Garbage collect dllimported symbols.

This is a different implementation than r303225 (which was reverted
in r303270, re-submitted in r303304 and then re-reverted in r303527).

In the previous patch, I tried to add Live bit to each dllimported
symbol. It turned out that it didn't work with "oldnames.lib" which
contains a lot of weak aliases to dllimported symbols.

The way we handle weak aliases is to check if undefined symbols
can be resolved using weak aliases, and if so, memcpy the Defined
symbols to weak Undefined symbols, so that any references to weak
aliases automatically see defined symbols instead of undefined ones.

This memcpy happens before MarkLive kicks in.

That means we may have multiple copies of dllimported symbols. So
turning on one instance's Live bit is not enough.

This patch moves the Live bit to dllimport file. Since multiple
copies of dllsymbols still point to the same file, we can use it as the
central repository to keep track of liveness.

Differential Revision: https://reviews.llvm.org/D33520

llvm-svn: 303814

lld/COFF/InputFiles.h

@@ -10,6 +10,7 @@
 #ifndef LLD_COFF_INPUT_FILES_H
 #define LLD_COFF_INPUT_FILES_H
 
+#include "Config.h"
 #include "lld/Core/LLVM.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseSet.h"
@@ -161,7 +162,9 @@ private:
 // for details about the format.
 class ImportFile : public InputFile {
 public:
-  explicit ImportFile(MemoryBufferRef M) : InputFile(ImportKind, M) {}
+  explicit ImportFile(MemoryBufferRef M)
+      : InputFile(ImportKind, M), Live(!Config->DoGC) {}
+
   static bool classof(const InputFile *F) { return F->kind() == ImportKind; }
 
   DefinedImportData *ImpSym = nullptr;
@@ -176,6 +179,14 @@ public:
   StringRef ExternalName;
   const coff_import_header *Hdr;
   Chunk *Location = nullptr;
+
+  // We want to eliminate dllimported symbols if no one actually refers them.
+  // This "Live" bit is used to keep track of which import library members
+  // are actually in use.
+  //
+  // If the Live bit is turned off by MarkLive, Writer will ignore dllimported
+  // symbols provided by this import library member.
+  bool Live;
 };
 
 // Used for LTO.

lld/COFF/MarkLive.cpp

@@ -37,19 +37,26 @@ void markLive(const std::vector<Chunk *> &Chunks) {
     Worklist.push_back(C);
   };
 
+  auto AddSym = [&](SymbolBody *B) {
+    if (auto *Sym = dyn_cast<DefinedRegular>(B))
+      Enqueue(Sym->getChunk());
+    else if (auto *Sym = dyn_cast<DefinedImportData>(B))
+      Sym->File->Live = true;
+    else if (auto *Sym = dyn_cast<DefinedImportThunk>(B))
+      Sym->WrappedSym->File->Live = true;
+  };
+
   // Add GC root chunks.
   for (SymbolBody *B : Config->GCRoot)
-    if (auto *D = dyn_cast<DefinedRegular>(B))
-      Enqueue(D->getChunk());
+    AddSym(B);
 
   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 *S : SC->symbols())
-      if (auto *D = dyn_cast<DefinedRegular>(S))
-        Enqueue(D->getChunk());
+    for (SymbolBody *B : SC->symbols())
+      AddSym(B);
 
     // Mark associative sections if any.
     for (SectionChunk *C : SC->children())

lld/COFF/Symbols.cpp

@@ -72,7 +72,7 @@ static Chunk *makeImportThunk(DefinedImportData *S, uint16_t Machine) {
 
 DefinedImportThunk::DefinedImportThunk(StringRef Name, DefinedImportData *S,
                                        uint16_t Machine)
-    : Defined(DefinedImportThunkKind, Name),
+    : Defined(DefinedImportThunkKind, Name), WrappedSym(S),
       Data(makeImportThunk(S, Machine)) {}
 
 Defined *Undefined::getWeakAlias() {

lld/COFF/Symbols.h

@@ -300,7 +300,6 @@ public:
   void setLocation(Chunk *AddressTable) { File->Location = AddressTable; }
   uint16_t getOrdinal() { return File->Hdr->OrdinalHint; }
 
-private:
   ImportFile *File;
 };
 
@@ -320,6 +319,8 @@ public:
   uint64_t getRVA() { return Data->getRVA(); }
   Chunk *getChunk() { return Data; }
 
+  DefinedImportData *WrappedSym;
+
 private:
   Chunk *Data;
 };

lld/COFF/Writer.cpp

@@ -365,6 +365,9 @@ void Writer::createImportTables() {
   // the same order as in the command line. (That affects DLL
   // initialization order, and this ordering is MSVC-compatible.)
   for (ImportFile *File : Symtab->ImportFiles) {
+    if (!File->Live)
+      continue;
+
     std::string DLL = StringRef(File->DLLName).lower();
     if (Config->DLLOrder.count(DLL) == 0)
       Config->DLLOrder[DLL] = Config->DLLOrder.size();
@@ -372,19 +375,25 @@ void Writer::createImportTables() {
 
   OutputSection *Text = createSection(".text");
   for (ImportFile *File : Symtab->ImportFiles) {
+    if (!File->Live)
+      continue;
+
     if (DefinedImportThunk *Thunk = File->ThunkSym)
       Text->addChunk(Thunk->getChunk());
+
     if (Config->DelayLoads.count(StringRef(File->DLLName).lower())) {
       DelayIdata.add(File->ImpSym);
     } else {
       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);
@@ -437,6 +446,14 @@ Optional<coff_symbol16> Writer::createSymbol(Defined *Def) {
     if (!D->getChunk()->isLive())
       return None;
 
+  if (auto *Sym = dyn_cast<DefinedImportData>(Def))
+    if (!Sym->File->Live)
+      return None;
+
+  if (auto *Sym = dyn_cast<DefinedImportThunk>(Def))
+    if (!Sym->WrappedSym->File->Live)
+      return None;
+
   coff_symbol16 Sym;
   StringRef Name = Def->getName();
   if (Name.size() > COFF::NameSize) {

lld/test/COFF/Inputs/import.yaml

@@ -7,6 +7,13 @@ 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
@@ -16,7 +23,7 @@ symbols:
     StorageClass:    IMAGE_SYM_CLASS_STATIC
     SectionDefinition:
       Length:          8
-      NumberOfRelocations: 0
+      NumberOfRelocations: 2
       NumberOfLinenumbers: 0
       CheckSum:        0
       Number:          0

lld/test/COFF/Inputs/oldname.yaml

@@ -0,0 +1,26 @@
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_UNKNOWN
+  Characteristics: [  ]
+sections:
+  - Name:            .drectve
+    Characteristics: [ IMAGE_SCN_LNK_INFO, IMAGE_SCN_LNK_REMOVE ]
+    Alignment:       1
+    SectionData:     ''
+symbols:
+  - Name:            exportfn1
+    Value:           0
+    SectionNumber:   0
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+  - Name:            exportfn1_alias
+    Value:           0
+    SectionNumber:   0
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_WEAK_EXTERNAL
+    WeakExternal:
+      TagIndex:        0
+      Characteristics: IMAGE_WEAK_EXTERN_SEARCH_ALIAS
+...

lld/test/COFF/dllimport-gc.test

@@ -0,0 +1,58 @@
+# 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 < %p/Inputs/oldname.yaml > %t-oldname.obj
+
+# RUN: yaml2obj < %s > %t.obj
+
+# RUN: lld-link /out:%t1.exe /entry:main %t.obj %t-oldname.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-oldname.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
+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: 0
+      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
+  - Name:            exportfn1_alias
+    Value:           0
+    SectionNumber:   0
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...