[Dwarf] Optimize getOrCreateSourceID() for repeated calls on same file (NFCI)

[nikic]

Summary

DwarfCompileUnit::getOrCreateSourceID() is often called many times in sequence with the same DIFile. This is currently very expensive, because it involves creating a string from directory and file name and looking it up in a string map. This patch remembers the last DIFile and its ID and directly returns that.

This gives a geomean -1.3% compile-time improvement on O0-g: http://llvm-compile-time-tracker.com/compare.php?from=0d1308a7b77c9ed87386c22a728a6c97e2fb4887&to=0524863ab347cbb6f897844203d066869b8bee31&stat=instructions

Disclaimer: I'm not familiar with the area, but this kind of caching looks legal to me.

[dblaikie]

Sure, sounds alright to me, thanks!

llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.h

class DwarfCompileUnit final : public DwarfUnit {
   const MCSymbol *BaseAddress = nullptr;
 
   DenseMap<const MDNode *, DIE *> AbstractSPDies;
   DenseMap<const DINode *, std::unique_ptr<DbgEntity>> AbstractEntities;
 
   /// DWO ID for correlating skeleton and split units.
   uint64_t DWOId = 0;
 
+  const DIFile *LastFile = nullptr;
+  unsigned LastFileID;
+
   /// Construct a DIE for the given DbgVariable without initializing the
   /// DbgVariable's DIE reference.
   DIE *constructVariableDIEImpl(const DbgVariable &DV, bool Abstract);
 
   bool isDwoUnit() const override;
 
   DenseMap<const MDNode *, DIE *> &getAbstractSPDies() {
     if (isDwoUnit() && !DD->shareAcrossDWOCUs())

llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

unsigned DwarfCompileUnit::getOrCreateSourceID(const DIFile *File) {
   // compile units. Thus all files will belong to the default compile unit.
 
   // FIXME: add a better feature test than hasRawTextSupport. Even better,
   // extend .file to support this.
   unsigned CUID = Asm->OutStreamer->hasRawTextSupport() ? 0 : getUniqueID();
   if (!File)
     return Asm->OutStreamer->emitDwarfFileDirective(0, "", "", None, None,
                                                     CUID);
-  return Asm->OutStreamer->emitDwarfFileDirective(
+
+  if (LastFile != File) {
+    LastFile = File;
+    LastFileID = Asm->OutStreamer->emitDwarfFileDirective(
         0, File->getDirectory(), File->getFilename(), DD->getMD5AsBytes(File),
         File->getSource(), CUID);
   }
+  return LastFileID;
+}
 
 DIE *DwarfCompileUnit::getOrCreateGlobalVariableDIE(
     const DIGlobalVariable *GV, ArrayRef<GlobalExpr> GlobalExprs) {
   // Check for pre-existence.
   if (DIE *Die = getDIE(GV))
     return Die;
 
   assert(GV);