ELF: Implement support for .gnu.linkonce.t.*.

[pcc]

Summary

The linkonce feature is a sort of proto-comdat. bfd and gold appear to
treat linkonce sections as if they were single-element comdat groups named
after the section name suffix (which means they can cause comdat groups to
be discarded), and this feature is necessary to link parts of the x86-32
CRT on Linux.

We only need support for .gnu.linkonce.t.* for the CRT (and I cannot find
any other uses of linkonce in the x86-32 prebuilt libraries on my system),
so this patch does only that.

Fixes PR31215.

[pcc]

• Simpler fix

lld/trunk/ELF/InputFiles.cpp

elf::ObjectFile<ELFT>::createInputSection(const Elf_Shdr &Sec,
   if (Name == ".note.GNU-split-stack") {
     error("objects using splitstacks are not supported");
     return &InputSection<ELFT>::Discarded;
   }
 
   if (Config->Strip != StripPolicy::None && Name.startswith(".debug"))
     return &InputSection<ELFT>::Discarded;
 
+  // The linkonce feature is a sort of proto-comdat. Some glibc i386 object
+  // files contain definitions of symbol "__x86.get_pc_thunk.bx" in linkonce
+  // sections. Drop those sections to avoid duplicate symbol errors.
+  // FIXME: This is glibc PR20543, we should remove this hack once that has been
+  // fixed for a while.
+  if (Name.startswith(".gnu.linkonce."))
+    return &InputSection<ELFT>::Discarded;
+
   // The linker merges EH (exception handling) frames and creates a
   // .eh_frame_hdr section for runtime. So we handle them with a special
   // class. For relocatable outputs, they are just passed through.
   if (Name == ".eh_frame" && !Config->Relocatable)
     return make<EhInputSection<ELFT>>(this, &Sec, Name);
 
   if (shouldMerge(Sec))
     return make<MergeInputSection<ELFT>>(this, &Sec, Name);

lld/trunk/test/ELF/comdat-linkonce.s

+// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %p/Inputs/comdat.s -o %t2.o
+// RUN: ld.lld -shared %t.o %t2.o -o %t
+// RUN: ld.lld -shared %t2.o %t.o -o %t
+
+.section .gnu.linkonce.t.zed
+.globl abc
+abc:
+nop