[PDB] Don't emit debug info associated with dead chunks

[rnk]

Summary

Previously we didn't add debug info chunks to the SparseChunks array, so
they didn't participate in section GC. Now we do.

[rnk]

• Reduce yaml files a little

[ruiu]

lld/COFF/Chunks.h

182

Since this function is not called very often, I'd remove the member variable and check the condition every time the function is called.

[rnk]

lld/COFF/Chunks.h

182

Isn't this function called by the Writer for every chunk written to the output file?

[ruiu]

lld/COFF/Chunks.h

182

Even if it is, I think it is still negligible.

[rnk]

• check section names in isCodeView predicate

[ruiu]

lld/COFF/Chunks.h

183–184

I'm sorry to be too picky, but isn't SectionName == ".debug" || SectionName.startswith(".debug$") enough? In LLD I'm really trying hard to make our code simple, boring and straightforward unless there's a strong reason to make it clever.

[rnk]

lld/COFF/Chunks.h

183–184

Sure, we can do that. I felt like if this was going to be on the main path, then we should at least try to be clever...

[rnk]

• don't be clever

[ruiu]

LGTM

lld/COFF/Chunks.h

public:
 
   // Returns true if this chunk was dropped by COMDAT deduplication.
   bool isDiscarded() const { return Discarded; }
 
   // Used by the SymbolTable when discarding unused comdat sections. This is
   // redundant when GC is enabled, as all comdat sections will start out dead.
   void markDiscarded() { Discarded = true; }
 
+  // True if this is a codeview debug info chunk. These will not be laid out in
+  // the image. Instead they will end up in the PDB, if one is requested.
+  bool isCodeView() const { return IsCodeViewDebug; }
+
   // Allow iteration over the bodies of this chunk's relocated symbols.
   llvm::iterator_range<symbol_iterator> symbols() const {
     return llvm::make_range(symbol_iterator(File, Relocs.begin()),
                             symbol_iterator(File, Relocs.end()));
   }
 
   // Allow iteration over the associated child chunks for this section.
   ArrayRef<SectionChunk *> children() const { return AssocChildren; }
 
private:
   size_t NumRelocs;
 
   // True if this chunk was discarded because it was a duplicate comdat section.
   bool Discarded;
 
   // Used by the garbage collector.
   bool Live;
 
+  bool IsCodeViewDebug;
+
   // Used for ICF (Identical COMDAT Folding)
   void replace(SectionChunk *Other);
   uint32_t Class[2] = {0, 0};
 
   // Sym points to a section symbol if this is a COMDAT chunk.
   DefinedRegular *Sym = nullptr;
 };
 

lld/COFF/Chunks.cpp

SectionChunk::SectionChunk(ObjectFile *F, const coff_section *H)
   // Chunks may be discarded during comdat merging.
   Discarded = false;
 
   // If linker GC is disabled, every chunk starts out alive.  If linker GC is
   // enabled, treat non-comdat sections as roots. Generally optimized object
   // files will be built with -ffunction-sections or /Gy, so most things worth
   // stripping will be in a comdat.
   Live = !Config->DoGC || !isCOMDAT();
+
+  IsCodeViewDebug =
+      SectionName == ".debug" || SectionName.startswith(".debug$");
 }
 
 static void add16(uint8_t *P, int16_t V) { write16le(P, read16le(P) + V); }
 static void add32(uint8_t *P, int32_t V) { write32le(P, read32le(P) + V); }
 static void add64(uint8_t *P, int64_t V) { write64le(P, read64le(P) + V); }
 static void or16(uint8_t *P, uint16_t V) { write16le(P, read16le(P) | V); }
 
 void SectionChunk::applyRelX64(uint8_t *Off, uint16_t Type, Defined *Sym,

lld/COFF/InputFiles.cpp

for (uint32_t I = 1; I < NumSections + 1; ++I) {
     //
     // CodeView needs a linker support. We need to interpret and debug
     // info, and then write it to a separate .pdb file.
 
     // Ignore debug info unless /debug is given.
     if (!Config->Debug && Name.startswith(".debug"))
       continue;
 
-    // CodeView sections are stored to a different vector because they are
-    // not linked in the regular manner.
-    if (Name == ".debug" || Name.startswith(".debug$")) {
-      DebugChunks.push_back(make<SectionChunk>(this, Sec));
-      continue;
-    }
-
     if (Sec->Characteristics & llvm::COFF::IMAGE_SCN_LNK_REMOVE)
       continue;
     auto *C = make<SectionChunk>(this, Sec);
+
+    // CodeView sections are stored to a different vector because they are not
+    // linked in the regular manner.
+    if (C->isCodeView())
+      DebugChunks.push_back(C);
+    else
       Chunks.push_back(C);
+
     SparseChunks[I] = C;
   }
 }
 
 void ObjectFile::initializeSymbols() {
   uint32_t NumSymbols = COFFObj->getNumberOfSymbols();
   SymbolBodies.reserve(NumSymbols);
   SparseSymbolBodies.resize(NumSymbols);

lld/COFF/PDB.cpp

for (ObjectFile *File : Symtab->ObjectFiles) {
     // type information, file checksums, and the string table.  Add type info to
     // the PDB first, so that we can get the map from object file type and item
     // indices to PDB type and item indices.
     SmallVector<TypeIndex, 128> TypeIndexMap;
     mergeDebugT(File, IDTable, TypeTable, TypeIndexMap, Handler);
 
     // Now do all line info.
     for (SectionChunk *DebugChunk : File->getDebugChunks()) {
-      // FIXME: Debug chunks do not have a correct isLive() bit.
-      // FIXME: When linker GC is off we need to ignore debug info whose
-      // associated symbol was discarded.
-      if (DebugChunk->getSectionName() != ".debug$S")
+      if (!DebugChunk->isLive() || DebugChunk->getSectionName() != ".debug$S")
         continue;
 
       ArrayRef<uint8_t> RelocatedDebugContents =
           relocateDebugChunk(Alloc, DebugChunk);
       if (RelocatedDebugContents.empty())
         continue;
 
       DebugSubsectionArray Subsections;

lld/COFF/Writer.cpp

size_t Writer::addEntryToStringTable(StringRef Str) {
   return OffsetOfEntry;
 }
 
 Optional<coff_symbol16> Writer::createSymbol(Defined *Def) {
   // Relative symbols are unrepresentable in a COFF symbol table.
   if (isa<DefinedRelative>(Def))
     return None;
 
-  if (auto *D = dyn_cast<DefinedRegular>(Def))
-    if (!D->getChunk()->isLive())
+  if (auto *D = dyn_cast<DefinedRegular>(Def)) {
+    // Don't write dead symbols or symbols in codeview sections to the symbol
+    // table.
+    if (!D->getChunk()->isLive() || D->getChunk()->isCodeView())
       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;

lld/test/COFF/Inputs/pdb_comdat_bar.yaml

+--- !COFF
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+  - Name:            '.debug$S'
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+...

lld/test/COFF/Inputs/pdb_comdat_main.yaml

+--- !COFF
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+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...

lld/test/COFF/pdb-comdat.test

+Consider this example program with an inline function "foo":
+
+==> foo.h <==
+extern int global;
+__inline void foo() {
+  ++global;
+}
+void bar();
+==> pdb_comdat_main.c <==
+#include "foo.h"
+int main(void) {
+  foo();
+  bar();
+  return 42;
+}
+==> pdb_comdat_bar.c <==
+#include "foo.h"
+void bar(void) {
+  foo();
+}
+
+Both object files will contain debug info for foo, but only the debug info from
+pdb_comdat_main.obj should be included in the PDB.
+
+RUN: rm -rf %t && mkdir -p %t && cd %t
+RUN: yaml2obj %S/Inputs/pdb_comdat_main.yaml -o pdb_comdat_main.obj
+RUN: yaml2obj %S/Inputs/pdb_comdat_bar.yaml -o pdb_comdat_bar.obj
+RUN: lld-link pdb_comdat_main.obj pdb_comdat_bar.obj -out:t.exe -debug -pdb:t.pdb -nodefaultlib -entry:main
+RUN: llvm-pdbutil raw -l t.pdb | FileCheck %s
+
+CHECK:                            Lines
+CHECK: ============================================================
+CHECK-LABEL:   Mod 0000 | `{{.*}}pdb_comdat_main.obj`:
+CHECK:       c:\src\llvm-project\build\pdb_comdat_main.c (MD5: F969E51BBE373436D81492EB61387F36)
+CHECK:       c:\src\llvm-project\build\foo.h (MD5: D74D834EFAC3AE2B45E606A8320B1D5C)
+CHECK-LABEL:   Mod 0001 | `{{.*}}pdb_comdat_bar.obj`:
+CHECK:       c:\src\llvm-project\build\pdb_comdat_bar.c (MD5: 365279DB4FCBEDD721BBFC3B14A953C2)
+CHECK-NOT:       c:\src\llvm-project\build\foo.h
+CHECK-LABEL:   Mod 0002 | `* Linker *`:
+
+Reorder the object files and verify that the other table is selected.
+
+RUN: lld-link pdb_comdat_bar.obj pdb_comdat_main.obj -out:t.exe -debug -pdb:t.pdb -nodefaultlib -entry:main
+RUN: llvm-pdbutil raw -l t.pdb | FileCheck %s --check-prefix=REORDER
+
+REORDER-LABEL:   Mod 0000 | `{{.*}}pdb_comdat_bar.obj`:
+REORDER:       c:\src\llvm-project\build\pdb_comdat_bar.c (MD5: 365279DB4FCBEDD721BBFC3B14A953C2)
+REORDER:       c:\src\llvm-project\build\foo.h (MD5: D74D834EFAC3AE2B45E606A8320B1D5C)
+REORDER-LABEL:   Mod 0001 | `{{.*}}pdb_comdat_main.obj`:
+REORDER:       c:\src\llvm-project\build\pdb_comdat_main.c
+REORDER-NOT:       c:\src\llvm-project\build\foo.h
+REORDER-LABEL:   Mod 0002 | `* Linker *`: