[PATCH] D36573: [ELF] - Treat .gnu.linkonce sections as COMDAT

[George Rimar via Phabricator via llvm-commits]

grimar created this revision.
Herald added a subscriber: emaste.

This is request from PR31586 comment.

.gnu.linkonce were invented before regular COMDAT sections
and should work in the same way (http://www.airs.com/blog/archives/52).

Previously we dropped such sections.
Patch makes them COMDATS.

It based on logic of https://reviews.llvm.org/D28430?id=83485 which
was changed in later diff.


https://reviews.llvm.org/D36573

Files:
  ELF/InputFiles.cpp
  ELF/InputFiles.h
  test/ELF/comdat-linkonce2.s


Index: test/ELF/comdat-linkonce2.s
===================================================================
--- test/ELF/comdat-linkonce2.s
+++ test/ELF/comdat-linkonce2.s
@@ -0,0 +1,11 @@
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: ld.lld %t.o -o %t
+
+.section .gnu.linkonce.t.zed
+.globl zed
+zed:
+
+.text
+.globl _start
+_start:
+ .quad zed
Index: ELF/InputFiles.h
===================================================================
--- ELF/InputFiles.h
+++ ELF/InputFiles.h
@@ -206,7 +206,9 @@
   void initializeSymbols();
   void initializeDwarfLine();
   InputSectionBase *getRelocTarget(const Elf_Shdr &Sec);
-  InputSectionBase *createInputSection(const Elf_Shdr &Sec);
+  InputSectionBase *
+  createInputSection(llvm::DenseSet<llvm::CachedHashStringRef> &ComdatGroups,
+                     const Elf_Shdr &Sec);
   StringRef getSectionName(const Elf_Shdr &Sec);
 
   bool shouldMerge(const Elf_Shdr &Sec);
Index: ELF/InputFiles.cpp
===================================================================
--- ELF/InputFiles.cpp
+++ ELF/InputFiles.cpp
@@ -305,7 +305,7 @@
       // object files, we want to pass through basically everything.
       if (IsNew) {
         if (Config->Relocatable)
-          this->Sections[I] = createInputSection(Sec);
+          this->Sections[I] = createInputSection(ComdatGroups, Sec);
         continue;
       }
 
@@ -329,7 +329,7 @@
     case SHT_NULL:
       break;
     default:
-      this->Sections[I] = createInputSection(Sec);
+      this->Sections[I] = createInputSection(ComdatGroups, Sec);
     }
 
     // .ARM.exidx sections have a reverse dependency on the InputSection they
@@ -372,7 +372,9 @@
 }
 
 template <class ELFT>
-InputSectionBase *ObjFile<ELFT>::createInputSection(const Elf_Shdr &Sec) {
+InputSectionBase *
+ObjFile<ELFT>::createInputSection(DenseSet<CachedHashStringRef> &ComdatGroups,
+                                  const Elf_Shdr &Sec) {
   StringRef Name = getSectionName(Sec);
 
   switch (Sec.sh_type) {
@@ -493,10 +495,15 @@
 
   // 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.
+  // sections. Section usually named as '.gnu.linkonce.TYPE.NAME', where NAME
+  // is a symbol name this section contains and also a name of comdat group.
+  // Usually NAME does not contain dots ('.'), but case above is special.
+  // Since this is temporary hack, we handle only ".gnu.linkonce.t.*" sections
+  // as minimal case for simplicity.
   // FIXME: This is glibc PR20543, we should remove this hack once that has been
   // fixed for a while.
-  if (Name.startswith(".gnu.linkonce."))
+  if (Name.startswith(".gnu.linkonce.t.") &&
+      !ComdatGroups.insert(CachedHashStringRef(Name.drop_front(16))).second)
     return &InputSection::Discarded;
 
   // The linker merges EH (exception handling) frames and creates a


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