[PATCH] D28430: ELF: Implement support for .gnu.linkonce.t.*.

[Peter Collingbourne via Phabricator via llvm-commits]

pcc created this revision.
pcc added reviewers: rafael, ruiu.
pcc added subscribers: llvm-commits, eugenis, grimar.

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.


https://reviews.llvm.org/D28430

Files:
  lld/ELF/InputFiles.cpp
  lld/ELF/InputFiles.h
  lld/test/ELF/comdat-linkonce.s


Index: lld/test/ELF/comdat-linkonce.s
===================================================================
--- /dev/null
+++ lld/test/ELF/comdat-linkonce.s
@@ -0,0 +1,9 @@
+// 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
Index: lld/ELF/InputFiles.h
===================================================================
--- lld/ELF/InputFiles.h
+++ lld/ELF/InputFiles.h
@@ -184,8 +184,9 @@
   void initializeSymbols();
   void initializeDwarfLine();
   InputSectionBase<ELFT> *getRelocTarget(const Elf_Shdr &Sec);
-  InputSectionBase<ELFT> *createInputSection(const Elf_Shdr &Sec,
-                                             StringRef SectionStringTable);
+  InputSectionBase<ELFT> *
+  createInputSection(const Elf_Shdr &Sec, StringRef SectionStringTable,
+                     llvm::DenseSet<llvm::CachedHashStringRef> &ComdatGroups);
 
   bool shouldMerge(const Elf_Shdr &Sec);
   SymbolBody *createSymbolBody(const Elf_Sym *Sym);
Index: lld/ELF/InputFiles.cpp
===================================================================
--- lld/ELF/InputFiles.cpp
+++ lld/ELF/InputFiles.cpp
@@ -294,7 +294,7 @@
     case SHT_NULL:
       break;
     default:
-      Sections[I] = createInputSection(Sec, SectionStringTable);
+      Sections[I] = createInputSection(Sec, SectionStringTable, ComdatGroups);
     }
 
     // .ARM.exidx sections have a reverse dependency on the InputSection they
@@ -329,12 +329,11 @@
 }
 
 template <class ELFT>
-InputSectionBase<ELFT> *
-elf::ObjectFile<ELFT>::createInputSection(const Elf_Shdr &Sec,
-                                          StringRef SectionStringTable) {
+InputSectionBase<ELFT> *elf::ObjectFile<ELFT>::createInputSection(
+    const Elf_Shdr &Sec, StringRef SectionStringTable,
+    DenseSet<CachedHashStringRef> &ComdatGroups) {
   StringRef Name =
       check(this->getObj().getSectionName(&Sec, SectionStringTable));
-
   switch (Sec.sh_type) {
   case SHT_ARM_ATTRIBUTES:
     // FIXME: ARM meta-data section. Retain the first attribute section
@@ -399,6 +398,15 @@
   if (Config->Strip != StripPolicy::None && Name.startswith(".debug"))
     return &InputSection<ELFT>::Discarded;
 
+  // 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.
+  if (Name.startswith(".gnu.linkonce.t.") &&
+      !ComdatGroups.insert(CachedHashStringRef(Name.substr(16))).second)
+    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.


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