[PATCH] D63182: [ELF][PPC64] Don't report "relocation refers to a discarded section" for .toc

[Fangrui Song via Phabricator via llvm-commits]

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

clang/gcc PPC64 may emit a .rela.toc which references an embedded switch table in
a discarded .rodata/.text section. The .toc and the .rela.toc are incorrectly not
placed in the comdat.

Technically a relocation from outside the group is not allowed by the ELF spec:

> A symbol table entry with STB_LOCAL binding that is defined relative
>  to one of a group's sections, and that is contained in a symbol table
>  section that is not part of the group, must be discarded if the group
>  members are discarded. References to this symbol table entry from
>  outside the group are not allowed.

Don't report errors to work around the bug.

This should fix the ppc64le-lld-multistage-test bot. Some other PPC
specific sections may have similar problems. We can blacklist more
section names when problems occur.


Repository:
  rLLD LLVM Linker

https://reviews.llvm.org/D63182

Files:
  ELF/InputSection.cpp
  ELF/Relocations.cpp
  test/ELF/comdat-discarded-ppc64.s


Index: test/ELF/comdat-discarded-ppc64.s
===================================================================
--- /dev/null
+++ test/ELF/comdat-discarded-ppc64.s
@@ -0,0 +1,16 @@
+# REQUIRES: ppc
+# RUN: llvm-mc -filetype=obj -triple=powerpc64le %s -o %t.o
+# RUN: ld.lld %t.o -o /dev/null
+
+## clang/gcc PPC64 may emit a .rela.toc which references a switch table in a
+## discarded .rodata/.text section. The .toc and the .rela.toc are incorrectly
+## not placed in the comdat.
+## Don't error "relocation refers to a discarded section".
+
+.section .text.foo,"axG", at progbits,foo,comdat
+.globl foo
+foo:
+.L0:
+
+.section .toc,"aw"
+.quad .L0
Index: ELF/Relocations.cpp
===================================================================
--- ELF/Relocations.cpp
+++ ELF/Relocations.cpp
@@ -718,6 +718,13 @@
   if (Config->UnresolvedSymbols == UnresolvedPolicy::Ignore && CanBeExternal)
     return false;
 
+  // clang/gcc PPC64 may emit a .rela.toc which references a switch table in a
+  // discarded .rodata/.text section. The .toc and the .rela.toc are incorrectly
+  // not placed in the comdat. Work around the bug.
+  if (Config->EMachine == EM_PPC64 &&
+      cast<Undefined>(Sym).DiscardedSecIdx != 0 && Sec.Name == ".toc")
+    return false;
+
   auto Visibility = [&]() -> std::string {
     switch (Sym.Visibility) {
     case STV_INTERNAL:
Index: ELF/InputSection.cpp
===================================================================
--- ELF/InputSection.cpp
+++ ELF/InputSection.cpp
@@ -442,7 +442,7 @@
       if (!D) {
         if (!Sec->Name.startswith(".debug") &&
             !Sec->Name.startswith(".zdebug") && Sec->Name != ".eh_frame" &&
-            Sec->Name != ".gcc_except_table") {
+            Sec->Name != ".gcc_except_table" && Sec->Name != ".toc") {
           uint32_t SecIdx = cast<Undefined>(Sym).DiscardedSecIdx;
           Elf_Shdr_Impl<ELFT> Sec =
               CHECK(File->getObj().sections(), File)[SecIdx];


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