[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

llvm-svn: 358794

1 files changed, 13 insertions, 5 deletions

diff --git a/lld/COFF/Symbols.cpp b/lld/COFF/Symbols.cpp
index e14785d8612..a23429ef835 100644
--- a/lld/COFF/Symbols.cpp
+++ b/lld/COFF/Symbols.cpp
@@ -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() {