Add a flag to the LLVMContext to disable name for Value other than GlobalValue

Summary:
This is intended to be a performance flag, on the same level as clang
cc1 option "--disable-free". LLVM will never initialize it by default,
it will be up to the client creating the LLVMContext to request this
behavior. Clang will do it by default in Release build (just like
--disable-free).

"opt" and "llc" can opt-in using -disable-named-value command line
option.

When performing LTO on llvm-tblgen, the initial merging of IR peaks
at 92MB without this patch, and 86MB after this patch,setNameImpl()
drops from 6.5MB to 0.5MB.
The total link time goes from ~29.5s to ~27.8s.

Compared to a compile-time flag (like the IRBuilder one), it performs
very close. I profiled on SROA and obtain these results:

 420ms with IRBuilder that preserve name
 372ms with IRBuilder that strip name
 375ms with IRBuilder that preserve name, and a runtime flag to strip

Reviewers: chandlerc, dexonsmith, bogner

Subscribers: joker.eph, llvm-commits

Differential Revision: http://reviews.llvm.org/D17946

From: Mehdi Amini <mehdi.amini@apple.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@263086 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/LLVMContext.h

@@ -104,6 +104,15 @@ public:
   /// Remove the GC for a function
   void deleteGC(const Function &Fn);
 
+  /// Return true if the Context runtime configuration is set to discard all
+  /// value names. When true, only GlobalValue names will be available in the
+  /// IR.
+  bool discardValueNames();
+
+  /// Set the Context runtime configuration to discard all value name (but
+  /// GlobalValue). Clients can use this flag to save memory and runtime,
+  /// especially in release mode.
+  void setDiscardValueNames(bool Discard);
 
   typedef void (*InlineAsmDiagHandlerTy)(const SMDiagnostic&, void *Context,
                                          unsigned LocCookie);

lib/AsmParser/LLParser.cpp

@@ -46,6 +46,11 @@ bool LLParser::Run() {
   // Prime the lexer.
   Lex.Lex();
 
+  if (Context.discardValueNames())
+    return Error(
+        Lex.getLoc(),
+        "Can't read textual IR with a Context that discards named Values");
+
   return ParseTopLevelEntities() ||
          ValidateEndOfModule();
 }

lib/IR/LLVMContext.cpp

@@ -325,3 +325,9 @@ const std::string &LLVMContext::getGC(const Function &Fn) {
 void LLVMContext::deleteGC(const Function &Fn) {
   pImpl->GCNames.erase(&Fn);
 }
+
+bool LLVMContext::discardValueNames() { return pImpl->DiscardValueNames; }
+
+void LLVMContext::setDiscardValueNames(bool Discard) {
+  pImpl->DiscardValueNames = Discard;
+}

lib/IR/LLVMContextImpl.h

@@ -1034,6 +1034,10 @@ public:
   /// clients which do use GC.
   DenseMap<const Function*, std::string> GCNames;
 
+  /// Flag to indicate if Value (other than GlobalValue) retains their name or
+  /// not.
+  bool DiscardValueNames = false;
+
   LLVMContextImpl(LLVMContext &C);
   ~LLVMContextImpl();
 

lib/IR/Value.cpp

@@ -198,6 +198,10 @@ StringRef Value::getName() const {
 }
 
 void Value::setNameImpl(const Twine &NewName) {
+  // Fast-path: LLVMContext can be set to strip out non-GlobalValue names
+  if (getContext().discardValueNames() && !isa<GlobalValue>(this))
+    return;
+
   // Fast path for common IRBuilder case of setName("") when there is no name.
   if (NewName.isTriviallyEmpty() && !hasName())
     return;

lib/LTO/LTOCodeGenerator.cpp

@@ -65,9 +65,22 @@ const char* LTOCodeGenerator::getVersionString() {
 #endif
 }
 
+namespace llvm {
+cl::opt<bool> LTODiscardValueNames(
+    "discard-value-names",
+    cl::desc("Strip names from Value (other than GlobalValue)."),
+#ifdef NDEBUG
+    cl::init(true),
+#else
+    cl::init(false),
+#endif
+    cl::Hidden);
+}
+
 LTOCodeGenerator::LTOCodeGenerator(LLVMContext &Context)
     : Context(Context), MergedModule(new Module("ld-temp.o", Context)),
       TheLinker(new Linker(*MergedModule)) {
+  Context.setDiscardValueNames(LTODiscardValueNames);
   initializeLTOPasses();
 }
 

lib/LTO/ThinLTOCodeGenerator.cpp

@@ -41,6 +41,11 @@
 
 using namespace llvm;
 
+namespace llvm {
+// Flags -discard-value-names, defined in LTOCodeGenerator.cpp
+extern cl::opt<bool> LTODiscardValueNames;
+}
+
 namespace {
 
 static cl::opt<int> ThreadCount("threads",
@@ -361,6 +366,7 @@ void ThinLTOCodeGenerator::run() {
     for (auto &ModuleBuffer : Modules) {
       Pool.async([&](int count) {
         LLVMContext Context;
+        Context.setDiscardValueNames(LTODiscardValueNames);
 
         // Parse module now
         auto TheModule = loadModuleFromBuffer(ModuleBuffer, Context, false);

test/Feature/strip_names.ll

@@ -0,0 +1,26 @@
+; RUN: opt < %s -S | FileCheck %s
+; RUN: opt < %s  | opt -S -discard-value-names | FileCheck --check-prefix=NONAME %s
+
+
+; CHECK: @GlobalValueName
+; CHECK: @foo(i32 %in)
+; CHECK: somelabel:
+; CHECK:  %GV = load i32, i32* @GlobalValueName
+; CHECK:  %add = add i32 %in, %GV
+; CHECK:  ret i32 %add
+
+; NONAME: @GlobalValueName
+; NONAME: @foo(i32)
+; NONAME-NOT: somelabel:
+; NONAME:  %2 = load i32, i32* @GlobalValueName
+; NONAME:  %3 = add i32 %0, %2
+; NONAME:  ret i32 %3
+
+@GlobalValueName = global i32 0
+
+define i32 @foo(i32 %in) {
+somelabel:
+  %GV = load i32, i32* @GlobalValueName
+  %add = add i32 %in, %GV
+  ret i32 %add
+}

tools/llc/llc.cpp

@@ -103,6 +103,11 @@ static cl::opt<bool>
                           "manager and verify the result is the same."),
                  cl::init(false));
 
+static cl::opt<bool> DiscardValueNames(
+    "discard-value-names",
+    cl::desc("Discard names from Value (other than GlobalValue)."),
+    cl::init(false), cl::Hidden);
+
 static int compileModule(char **, LLVMContext &);
 
 static std::unique_ptr<tool_output_file>
@@ -205,6 +210,8 @@ int main(int argc, char **argv) {
 
   cl::ParseCommandLineOptions(argc, argv, "llvm system compiler\n");
 
+  Context.setDiscardValueNames(DiscardValueNames);
+
   // Compile the module TimeCompilations times to give better compile time
   // metrics.
   for (unsigned I = TimeCompilations; I; --I)

tools/opt/opt.cpp

@@ -196,6 +196,11 @@ static cl::opt<bool>
              cl::desc("Run all passes twice, re-using the same pass manager."),
              cl::init(false), cl::Hidden);
 
+static cl::opt<bool> DiscardValueNames(
+    "discard-value-names",
+    cl::desc("Discard names from Value (other than GlobalValue)."),
+    cl::init(false), cl::Hidden);
+
 static inline void addPass(legacy::PassManagerBase &PM, Pass *P) {
   // Add the pass to the pass manager...
   PM.add(P);
@@ -346,6 +351,8 @@ int main(int argc, char **argv) {
 
   SMDiagnostic Err;
 
+  Context.setDiscardValueNames(DiscardValueNames);
+
   // Load the input module...
   std::unique_ptr<Module> M = parseIRFile(InputFilename, Err, Context);