[lld] r358794 - [COFF] Pack Name in Symbol as is done in ELF

[Reid Kleckner via llvm-commits]

Author: rnk
Date: Fri Apr 19 15:51:49 2019
New Revision: 358794

URL: http://llvm.org/viewvc/llvm-project?rev=358794&view=rev
Log:
[COFF] Pack Name in Symbol as is done in ELF

Summary:
This assumes all symbols are <4GB long, so we can store them as a 32-bit
integer. This reorders the fields so the length appears first, packing
with the other bitfield data in the base Symbol object.

This saved 70MB / 3.60% of heap allocations when linking
browser_tests.exe with no PDB. It's not much as a percentage, but worth
doing. I didn't do performance measurements, I don't think it will be
measurable in time.

Reviewers: ruiu, inglorion, amccarth, aganea

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60297

Modified:
    lld/trunk/COFF/Symbols.cpp
    lld/trunk/COFF/Symbols.h

Modified: lld/trunk/COFF/Symbols.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/COFF/Symbols.cpp?rev=358794&r1=358793&r2=358794&view=diff
==============================================================================
--- lld/trunk/COFF/Symbols.cpp (original)
+++ lld/trunk/COFF/Symbols.cpp Fri Apr 19 15:51:49 2019
@@ -20,6 +20,9 @@ using namespace llvm::object;
 
 using namespace lld::coff;
 
+static_assert(sizeof(SymbolUnion) <= 48,
+              "symbols should be optimized for memory usage");
+
 // Returns a symbol name for an error message.
 std::string lld::toString(coff::Symbol &B) {
   if (Config->Demangle)
@@ -39,11 +42,15 @@ StringRef Symbol::getName() {
   // name. Object files contain lots of non-external symbols, and creating
   // StringRefs for them (which involves lots of strlen() on the string table)
   // is a waste of time.
-  if (Name.empty()) {
+  if (NameData == nullptr) {
     auto *D = cast<DefinedCOFF>(this);
-    cast<ObjFile>(D->File)->getCOFFObj()->getSymbolName(D->Sym, Name);
+    StringRef NameStr;
+    cast<ObjFile>(D->File)->getCOFFObj()->getSymbolName(D->Sym, NameStr);
+    NameData = NameStr.data();
+    NameSize = NameStr.size();
+    assert(NameSize == NameStr.size() && "name length truncated");
   }
-  return Name;
+  return StringRef(NameData, NameSize);
 }
 
 InputFile *Symbol::getFile() {
@@ -67,9 +74,10 @@ bool Symbol::isLive() const {
 
 // MinGW specific.
 void Symbol::replaceKeepingName(Symbol *Other, size_t Size) {
-  StringRef OrigName = Name;
+  StringRef OrigName = getName();
   memcpy(this, Other, Size);
-  Name = OrigName;
+  NameData = OrigName.data();
+  NameSize = OrigName.size();
 }
 
 COFFSymbolRef DefinedCOFF::getCOFFSymbol() {

Modified: lld/trunk/COFF/Symbols.h
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/COFF/Symbols.h?rev=358794&r1=358793&r2=358794&view=diff
==============================================================================
--- lld/trunk/COFF/Symbols.h (original)
+++ lld/trunk/COFF/Symbols.h Fri Apr 19 15:51:49 2019
@@ -79,7 +79,8 @@ protected:
   explicit Symbol(Kind K, StringRef N = "")
       : SymbolKind(K), IsExternal(true), IsCOMDAT(false),
         WrittenToSymtab(false), PendingArchiveLoad(false), IsGCRoot(false),
-        IsRuntimePseudoReloc(false), Name(N) {}
+        IsRuntimePseudoReloc(false), NameSize(N.size()),
+        NameData(N.empty() ? nullptr : N.data()) {}
 
   const unsigned SymbolKind : 8;
   unsigned IsExternal : 1;
@@ -106,7 +107,10 @@ public:
   unsigned IsRuntimePseudoReloc : 1;
 
 protected:
-  StringRef Name;
+  // Symbol name length. Assume symbol lengths fit in a 32-bit integer.
+  uint32_t NameSize;
+
+  const char *NameData;
 };
 
 // The base class for any defined symbols, including absolute symbols,
@@ -129,7 +133,7 @@ public:
 // Symbols defined via a COFF object file or bitcode file.  For COFF files, this
 // stores a coff_symbol_generic*, and names of internal symbols are lazily
 // loaded through that. For bitcode files, Sym is nullptr and the name is stored
-// as a StringRef.
+// as a decomposed StringRef.
 class DefinedCOFF : public Defined {
   friend Symbol;