From 99111287fc97eb95f035e2ad867165f563caa5fd Mon Sep 17 00:00:00 2001
From: Peter Collingbourne <peter@pcc.me.uk>
Date: Sun, 11 Dec 2016 22:15:20 +0000
Subject: [PATCH] COFF: Use a bit in SymbolBody to track which symbols are
 written to the symbol table.

Using a set here caused us to take about 1 second longer to write the symbol
table when linking chrome_child.dll. With this I consistently get better
performance on Windows with the new symbol table.

Before r289280 and with r289183 reverted (median of 5 runs): 17.65s
After this change: 17.33s

On Linux things look even better:

Before: 10.700480444s
After: 5.735681610s

Differential Revision: https://reviews.llvm.org/D27648

llvm-svn: 289408
---
 lld/COFF/Symbols.h  | 7 ++++++-
 lld/COFF/Writer.cpp | 5 +++--
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/lld/COFF/Symbols.h b/lld/COFF/Symbols.h
index 240a67aa1f14..560010b63a43 100644
--- a/lld/COFF/Symbols.h
+++ b/lld/COFF/Symbols.h
@@ -80,7 +80,7 @@ protected:
   friend SymbolTable;
   explicit SymbolBody(Kind K, StringRef N = "")
       : SymbolKind(K), IsExternal(true), IsCOMDAT(false),
-        IsReplaceable(false), Name(N) {}
+        IsReplaceable(false), WrittenToSymtab(false), Name(N) {}
 
   const unsigned SymbolKind : 8;
   unsigned IsExternal : 1;
@@ -91,6 +91,11 @@ protected:
   // This bit is used by the \c DefinedBitcode subclass.
   unsigned IsReplaceable : 1;
 
+public:
+  // This bit is used by Writer::createSymbolAndStringTable().
+  unsigned WrittenToSymtab : 1;
+
+protected:
   StringRef Name;
 };
 
diff --git a/lld/COFF/Writer.cpp b/lld/COFF/Writer.cpp
index 13d95c6ea778..98fd6c4470cd 100644
--- a/lld/COFF/Writer.cpp
+++ b/lld/COFF/Writer.cpp
@@ -533,13 +533,14 @@ void Writer::createSymbolAndStringTable() {
     Sec->setStringTableOff(addEntryToStringTable(Name));
   }
 
-  std::set<SymbolBody *> SeenSymbols;
   for (lld::coff::ObjectFile *File : Symtab->ObjectFiles)
     for (SymbolBody *B : File->getSymbols())
       if (auto *D = dyn_cast<Defined>(B))
-        if (SeenSymbols.insert(D).second)
+        if (!D->WrittenToSymtab) {
+          D->WrittenToSymtab = true;
           if (Optional<coff_symbol16> Sym = createSymbol(D))
             OutputSymtab.push_back(*Sym);
+        }
 
   OutputSection *LastSection = OutputSections.back();
   // We position the symbol table to be adjacent to the end of the last section.