Revert r303304: Re-submit r303225: Garbage collect dllimported symbols.

This reverts commit r303304 because it looks like the change
introduced a crash bug. At least after that change, LLD with thinlto
crashes when linking Chromium.

llvm-svn: 303527

7 files changed, 27 insertions, 147 deletions

diff --git a/lld/COFF/InputFiles.cpp b/lld/COFF/InputFiles.cpp
index 2371616c850..6e6465cd5d6 100644
--- a/lld/COFF/InputFiles.cpp
+++ b/lld/COFF/InputFiles.cpp
@@ -325,21 +325,12 @@ void ImportFile::parse() {
   this->Hdr = Hdr;
   ExternalName = ExtName;
 
-  // Instantiate symbol objects.
   ImpSym = cast<DefinedImportData>(
       Symtab->addImportData(ImpName, this)->body());
-
-  if (Hdr->getType() == llvm::COFF::IMPORT_CONST) {
+  if (Hdr->getType() == llvm::COFF::IMPORT_CONST)
     ConstSym =
         cast<DefinedImportData>(Symtab->addImportData(Name, this)->body());
 
-    // A __imp_ and non-__imp_ symbols for the same dllimport'ed symbol
-    // should be gc'ed as a group. Add a bidirectional edge.
-    // Used by MarkLive.cpp.
-    ImpSym->Sibling = ConstSym;
-    ConstSym->Sibling = ImpSym;
-  }
-
   // If type is function, we need to create a thunk which jump to an
   // address pointed by the __imp_ symbol. (This allows you to call
   // DLL functions just like regular non-DLL functions.)
diff --git a/lld/COFF/MarkLive.cpp b/lld/COFF/MarkLive.cpp
index 4c53c9a9885..0156d238b67 100644
--- a/lld/COFF/MarkLive.cpp
+++ b/lld/COFF/MarkLive.cpp
@@ -15,16 +15,16 @@
 namespace lld {
 namespace coff {
 
-// Set live bit for each reachable COMDAT chunk and dllimported symbol.
-// Unmarked (or unreachable) chunks or symbols are ignored by Writer,
-// so they are not included in the final output.
+// Set live bit on for each reachable chunk. Unmarked (unreachable)
+// COMDAT chunks will be ignored by Writer, so they will be excluded
+// from the final output.
 void markLive(const std::vector<Chunk *> &Chunks) {
   // We build up a worklist of sections which have been marked as live. We only
   // push into the worklist when we discover an unmarked section, and we mark
   // as we push, so sections never appear twice in the list.
   SmallVector<SectionChunk *, 256> Worklist;
 
-  // Non-COMDAT chunks are never be gc'ed, so they are gc-root.
+  // COMDAT section chunks are dead by default. Add non-COMDAT chunks.
   for (Chunk *C : Chunks)
     if (auto *SC = dyn_cast<SectionChunk>(C))
       if (SC->isLive())
@@ -37,37 +37,19 @@ void markLive(const std::vector<Chunk *> &Chunks) {
     Worklist.push_back(C);
   };
 
-  // Mark a given symbol as reachable.
-  std::function<void(SymbolBody * B)> AddSym = [&](SymbolBody *B) {
-    if (auto *Sym = dyn_cast<DefinedRegular>(B)) {
-      Enqueue(Sym->getChunk());
-    } else if (auto *Sym = dyn_cast<DefinedImportData>(B)) {
-      if (Sym->Live)
-        return;
-      Sym->Live = true;
-      if (Sym->Sibling)
-        Sym->Sibling->Live = true;
-    } else if (auto *Sym = dyn_cast<DefinedImportThunk>(B)) {
-      if (Sym->Live)
-        return;
-      Sym->Live = true;
-      AddSym(Sym->WrappedSym);
-    } else if (auto *Sym = dyn_cast<DefinedLocalImport>(B)) {
-      AddSym(Sym->WrappedSym);
-    }
-  };
-
-  // Add gc-root symbols.
+  // Add GC root chunks.
   for (SymbolBody *B : Config->GCRoot)
-    AddSym(B);
+    if (auto *D = dyn_cast<DefinedRegular>(B))
+      Enqueue(D->getChunk());
 
   while (!Worklist.empty()) {
     SectionChunk *SC = Worklist.pop_back_val();
     assert(SC->isLive() && "We mark as live when pushing onto the worklist!");
 
     // Mark all symbols listed in the relocation table for this section.
-    for (SymbolBody *B : SC->symbols())
-      AddSym(B);
+    for (SymbolBody *S : SC->symbols())
+      if (auto *D = dyn_cast<DefinedRegular>(S))
+        Enqueue(D->getChunk());
 
     // Mark associative sections if any.
     for (SectionChunk *C : SC->children())
diff --git a/lld/COFF/Symbols.cpp b/lld/COFF/Symbols.cpp
index f63c381ca34..993e920ce7f 100644
--- a/lld/COFF/Symbols.cpp
+++ b/lld/COFF/Symbols.cpp
@@ -63,8 +63,7 @@ COFFSymbolRef DefinedCOFF::getCOFFSymbol() {
 
 DefinedImportThunk::DefinedImportThunk(StringRef Name, DefinedImportData *S,
                                        uint16_t Machine)
-    : Defined(DefinedImportThunkKind, Name), Live(!Config->DoGC),
-      WrappedSym(S) {
+    : Defined(DefinedImportThunkKind, Name) {
   switch (Machine) {
   case AMD64: Data = make<ImportThunkChunkX64>(S); return;
   case I386:  Data = make<ImportThunkChunkX86>(S); return;
diff --git a/lld/COFF/Symbols.h b/lld/COFF/Symbols.h
index 141b40f2d0e..1b83f73ff20 100644
--- a/lld/COFF/Symbols.h
+++ b/lld/COFF/Symbols.h
@@ -287,29 +287,19 @@ public:
 class DefinedImportData : public Defined {
 public:
   DefinedImportData(StringRef N, ImportFile *F)
-      : Defined(DefinedImportDataKind, N), Live(!Config->DoGC), File(F) {}
+      : Defined(DefinedImportDataKind, N), File(F) {
+  }
 
   static bool classof(const SymbolBody *S) {
     return S->kind() == DefinedImportDataKind;
   }
 
   uint64_t getRVA() { return File->Location->getRVA(); }
-
   StringRef getDLLName() { return File->DLLName; }
   StringRef getExternalName() { return File->ExternalName; }
   void setLocation(Chunk *AddressTable) { File->Location = AddressTable; }
   uint16_t getOrdinal() { return File->Hdr->OrdinalHint; }
 
-  // If all sections referring a dllimported symbol become dead by gc,
-  // we want to kill the symbol as well, so that a resulting binary has
-  // fewer number of dependencies to DLLs. "Live" bit manages reachability.
-  bool Live;
-
-  // For a dllimported data symbol, we create two symbols.
-  // They should be considered as a unit by gc. This pointer points
-  // to the other symbol.
-  DefinedImportData *Sibling = nullptr;
-
 private:
   ImportFile *File;
 };
@@ -330,10 +320,6 @@ public:
   uint64_t getRVA() { return Data->getRVA(); }
   Chunk *getChunk() { return Data; }
 
-  // For GC.
-  bool Live;
-  DefinedImportData *WrappedSym;
-
 private:
   Chunk *Data;
 };
@@ -346,8 +332,7 @@ private:
 class DefinedLocalImport : public Defined {
 public:
   DefinedLocalImport(StringRef N, Defined *S)
-      : Defined(DefinedLocalImportKind, N), WrappedSym(S),
-        Data(make<LocalImportChunk>(S)) {}
+      : Defined(DefinedLocalImportKind, N), Data(make<LocalImportChunk>(S)) {}
 
   static bool classof(const SymbolBody *S) {
     return S->kind() == DefinedLocalImportKind;
@@ -356,9 +341,6 @@ public:
   uint64_t getRVA() { return Data->getRVA(); }
   Chunk *getChunk() { return Data; }
 
-  // For GC.
-  Defined *WrappedSym;
-
 private:
   LocalImportChunk *Data;
 };
diff --git a/lld/COFF/Writer.cpp b/lld/COFF/Writer.cpp
index 5cadb739cc4..cf3ad7ef045 100644
--- a/lld/COFF/Writer.cpp
+++ b/lld/COFF/Writer.cpp
@@ -373,24 +373,18 @@ void Writer::createImportTables() {
   OutputSection *Text = createSection(".text");
   for (ImportFile *File : Symtab->ImportFiles) {
     if (DefinedImportThunk *Thunk = File->ThunkSym)
-      if (Thunk->Live)
-        Text->addChunk(Thunk->getChunk());
-
+      Text->addChunk(Thunk->getChunk());
     if (Config->DelayLoads.count(StringRef(File->DLLName).lower())) {
-      if (File->ImpSym->Live)
-        DelayIdata.add(File->ImpSym);
+      DelayIdata.add(File->ImpSym);
     } else {
-      if (File->ImpSym->Live)
-        Idata.add(File->ImpSym);
+      Idata.add(File->ImpSym);
     }
   }
-
   if (!Idata.empty()) {
     OutputSection *Sec = createSection(".idata");
     for (Chunk *C : Idata.getChunks())
       Sec->addChunk(C);
   }
-
   if (!DelayIdata.empty()) {
     Defined *Helper = cast<Defined>(Config->DelayLoadHelper);
     DelayIdata.create(Helper);
@@ -497,24 +491,14 @@ void Writer::createSymbolAndStringTable() {
     Sec->setStringTableOff(addEntryToStringTable(Name));
   }
 
-  for (lld::coff::ObjectFile *File : Symtab->ObjectFiles) {
-    for (SymbolBody *B : File->getSymbols()) {
-      auto *D = dyn_cast<Defined>(B);
-      if (!D || D->WrittenToSymtab)
-        continue;
-
-      if (auto *S = dyn_cast<DefinedImportData>(D))
-        if (!S->Live)
-          continue;
-      if (auto *S = dyn_cast<DefinedImportThunk>(D))
-        if (!S->Live)
-          continue;
-
-      D->WrittenToSymtab = true;
-      if (Optional<coff_symbol16> Sym = createSymbol(D))
-        OutputSymtab.push_back(*Sym);
-    }
-  }
+  for (lld::coff::ObjectFile *File : Symtab->ObjectFiles)
+    for (SymbolBody *B : File->getSymbols())
+      if (auto *D = dyn_cast<Defined>(B))
+        if (!D->WrittenToSymtab) {
+          D->WrittenToSymtab = true;
+          if (Optional<coff_symbol16> Sym = createSymbol(D))
+            OutputSymtab.push_back(*Sym);
+        }
 
   OutputSection *LastSection = OutputSections.back();
   // We position the symbol table to be adjacent to the end of the last section.
diff --git a/lld/test/COFF/Inputs/import.yaml b/lld/test/COFF/Inputs/import.yaml
index b7ae026de29..493400143d6 100644
--- a/lld/test/COFF/Inputs/import.yaml
+++ b/lld/test/COFF/Inputs/import.yaml
@@ -7,13 +7,6 @@ sections:
     Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
     Alignment:       4
     SectionData:     0000000000000000
-    Relocations:
-      - VirtualAddress:  0
-        SymbolName:      exportfn1
-        Type:            IMAGE_REL_AMD64_ADDR32NB
-      - VirtualAddress:  4
-        SymbolName:      exportfn2
-        Type:            IMAGE_REL_AMD64_ADDR32NB
 symbols:
   - Name:            .text
     Value:           0
@@ -23,7 +16,7 @@ symbols:
     StorageClass:    IMAGE_SYM_CLASS_STATIC
     SectionDefinition:
       Length:          8
-      NumberOfRelocations: 2
+      NumberOfRelocations: 0
       NumberOfLinenumbers: 0
       CheckSum:        0
       Number:          0
diff --git a/lld/test/COFF/dllimport-gc.test b/lld/test/COFF/dllimport-gc.test
deleted file mode 100644
index acc604a9de1..00000000000
--- a/lld/test/COFF/dllimport-gc.test
+++ /dev/null
@@ -1,51 +0,0 @@
-# REQUIRES: winres
-
-# RUN: yaml2obj < %p/Inputs/export.yaml > %t-lib.obj
-# RUN: lld-link /out:%t.dll /dll %t-lib.obj /implib:%t.lib /export:exportfn1
-
-# RUN: yaml2obj < %s > %t.obj
-
-# RUN: lld-link /out:%t1.exe /entry:main %t.obj %t.lib
-# RUN: llvm-readobj -coff-imports %t1.exe | FileCheck -check-prefix=REF %s
-# REF-NOT: Symbol: exportfn1
-
-# RUN: lld-link /out:%t2.exe /entry:main %t.obj %t.lib /opt:noref
-# RUN: llvm-readobj -coff-imports %t2.exe | FileCheck -check-prefix=NOREF %s
-# NOREF: Symbol: exportfn1
-
---- !COFF
-header:
-  Machine:         IMAGE_FILE_MACHINE_AMD64
-  Characteristics: []
-sections:
-  - Name:            .text
-    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
-    Alignment:       4
-    SectionData:     0000000000000000
-    Relocations:
-symbols:
-  - Name:            .text
-    Value:           0
-    SectionNumber:   1
-    SimpleType:      IMAGE_SYM_TYPE_NULL
-    ComplexType:     IMAGE_SYM_DTYPE_NULL
-    StorageClass:    IMAGE_SYM_CLASS_STATIC
-    SectionDefinition:
-      Length:          8
-      NumberOfRelocations: 2
-      NumberOfLinenumbers: 0
-      CheckSum:        0
-      Number:          0
-  - Name:            main
-    Value:           0
-    SectionNumber:   1
-    SimpleType:      IMAGE_SYM_TYPE_NULL
-    ComplexType:     IMAGE_SYM_DTYPE_FUNCTION
-    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
-  - Name:            exportfn1
-    Value:           0
-    SectionNumber:   0
-    SimpleType:      IMAGE_SYM_TYPE_NULL
-    ComplexType:     IMAGE_SYM_DTYPE_NULL
-    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
-...