[PATCH] D118041: [Dwarf] Optimize getOrCreateSourceID() for repeated calls on same file (NFCI)

[Nikita Popov via Phabricator via llvm-commits]

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`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.


https://reviews.llvm.org/D118041

Files:
  llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
  llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.h


Index: llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.h
===================================================================
--- llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.h
+++ llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.h
@@ -86,6 +86,9 @@
   /// 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);
Index: llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
===================================================================
--- llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
+++ llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp
@@ -127,9 +127,14 @@
   if (!File)
     return Asm->OutStreamer->emitDwarfFileDirective(0, "", "", None, None,
                                                     CUID);
-  return Asm->OutStreamer->emitDwarfFileDirective(
-      0, File->getDirectory(), File->getFilename(), DD->getMD5AsBytes(File),
-      File->getSource(), CUID);
+
+  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(


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