[Debuginfo][NFC] Avoid double calling of DWARFDie::find(DW_AT_name).

[avl]

Summary

Current implementation of DWARFDie::getName(DINameKind Kind) could
lead to double call to DWARFDie::find(DW_AT_name) in following
scenario:

getName(LinkageName);
getName(ShortName);

getName(LinkageName) calls find(DW_AT_name) if linkage name is not
found. Then, it is called again in getName(ShortName). This patch
alows to request LinkageName and ShortName separately
to avoid extra call to find(DW_AT_name).

It helps D74169 to parse clang debuginfo faster(~1%).

[dblaikie]

llvm/lib/DebugInfo/DWARF/DWARFDie.cpp

529–542

Are there any callers left relying on this fallback behavior? Could they be ported to an explicit function dedicated to this task ("getLinkageNameOrShortName") so it's clear?

[avl]

llvm/lib/DebugInfo/DWARF/DWARFDie.cpp

529–542

There are some callers relying on that fallback behavior which could use function with clearer name :

D.getName(DINameKind::LinkageName) -> D.getLinkageNameOrShortName()

But there are callers which receive name kind through parameters. In those cases it could not be replaced to getLinkageNameOrShortName:

const char *DWARFDie::getSubroutineName(DINameKind Kind) const {
  if (!isSubroutineDIE())
    return nullptr;
  return getName(Kind);
}
...
(Name = DIE.getSubroutineName(Kind)
...

I assume that this fallback behavior is important for all who currently specify DINameKind::LinkageName. Probably they could be evaluated and not all would finally require fallback behavior.

llvm/include/llvm/DebugInfo/DWARF/DWARFDie.h

public:
   getLocations(dwarf::Attribute Attr) const;
 
   /// If a DIE represents a subprogram (or inlined subroutine), returns its
   /// mangled name (or short name, if mangled is missing). This name may be
   /// fetched from specification or abstract origin for this subprogram.
   /// Returns null if no name is found.
   const char *getSubroutineName(DINameKind Kind) const;
 
-  /// Return the DIE name resolving DW_AT_sepcification or DW_AT_abstract_origin
-  /// references if necessary. Returns null if no name is found.
+  /// Return the DIE name resolving DW_AT_specification or DW_AT_abstract_origin
+  /// references if necessary. For the LinkageName case it additionaly searches
+  /// for ShortName if LinkageName is not found.
+  /// Returns null if no name is found.
   const char *getName(DINameKind Kind) const;
 
+  /// Return the DIE short name resolving DW_AT_specification or
+  /// DW_AT_abstract_origin references if necessary. Returns null if no name
+  /// is found.
+  const char *getShortName() const;
+
+  /// Return the DIE linkage name resolving DW_AT_specification or
+  /// DW_AT_abstract_origin references if necessary. Returns null if no name
+  /// is found.
+  const char *getLinkageName() const;
+
   /// Returns the declaration line (start line) for a DIE, assuming it specifies
   /// a subprogram. This may be fetched from specification or abstract origin
   /// for this subprogram by resolving DW_AT_sepcification or
   /// DW_AT_abstract_origin references if necessary.
   uint64_t getDeclLine() const;
 
   /// Retrieves values of DW_AT_call_file, DW_AT_call_line and DW_AT_call_column
   /// from DIE (or zeroes if they are missing). This function looks for

llvm/lib/DWARFLinker/DWARFLinker.cpp

bool DWARFLinker::DIECloner::getDIENames(const DWARFDie &Die,
                                          OffsetsStringPool &StringPool,
                                          bool StripTemplate) {
   // This function will be called on DIEs having low_pcs and
   // ranges. As getting the name might be more expansive, filter out
   // blocks directly.
   if (Die.getTag() == dwarf::DW_TAG_lexical_block)
     return false;
 
-  // FIXME: a bit wasteful as the first getName might return the
-  // short name.
   if (!Info.MangledName)
-    if (const char *MangledName = Die.getName(DINameKind::LinkageName))
+    if (const char *MangledName = Die.getLinkageName())
       Info.MangledName = StringPool.getEntry(MangledName);
 
   if (!Info.Name)
-    if (const char *Name = Die.getName(DINameKind::ShortName))
+    if (const char *Name = Die.getShortName())
       Info.Name = StringPool.getEntry(Name);
 
+  if (!Info.MangledName)
+    Info.MangledName = Info.Name;
+
   if (StripTemplate && Info.Name && Info.MangledName != Info.Name) {
     StringRef Name = Info.Name.getString();
     if (Optional<StringRef> StrippedName = StripTemplateParameters(Name))
       Info.NameWithoutTemplate = StringPool.getEntry(*StrippedName);
   }
 
   return Info.Name || Info.MangledName;
 }

llvm/lib/DWARFLinker/DWARFLinkerDeclContext.cpp

case dwarf::DW_TAG_typedef:
     // demand. For example implicitly defined constructors are ambiguous
     // because of the way we identify contexts, and they won't be generated
     // every time everywhere.
     if (dwarf::toUnsigned(DIE.find(dwarf::DW_AT_artificial), 0))
       return PointerIntPair<DeclContext *, 1>(nullptr);
     break;
   }
 
-  const char *Name = DIE.getName(DINameKind::LinkageName);
-  const char *ShortName = DIE.getName(DINameKind::ShortName);
+  const char *Name = DIE.getLinkageName();
+  const char *ShortName = DIE.getShortName();
+
+  if (!Name)
+    Name = ShortName;
+
   StringRef NameRef;
   StringRef ShortNameRef;
   StringRef FileRef;
 
   if (Name)
     NameRef = StringPool.internString(Name);
   else if (Tag == dwarf::DW_TAG_namespace)
     // FIXME: For dsymutil-classic compatibility. I think uniquing within

llvm/lib/DebugInfo/DWARF/DWARFDie.cpp

 }
 
 const char *DWARFDie::getSubroutineName(DINameKind Kind) const {
   if (!isSubroutineDIE())
     return nullptr;
   return getName(Kind);
 }
 
 const char *DWARFDie::getName(DINameKind Kind) const {
   if (!isValid() || Kind == DINameKind::None)
     return nullptr;
   // Try to get mangled name only if it was asked for.
   if (Kind == DINameKind::LinkageName) {
-    if (auto Name = dwarf::toString(
-            findRecursively({DW_AT_MIPS_linkage_name, DW_AT_linkage_name}),
-            nullptr))
+    if (auto Name = getLinkageName())
       return Name;
   }
-  if (auto Name = dwarf::toString(findRecursively(DW_AT_name), nullptr))
-    return Name;
+  return getShortName();
+}
+
+const char *DWARFDie::getShortName() const {
+  if (!isValid())
+    return nullptr;
+
+  return dwarf::toString(findRecursively(dwarf::DW_AT_name), nullptr);
+}
+
+const char *DWARFDie::getLinkageName() const {
+  if (!isValid())
     return nullptr;
+
+  return dwarf::toString(findRecursively({dwarf::DW_AT_MIPS_linkage_name,
+                                          dwarf::DW_AT_linkage_name}),
+                         nullptr);
 }
 
 uint64_t DWARFDie::getDeclLine() const {
   return toUnsigned(findRecursively(DW_AT_decl_line), 0);
 }
 
 void DWARFDie::getCallerFrame(uint32_t &CallFile, uint32_t &CallLine,
                               uint32_t &CallColumn,
                               uint32_t &CallDiscriminator) const {