj -triple=powerpc64le %s -o %t.o
+# RUN: echo '.globl ifunc; .type ifunc, %gnu_indirect_function; ifunc:' | \
+# RUN:   llvm-mc -filetype=obj -triple=powerpc64le - -o %t1.o
+# RUN: ld.lld %t.o %t1.o -o %t
+
+## ifunc is a non-preemptable STT_GNU_IFUNC. Its toc entry will be
+## relocated by R_PPC64_IRELATIVE, not representable by a toc-relative value.
+## Check the toc-indirect access is not relaxed.
+
+# CHECK:      nop
+# CHECK-NEXT: ld 3, -32768(2)
+
+addis 3, 2, .toc at toc@ha
+ld 3, .toc at toc@l(3)
+
+.section .toc,"aw", at progbits
+  .quad ifunc
Index: ELF/Arch/PPC64.cpp
===================================================================
--- ELF/Arch/PPC64.cpp
+++ ELF/Arch/PPC64.cpp
@@ -172,7 +172,10 @@
                    : getRelaTocSymAndAddend<ELF64BE>(tocISB, rel.addend);
 
   // Only non-preemptable defined symbols can be relaxed.
-  if (!d || d->isPreemptible)
+  //
+  // The toc entry of a non-preemptable ifunc is relocated by R_PPC64_IRELATIVE,
+  // not representable as a toc-relative value. Exclude ifunc.
+  if (!d || d->isPreemptible || d->isGnuIFunc())
     return false;
 
   // Two instructions can materialize a 32-bit signed offset from the toc base.


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