DebugInfo: Avoid using split DWARF when the split unit would be empty.

In ThinLTO many split CUs may be effectively empty because of the lack
of support for cross-unit references in split DWARF.

Using a split unit in those cases is just a waste/overhead - and turned
out to be one contributor to a significant symbolizer performance issue
when global variable debug info was being imported (see r348416 for the
primary fix) due to symbolizers seeing CUs with no ranges, assuming
there might still be addresses covered and walking into the split CU to
see if there are any ranges (when that split CU was in a DWP file, that
meant loading the DWP and its index, the index was extra large because
of all these fractured/empty CUs... and so was very expensive to load).

(the 3rd fix which will follow, is to assume that a CU with no ranges is
empty rather than merely missing its CU level range data - and to not
walk into its DIEs (split or otherwise) in search of address information
that is generally not present)

llvm-svn: 349207

1 files changed, 37 insertions, 28 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index 774f7d1df4e..5fc8d0622a4 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -568,25 +568,10 @@ void DwarfDebug::addGnuPubAttributes(DwarfCompileUnit &U, DIE &D) const {
   U.addFlag(D, dwarf::DW_AT_GNU_pubnames);
 }
 
-// Create new DwarfCompileUnit for the given metadata node with tag
-// DW_TAG_compile_unit.
-DwarfCompileUnit &
-DwarfDebug::getOrCreateDwarfCompileUnit(const DICompileUnit *DIUnit) {
-  if (auto *CU = CUMap.lookup(DIUnit))
-    return *CU;
-  StringRef FN = DIUnit->getFilename();
-  CompilationDir = DIUnit->getDirectory();
-
-  auto OwnedUnit = llvm::make_unique<DwarfCompileUnit>(
-      InfoHolder.getUnits().size(), DIUnit, Asm, this, &InfoHolder);
-  DwarfCompileUnit &NewCU = *OwnedUnit;
+void DwarfDebug::finishUnitAttributes(const DICompileUnit *DIUnit,
+                                      DwarfCompileUnit &NewCU) {
   DIE &Die = NewCU.getUnitDie();
-  InfoHolder.addUnit(std::move(OwnedUnit));
-  if (useSplitDwarf()) {
-    NewCU.setSkeleton(constructSkeletonCU(NewCU));
-    NewCU.addString(Die, dwarf::DW_AT_GNU_dwo_name,
-                  Asm->TM.Options.MCOptions.SplitDwarfFile);
-  }
+  StringRef FN = DIUnit->getFilename();
 
   for (auto *IE : DIUnit->getImportedEntities())
     NewCU.addImportedEntity(IE);
@@ -640,11 +625,6 @@ DwarfDebug::getOrCreateDwarfCompileUnit(const DICompileUnit *DIUnit) {
                     dwarf::DW_FORM_data1, RVer);
   }
 
-  if (useSplitDwarf())
-    NewCU.setSection(Asm->getObjFileLowering().getDwarfInfoDWOSection());
-  else
-    NewCU.setSection(Asm->getObjFileLowering().getDwarfInfoSection());
-
   if (DIUnit->getDWOId()) {
     // This CU is either a clang module DWO or a skeleton CU.
     NewCU.addUInt(Die, dwarf::DW_AT_GNU_dwo_id, dwarf::DW_FORM_data8,
@@ -654,9 +634,31 @@ DwarfDebug::getOrCreateDwarfCompileUnit(const DICompileUnit *DIUnit) {
       NewCU.addString(Die, dwarf::DW_AT_GNU_dwo_name,
                       DIUnit->getSplitDebugFilename());
   }
+}
+// Create new DwarfCompileUnit for the given metadata node with tag
+// DW_TAG_compile_unit.
+DwarfCompileUnit &
+DwarfDebug::getOrCreateDwarfCompileUnit(const DICompileUnit *DIUnit) {
+  if (auto *CU = CUMap.lookup(DIUnit))
+    return *CU;
+
+  CompilationDir = DIUnit->getDirectory();
+
+  auto OwnedUnit = llvm::make_unique<DwarfCompileUnit>(
+      InfoHolder.getUnits().size(), DIUnit, Asm, this, &InfoHolder);
+  DwarfCompileUnit &NewCU = *OwnedUnit;
+  InfoHolder.addUnit(std::move(OwnedUnit));
+
+  if (useSplitDwarf()) {
+    NewCU.setSkeleton(constructSkeletonCU(NewCU));
+    NewCU.setSection(Asm->getObjFileLowering().getDwarfInfoDWOSection());
+  } else {
+    finishUnitAttributes(DIUnit, NewCU);
+    NewCU.setSection(Asm->getObjFileLowering().getDwarfInfoSection());
+  }
 
   CUMap.insert({DIUnit, &NewCU});
-  CUDieMap.insert({&Die, &NewCU});
+  CUDieMap.insert({&NewCU.getUnitDie(), &NewCU});
   return NewCU;
 }
 
@@ -851,7 +853,12 @@ void DwarfDebug::finalizeModuleInfo() {
     // If we're splitting the dwarf out now that we've got the entire
     // CU then add the dwo id to it.
     auto *SkCU = TheCU.getSkeleton();
-    if (useSplitDwarf()) {
+    if (useSplitDwarf() && !empty(TheCU.getUnitDie().children())) {
+      finishUnitAttributes(TheCU.getCUNode(), TheCU);
+      TheCU.addString(TheCU.getUnitDie(), dwarf::DW_AT_GNU_dwo_name,
+                      Asm->TM.Options.MCOptions.SplitDwarfFile);
+      SkCU->addString(SkCU->getUnitDie(), dwarf::DW_AT_GNU_dwo_name,
+                      Asm->TM.Options.MCOptions.SplitDwarfFile);
       // Emit a unique identifier for this CU.
       uint64_t ID =
           DIEHash(Asm).computeCUSignature(DWOName, TheCU.getUnitDie());
@@ -870,6 +877,8 @@ void DwarfDebug::finalizeModuleInfo() {
         SkCU->addSectionLabel(SkCU->getUnitDie(), dwarf::DW_AT_GNU_ranges_base,
                               Sym, Sym);
       }
+    } else if (SkCU) {
+      finishUnitAttributes(SkCU->getCUNode(), *SkCU);
     }
 
     // If we have code split among multiple sections or non-contiguous
@@ -882,7 +891,9 @@ void DwarfDebug::finalizeModuleInfo() {
 
     // We don't keep track of which addresses are used in which CU so this
     // is a bit pessimistic under LTO.
-    if (!AddrPool.isEmpty())
+    if (!AddrPool.isEmpty() &&
+        (getDwarfVersion() >= 5 ||
+         (SkCU && !empty(TheCU.getUnitDie().children()))))
       U.addAddrTableBase();
 
     if (unsigned NumRanges = TheCU.getRanges().size()) {
@@ -2483,8 +2494,6 @@ void DwarfDebug::emitDebugMacinfo() {
 
 void DwarfDebug::initSkeletonUnit(const DwarfUnit &U, DIE &Die,
                                   std::unique_ptr<DwarfCompileUnit> NewU) {
-  NewU->addString(Die, dwarf::DW_AT_GNU_dwo_name,
-                  Asm->TM.Options.MCOptions.SplitDwarfFile);
 
   if (!CompilationDir.empty())
     NewU->addString(Die, dwarf::DW_AT_comp_dir, CompilationDir);