From 40fae4d8fcbd6224f16fdf3ba84a0d89b713e7d5 Mon Sep 17 00:00:00 2001
From: Fangrui Song <i@maskray.me>
Date: Fri, 24 Dec 2021 19:01:50 -0800
Subject: [PATCH] [ELF] Optimize replaceCommonSymbols

This decreases the 0.2% time (no debug info) to nearly no.
---
 lld/ELF/Driver.cpp     | 22 +++++++++++++---------
 lld/ELF/InputFiles.cpp |  1 +
 lld/ELF/InputFiles.h   |  5 ++++-
 3 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/lld/ELF/Driver.cpp b/lld/ELF/Driver.cpp
index 19266cb..6b689f5 100644
--- a/lld/ELF/Driver.cpp
+++ b/lld/ELF/Driver.cpp
@@ -1821,17 +1821,21 @@ static void writeDependencyFile() {
 // symbols of type CommonSymbol.
 static void replaceCommonSymbols() {
   llvm::TimeTraceScope timeScope("Replace common symbols");
-  for (Symbol *sym : symtab->symbols()) {
-    auto *s = dyn_cast<CommonSymbol>(sym);
-    if (!s)
+  for (ELFFileBase *file : objectFiles) {
+    if (!file->hasCommonSyms)
       continue;
+    for (Symbol *sym : file->getGlobalSymbols()) {
+      auto *s = dyn_cast<CommonSymbol>(sym);
+      if (!s)
+        continue;
 
-    auto *bss = make<BssSection>("COMMON", s->size, s->alignment);
-    bss->file = s->file;
-    bss->markDead();
-    inputSections.push_back(bss);
-    s->replace(Defined{s->file, s->getName(), s->binding, s->stOther, s->type,
-                       /*value=*/0, s->size, bss});
+      auto *bss = make<BssSection>("COMMON", s->size, s->alignment);
+      bss->file = s->file;
+      bss->markDead();
+      inputSections.push_back(bss);
+      s->replace(Defined{s->file, s->getName(), s->binding, s->stOther, s->type,
+                         /*value=*/0, s->size, bss});
+    }
   }
 }
 
diff --git a/lld/ELF/InputFiles.cpp b/lld/ELF/InputFiles.cpp
index 0badf2c..e321b0d 100644
--- a/lld/ELF/InputFiles.cpp
+++ b/lld/ELF/InputFiles.cpp
@@ -1123,6 +1123,7 @@ template <class ELFT> void ObjFile<ELFT>::initializeSymbols() {
       if (value == 0 || value >= UINT32_MAX)
         fatal(toString(this) + ": common symbol '" + name +
               "' has invalid alignment: " + Twine(value));
+      hasCommonSyms = true;
       sym->resolve(
           CommonSymbol{this, name, binding, stOther, type, value, size});
       continue;
diff --git a/lld/ELF/InputFiles.h b/lld/ELF/InputFiles.h
index f58e76e..d622390 100644
--- a/lld/ELF/InputFiles.h
+++ b/lld/ELF/InputFiles.h
@@ -205,12 +205,15 @@ protected:
   // Initializes this class's member variables.
   template <typename ELFT> void init();
 
+  StringRef stringTable;
   const void *elfShdrs = nullptr;
   const void *elfSyms = nullptr;
   uint32_t numELFShdrs = 0;
   uint32_t numELFSyms = 0;
   uint32_t firstGlobal = 0;
-  StringRef stringTable;
+
+public:
+  bool hasCommonSyms = false;
 };
 
 // .o file.
-- 
2.7.4