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>
llvm-svn: 263086

2 files changed, 19 insertions, 0 deletions

diff --git a/llvm/lib/LTO/LTOCodeGenerator.cpp b/llvm/lib/LTO/LTOCodeGenerator.cpp
index f36dc56774b..a85cf407e6a 100644
--- a/llvm/lib/LTO/LTOCodeGenerator.cpp
+++ b/llvm/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();
 }
 
diff --git a/llvm/lib/LTO/ThinLTOCodeGenerator.cpp b/llvm/lib/LTO/ThinLTOCodeGenerator.cpp
index b17ec2e916c..a99c6e4cf73 100644
--- a/llvm/lib/LTO/ThinLTOCodeGenerator.cpp
+++ b/llvm/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);