[ELF] - Change the way of sorting local symbols.

rLLD329787 added the stable sorting to SymbolTableBaseSection::postThunkContents.

I profiled the Mozilla (response-O0.txt) from lld-speed-test package and found
std::stable_sort is showing up in profile results and consuming the 3.1% of the total
CPU time in the RelWithDebug build. Total time of postThunkContents is 3.54%, 238ms.

This change reduces postTimeContents time to 50ms, making it to take 0.73% of Total CPU time.

So, instead of sorting the local part I suggest to just rebuild it.
That is what this patch does.

Differential revision: https://reviews.llvm.org/D45519

llvm-svn: 335583

lld/ELF/SyntheticSections.cpp

@@ -1762,19 +1762,19 @@ void SymbolTableBaseSection::postThunkContents() {
   size_t NumLocals = E - Symbols.begin();
   getParent()->Info = NumLocals + 1;
 
-  // Assign the growing unique ID for each local symbol's file.
-  DenseMap<InputFile *, unsigned> FileIDs;
-  for (auto I = Symbols.begin(); I != E; ++I)
-    FileIDs.insert({I->Sym->File, FileIDs.size()});
-
-  // Sort the local symbols to group them by file. We do not need to care about
-  // the STT_FILE symbols, they are already naturally placed first in each group.
-  // That happens because STT_FILE is always the first symbol in the object and
-  // hence precede all other local symbols we add for a file.
-  std::stable_sort(Symbols.begin(), E,
-                   [&](const SymbolTableEntry &L, const SymbolTableEntry &R) {
-                     return FileIDs[L.Sym->File] < FileIDs[R.Sym->File];
-                   });
+  // We want to group the local symbols by file. For that we rebuild the local
+  // part of the symbols vector. We do not need to care about the STT_FILE
+  // symbols, they are already naturally placed first in each group. That
+  // happens because STT_FILE is always the first symbol in the object and hence
+  // precede all other local symbols we add for a file.
+  MapVector<InputFile *, std::vector<SymbolTableEntry>> Arr;
+  for (const SymbolTableEntry &S : llvm::make_range(Symbols.begin(), E))
+    Arr[S.Sym->File].push_back(S);
+
+  auto I = Symbols.begin();
+  for (std::pair<InputFile *, std::vector<SymbolTableEntry>> &P : Arr)
+    for (SymbolTableEntry &Entry : P.second)
+      *I++ = Entry;
 }
 
 void SymbolTableBaseSection::addSymbol(Symbol *B) {