[DebugInfo] Generate .debug_names section when it makes sense

Summary:
This patch makes us generate the debug_names section in response to some
user-facing commands (previously it was only generated if explicitly
selected via the -accel-tables option).

My goal was to make this work for DWARF>=5 (as it's an official part of
that standard), and also, as an extension, for DWARF<5 if one is
explicitly tuning for lldb as a debugger (because it brings a large
performance improvement there).

This is slightly complicated by the fact that the debug_names tables are
incompatible with the DWARF v4 type units (they assume that the type
units are in the debug_info section), and unfortunately, right now we
generate DWARF v4-style type units even for -gdwarf-5. For this reason,
I disable all accelerator tables if the user requested type unit
generation. I do this even for apple tables, as they have the same
problem (in fact generating type units for apple targets makes us crash
even before we get around to emitting the accelerator tables).

Reviewers: JDevlieghere, aprantl, dblaikie, echristo, probinson

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D49420

llvm-svn: 337544

3 files changed, 41 insertions, 17 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index 2fe6a2dfb7e..40fe0e9cb49 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -94,6 +94,11 @@ static cl::opt<bool> GenerateARangeSection("generate-arange-section",
                                            cl::desc("Generate dwarf aranges"),
                                            cl::init(false));
 
+static cl::opt<bool>
+    GenerateDwarfTypeUnits("generate-type-units", cl::Hidden,
+                           cl::desc("Generate DWARF4 type units."),
+                           cl::init(false));
+
 static cl::opt<bool> SplitDwarfCrossCuReferences(
     "split-dwarf-cross-cu-references", cl::Hidden,
     cl::desc("Enable cross-cu references in DWO files"), cl::init(false));
@@ -284,6 +289,29 @@ void DbgVariable::addMMIEntry(const DbgVariable &V) {
          "conflicting locations for variable");
 }
 
+static AccelTableKind computeAccelTableKind(unsigned DwarfVersion,
+                                            bool GenerateTypeUnits,
+                                            DebuggerKind Tuning,
+                                            const Triple &TT) {
+  // Honor an explicit request.
+  if (AccelTables != AccelTableKind::Default)
+    return AccelTables;
+
+  // Accelerator tables with type units are currently not supported.
+  if (GenerateTypeUnits)
+    return AccelTableKind::None;
+
+  // Accelerator tables get emitted if targetting DWARF v5 or LLDB.  DWARF v5
+  // always implies debug_names. For lower standard versions we use apple
+  // accelerator tables on apple platforms and debug_names elsewhere.
+  if (DwarfVersion >= 5)
+    return AccelTableKind::Dwarf;
+  if (Tuning == DebuggerKind::LLDB)
+    return TT.isOSBinFormatMachO() ? AccelTableKind::Apple
+                                   : AccelTableKind::Dwarf;
+  return AccelTableKind::None;
+}
+
 DwarfDebug::DwarfDebug(AsmPrinter *A, Module *M)
     : DebugHandlerBase(A), DebugLocs(A->OutStreamer->isVerboseAsm()),
       InfoHolder(A, "info_string", DIEValueAllocator),
@@ -302,16 +330,6 @@ DwarfDebug::DwarfDebug(AsmPrinter *A, Module *M)
   else
     DebuggerTuning = DebuggerKind::GDB;
 
-  // Turn on accelerator tables by default, if tuning for LLDB and the target is
-  // supported.
-  if (AccelTables == AccelTableKind::Default) {
-    if (tuneForLLDB() && A->TM.getTargetTriple().isOSBinFormatMachO())
-      TheAccelTableKind = AccelTableKind::Apple;
-    else
-      TheAccelTableKind = AccelTableKind::None;
-  } else
-    TheAccelTableKind = AccelTables;
-
   if (DwarfInlinedStrings == Default)
     UseInlineStrings = TT.isNVPTX();
   else
@@ -346,6 +364,11 @@ DwarfDebug::DwarfDebug(AsmPrinter *A, Module *M)
   else
     UseSectionsAsReferences = DwarfSectionsAsReferences == Enable;
 
+  GenerateTypeUnits = GenerateDwarfTypeUnits;
+
+  TheAccelTableKind = computeAccelTableKind(
+      DwarfVersion, GenerateTypeUnits, DebuggerTuning, A->TM.getTargetTriple());
+
   // Work around a GDB bug. GDB doesn't support the standard opcode;
   // SCE doesn't support GNU's; LLDB prefers the standard opcode, which
   // is defined as of DWARF 3.
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h
index 42988d53b8b..6f4cefe6107 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h
@@ -279,6 +279,9 @@ class DwarfDebug : public DebugHandlerBase {
   ///Allow emission of the .debug_loc section.
   bool UseLocSection = true;
 
+  /// Generate DWARF v4 type units.
+  bool GenerateTypeUnits;
+
   /// DWARF5 Experimental Options
   /// @{
   AccelTableKind TheAccelTableKind;
@@ -546,6 +549,9 @@ public:
   /// Returns whether .debug_loc section should be emitted.
   bool useLocSection() const { return UseLocSection; }
 
+  /// Returns whether to generate DWARF v4 type units.
+  bool generateTypeUnits() const { return GenerateTypeUnits; }
+
   // Experimental DWARF5 features.
 
   /// Returns what kind (if any) of accelerator tables to emit.
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
index 9af555464d4..43b835b2c4a 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -47,11 +47,6 @@ using namespace llvm;
 
 #define DEBUG_TYPE "dwarfdebug"
 
-static cl::opt<bool>
-GenerateDwarfTypeUnits("generate-type-units", cl::Hidden,
-                       cl::desc("Generate DWARF4 type units."),
-                       cl::init(false));
-
 DIEDwarfExpression::DIEDwarfExpression(const AsmPrinter &AP, DwarfUnit &DU,
                                        DIELoc &DIE)
     : DwarfExpression(AP.getDwarfVersion()), AP(AP), DU(DU),
@@ -185,7 +180,7 @@ bool DwarfUnit::isShareableAcrossCUs(const DINode *D) const {
     return false;
   return (isa<DIType>(D) ||
           (isa<DISubprogram>(D) && !cast<DISubprogram>(D)->isDefinition())) &&
-         !GenerateDwarfTypeUnits;
+         !DD->generateTypeUnits();
 }
 
 DIE *DwarfUnit::getDIE(const DINode *D) const {
@@ -768,7 +763,7 @@ DIE *DwarfUnit::getOrCreateTypeDIE(const MDNode *TyNode) {
   else if (auto *STy = dyn_cast<DISubroutineType>(Ty))
     constructTypeDIE(TyDIE, STy);
   else if (auto *CTy = dyn_cast<DICompositeType>(Ty)) {
-    if (GenerateDwarfTypeUnits && !Ty->isForwardDecl())
+    if (DD->generateTypeUnits() && !Ty->isForwardDecl())
       if (MDString *TypeId = CTy->getRawIdentifier()) {
         DD->addDwarfTypeUnitType(getCU(), TypeId->getString(), TyDIE, CTy);
         // Skip updating the accelerator tables since this is not the full type.