[PECOFF] Support COMDAT associative sections.

COFF supports a feature similar to ELF's section groups. This
patch implements it.

In ELF, section groups are identified by their names, and they are
treated somewhat differently from regular symbols. In COFF, the
feature is realized in a more straightforward way. A section can
have an annotation saying "if Nth section is linked, link this
section too."

Implementing such feature is easy. We can add a reference from a
target atom to an original atom, so that if the target is linked,
the original atom is also linked. If not linked, both will be
dead-stripped. So they are treated as a group.

I added a new reference type, kindAssociate. It does nothing except
preventing referenced atoms from being dead-stripped.

No change to the Resolver is needed.

Reviewers: Bigcheese, shankarke, atanasyan

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D3946

llvm-svn: 210240

7 files changed, 146 insertions, 11 deletions

diff --git a/lld/include/lld/Core/Reference.h b/lld/include/lld/Core/Reference.h
index c5580efb39e..ee0d88b9e03 100644
--- a/lld/include/lld/Core/Reference.h
+++ b/lld/include/lld/Core/Reference.h
@@ -91,6 +91,8 @@ public:
     kindLayoutBefore = 3,
     // kindGroupChild is treated as a bidirected edge too.
     kindGroupChild = 4,
+    // kindAssociate prevents a referenced atom from being dead-stripped.
+    kindAssociate = 5,
   };
 
   // A value to be added to the value of a target
diff --git a/lld/lib/ReaderWriter/PECOFF/Atoms.h b/lld/lib/ReaderWriter/PECOFF/Atoms.h
index c4ae1271c0b..7cd74951c0c 100644
--- a/lld/lib/ReaderWriter/PECOFF/Atoms.h
+++ b/lld/lib/ReaderWriter/PECOFF/Atoms.h
@@ -167,6 +167,13 @@ public:
   uint64_t ordinal() const override { return _ordinal; }
   Alignment alignment() const override { return _alignment; }
 
+  void associate(COFFDefinedFileAtom *other) {
+    auto *r = new COFFReference(other, 0, Reference::kindAssociate,
+				Reference::KindNamespace::all,
+				Reference::KindArch::all);
+    addReference(std::unique_ptr<COFFReference>(r));
+  }
+
 private:
   StringRef _sectionName;
   Scope _scope;
diff --git a/lld/lib/ReaderWriter/PECOFF/ReaderCOFF.cpp b/lld/lib/ReaderWriter/PECOFF/ReaderCOFF.cpp
index 47520972bac..069c710ac3e 100644
--- a/lld/lib/ReaderWriter/PECOFF/ReaderCOFF.cpp
+++ b/lld/lib/ReaderWriter/PECOFF/ReaderCOFF.cpp
@@ -172,6 +172,9 @@ private:
   // A map to get whether the section allows its contents to be merged or not.
   std::map<const coff_section *, DefinedAtom::Merge> _merge;
 
+  // COMDAT associative sections
+  std::map<const coff_section *, std::set<const coff_section *>> _association;
+
   // A sorted map to find an atom from a section and an offset within
   // the section.
   std::map<const coff_section *,
@@ -540,6 +543,7 @@ error_code FileCOFF::cacheSectionAttributes() {
   // section. It feels to me that it's unnecessarily complicated, but this is
   // how COFF works.
   for (auto i : _auxSymbol) {
+    // Read a section from the file
     const coff_symbol *sym = i.first;
     if (sym->SectionNumber == llvm::COFF::IMAGE_SYM_ABSOLUTE ||
         sym->SectionNumber == llvm::COFF::IMAGE_SYM_UNDEFINED)
@@ -548,19 +552,22 @@ error_code FileCOFF::cacheSectionAttributes() {
     const coff_section *sec;
     if (error_code ec = _obj->getSection(sym->SectionNumber, sec))
       return ec;
+    const coff_aux_section_definition *aux =
+      reinterpret_cast<const coff_aux_section_definition *>(i.second);
 
-    if (_merge.count(sec))
-      continue;
-    if (!(sec->Characteristics & llvm::COFF::IMAGE_SCN_LNK_COMDAT))
-      continue;
-
-    _comdatSections.insert(sec);
+    if (sec->Characteristics & llvm::COFF::IMAGE_SCN_LNK_COMDAT) {
+      // Read aux symbol data.
+      _comdatSections.insert(sec);
+      _merge[sec] = getMerge(aux);
+    }
 
-    if (sym->NumberOfAuxSymbols == 0)
-      return llvm::object::object_error::parse_failed;
-    const coff_aux_section_definition *aux =
-        reinterpret_cast<const coff_aux_section_definition *>(i.second);
-    _merge[sec] = getMerge(aux);
+    // Handle associative sections.
+    if (aux->Selection == llvm::COFF::IMAGE_COMDAT_SELECT_ASSOCIATIVE) {
+      const coff_section *parent;
+      if (error_code ec = _obj->getSection(aux->Number, parent))
+	return ec;
+      _association[parent].insert(sec);
+    }
   }
 
   // The sections that does not have auxiliary symbol are regular sections, in
@@ -694,6 +701,28 @@ error_code FileCOFF::AtomizeDefinedSymbols(
       definedAtoms.push_back(atom);
     }
   }
+
+  // A COMDAT section with SELECT_ASSOCIATIVE attribute refer to other
+  // section. They need to be linked to a binary or dead stripped as a group.
+  // Here We add a kindAssociate edge between them, so that if a target section
+  // is linked, the other is also linked.
+  //
+  // In a typical use case, parent is a comdat BSS section, and a child is a
+  // static initializer for the data.
+  for (auto i : _association) {
+    const coff_section *parent = i.first;
+    const std::set<const coff_section *> &childSections = i.second;
+    assert(_sectionAtoms[parent].size() > 0);
+
+    COFFDefinedFileAtom *p = _sectionAtoms[parent][0];
+    for (const coff_section *sec : childSections) {
+      if (_sectionAtoms.count(sec)) {
+	assert(_sectionAtoms[sec].size() > 0);
+	p->associate(_sectionAtoms[sec][0]);
+      }
+    }
+  }
+
   return error_code();
 }
 
diff --git a/lld/lib/ReaderWriter/Reader.cpp b/lld/lib/ReaderWriter/Reader.cpp
index ce81c69186e..644acd7fe96 100644
--- a/lld/lib/ReaderWriter/Reader.cpp
+++ b/lld/lib/ReaderWriter/Reader.cpp
@@ -52,6 +52,7 @@ static const Registry::KindStrings kindStrings[] = {
     {Reference::kindLayoutAfter, "layout-after"},
     {Reference::kindLayoutBefore, "layout-before"},
     {Reference::kindGroupChild, "group-child"},
+    {Reference::kindAssociate, "associate"},
     LLD_KIND_STRING_END};
 
 Registry::Registry() {
diff --git a/lld/test/pecoff/Inputs/associative1.obj.yaml b/lld/test/pecoff/Inputs/associative1.obj.yaml
new file mode 100644
index 00000000000..51a7be0b34d
--- /dev/null
+++ b/lld/test/pecoff/Inputs/associative1.obj.yaml
@@ -0,0 +1,53 @@
+---
+header:
+  Machine:         IMAGE_FILE_MACHINE_I386
+  Characteristics: []
+sections:
+  - Name:            .data
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_LNK_COMDAT, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
+    Alignment:       4
+    SectionData:     00000000
+  - Name:            '.CRT$XCU'
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_LNK_COMDAT, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    Alignment:       4
+    SectionData:     77777777
+symbols:
+  - Name:            .data
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          4
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          0
+      Selection:       IMAGE_COMDAT_SELECT_ANY
+  - Name:            _var
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+  - Name:            '.CRT$XCU'
+    Value:           0
+    SectionNumber:   2
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          4
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          1
+      Selection:       IMAGE_COMDAT_SELECT_ASSOCIATIVE
+  - Name:            _init
+    Value:           0
+    SectionNumber:   2
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...
diff --git a/lld/test/pecoff/Inputs/associative3.obj.yaml b/lld/test/pecoff/Inputs/associative3.obj.yaml
new file mode 100644
index 00000000000..ffea761a357
--- /dev/null
+++ b/lld/test/pecoff/Inputs/associative3.obj.yaml
@@ -0,0 +1,33 @@
+---
+header:
+  Machine:         IMAGE_FILE_MACHINE_I386
+  Characteristics: []
+sections:
+  - Name:            .text
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    Alignment:       4
+    SectionData:     0000000000000000
+    Relocations:
+      - VirtualAddress:  4
+        SymbolName:      _var
+        Type:            IMAGE_REL_I386_DIR32
+symbols:
+  - Name:            .text
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+  - Name:            _main
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+  - Name:            _var
+    Value:           0
+    SectionNumber:   0
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...
diff --git a/lld/test/pecoff/associative.test b/lld/test/pecoff/associative.test
new file mode 100644
index 00000000000..f998befd007
--- /dev/null
+++ b/lld/test/pecoff/associative.test
@@ -0,0 +1,10 @@
+# RUN: yaml2obj %p/Inputs/associative1.obj.yaml > %t1.obj
+# RUN: yaml2obj %p/Inputs/associative1.obj.yaml > %t2.obj
+# RUN: yaml2obj %p/Inputs/associative3.obj.yaml > %t3.obj
+#
+# RUN: lld -flavor link /machine:x86 /subsystem:console /entry:main \
+# RUN:   /out:%t.exe -- %t1.obj %t2.obj %t3.obj
+# RUN: obj2yaml %t.exe | FileCheck %s
+
+CHECK: - Name: .CRT
+CHECK:   SectionData: '77777777'