[codeview] Use comdats for debug info describing comdat functions

Summary:
This allows the linker to discard unused symbol information for comdat
functions that were discarded during the link. Before this change,
searching for the name of an inline function in the debugger would
return multiple results, one per symbol subsection in the object file.
After this change, there is only one result, the result for the function
chosen by the linker.

Reviewers: zturner, majnemer

Subscribers: aaboud, amccarth, llvm-commits

Differential Revision: http://reviews.llvm.org/D20642

llvm-svn: 270792

2 files changed, 57 insertions, 12 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
index 0eb3e44fba1..3f3408eb368 100644
--- a/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp
@@ -18,6 +18,7 @@
 #include "llvm/DebugInfo/CodeView/TypeIndex.h"
 #include "llvm/DebugInfo/CodeView/TypeRecord.h"
 #include "llvm/MC/MCExpr.h"
+#include "llvm/MC/MCSectionCOFF.h"
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/Support/COFF.h"
 #include "llvm/Target/TargetSubtargetInfo.h"
@@ -217,19 +218,19 @@ void CodeViewDebug::maybeRecordLocation(DebugLoc DL,
                         /*IsStmt=*/false, DL->getFilename());
 }
 
+void CodeViewDebug::emitCodeViewMagicVersion() {
+  OS.EmitValueToAlignment(4);
+  OS.AddComment("Debug section magic");
+  OS.EmitIntValue(COFF::DEBUG_SECTION_MAGIC, 4);
+}
+
 void CodeViewDebug::endModule() {
   if (FnDebugInfo.empty())
     return;
 
   emitTypeInformation();
 
-  // FIXME: For functions that are comdat, we should emit separate .debug$S
-  // sections that are comdat associative with the main function instead of
-  // having one big .debug$S section.
   assert(Asm != nullptr);
-  OS.SwitchSection(Asm->getObjFileLowering().getCOFFDebugSymbolsSection());
-  OS.AddComment("Debug section magic");
-  OS.EmitIntValue(COFF::DEBUG_SECTION_MAGIC, 4);
 
   // The COFF .debug$S section consists of several subsections, each starting
   // with a 4-byte control code (e.g. 0xF1, 0xF2, etc) and then a 4-byte length
@@ -237,12 +238,16 @@ void CodeViewDebug::endModule() {
   // aligned.
 
   // Make a subsection for all the inlined subprograms.
-  emitInlineeFuncIdsAndLines();
+  emitInlineeLinesSubsection();
 
   // Emit per-function debug information.
   for (auto &P : FnDebugInfo)
     emitDebugInfoForFunction(P.first, P.second);
 
+  // Switch back to the generic .debug$S section after potentially processing
+  // comdat symbol sections.
+  switchToDebugSectionForSymbol(nullptr);
+
   // This subsection holds a file index to offset in string table table.
   OS.AddComment("File index to string table offset subsection");
   OS.EmitCVFileChecksumsDirective();
@@ -275,9 +280,7 @@ void CodeViewDebug::emitTypeInformation() {
 
   // Start the .debug$T section with 0x4.
   OS.SwitchSection(Asm->getObjFileLowering().getCOFFDebugTypesSection());
-  OS.AddComment("Debug section magic");
-  OS.EmitValueToAlignment(4);
-  OS.EmitIntValue(COFF::DEBUG_SECTION_MAGIC, 4);
+  emitCodeViewMagicVersion();
 
   TypeTable.ForEachRecord(
       [&](TypeIndex Index, const MemoryTypeTableBuilder::Record *R) {
@@ -298,10 +301,13 @@ void CodeViewDebug::emitTypeInformation() {
       });
 }
 
-void CodeViewDebug::emitInlineeFuncIdsAndLines() {
+void CodeViewDebug::emitInlineeLinesSubsection() {
   if (InlinedSubprograms.empty())
     return;
 
+  // Use the generic .debug$S section.
+  switchToDebugSectionForSymbol(nullptr);
+
   MCSymbol *InlineBegin = MMI->getContext().createTempSymbol(),
            *InlineEnd = MMI->getContext().createTempSymbol();
 
@@ -401,6 +407,26 @@ void CodeViewDebug::emitInlinedCallSite(const FunctionInfo &FI,
   OS.EmitIntValue(SymbolKind::S_INLINESITE_END, 2); // RecordKind
 }
 
+void CodeViewDebug::switchToDebugSectionForSymbol(const MCSymbol *GVSym) {
+  // If we have a symbol, it may be in a section that is COMDAT. If so, find the
+  // comdat key. A section may be comdat because of -ffunction-sections or
+  // because it is comdat in the IR.
+  MCSectionCOFF *GVSec =
+      GVSym ? dyn_cast<MCSectionCOFF>(&GVSym->getSection()) : nullptr;
+  const MCSymbol *KeySym = GVSec ? GVSec->getCOMDATSymbol() : nullptr;
+
+  MCSectionCOFF *DebugSec = cast<MCSectionCOFF>(
+      Asm->getObjFileLowering().getCOFFDebugSymbolsSection());
+  DebugSec = OS.getContext().getAssociativeCOFFSection(DebugSec, KeySym);
+
+  OS.SwitchSection(DebugSec);
+
+  // Emit the magic version number if this is the first time we've switched to
+  // this section.
+  if (ComdatDebugSections.insert(DebugSec).second)
+    emitCodeViewMagicVersion();
+}
+
 void CodeViewDebug::emitDebugInfoForFunction(const Function *GV,
                                              FunctionInfo &FI) {
   // For each function there is a separate subsection
@@ -408,6 +434,9 @@ void CodeViewDebug::emitDebugInfoForFunction(const Function *GV,
   const MCSymbol *Fn = Asm->getSymbol(GV);
   assert(Fn);
 
+  // Switch to the to a comdat section, if appropriate.
+  switchToDebugSectionForSymbol(Fn);
+
   StringRef FuncName;
   if (auto *SP = GV->getSubprogram())
     FuncName = SP->getDisplayName();
diff --git a/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.h b/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.h
index 53e1aa0b422..df7c85c0007 100644
--- a/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.h
+++ b/llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.h
@@ -101,6 +101,18 @@ class LLVM_LIBRARY_VISIBILITY CodeViewDebug : public DebugHandlerBase {
   };
   FunctionInfo *CurFn;
 
+  /// The set of comdat .debug$S sections that we've seen so far. Each section
+  /// must start with a magic version number that must only be emitted once.
+  /// This set tracks which sections we've already opened.
+  DenseSet<MCSectionCOFF *> ComdatDebugSections;
+
+  /// Switch to the appropriate .debug$S section for GVSym. If GVSym, the symbol
+  /// of an emitted global value, is in a comdat COFF section, this will switch
+  /// to a new .debug$S section in that comdat. This method ensures that the
+  /// section starts with the magic version number on first use. If GVSym is
+  /// null, uses the main .debug$S section.
+  void switchToDebugSectionForSymbol(const MCSymbol *GVSym);
+
   /// The next available function index for use with our .cv_* directives. Not
   /// to be confused with type indices for LF_FUNC_ID records.
   unsigned NextFuncId = 0;
@@ -148,9 +160,13 @@ class LLVM_LIBRARY_VISIBILITY CodeViewDebug : public DebugHandlerBase {
     FileToFilepathMap.clear();
   }
 
+  /// Emit the magic version number at the start of a CodeView type or symbol
+  /// section. Appears at the front of every .debug$S or .debug$T section.
+  void emitCodeViewMagicVersion();
+
   void emitTypeInformation();
 
-  void emitInlineeFuncIdsAndLines();
+  void emitInlineeLinesSubsection();
 
   void emitDebugInfoForFunction(const Function *GV, FunctionInfo &FI);