[gn build] Separate debug and optimization settings

[dmajor]

Summary

This patch adds an is_optimized variable, orthogonal to is_debug, to allow for a gn analogue to RelWithDebInfo builds.

As part of this we'll want to explicitly enable GC+ICF, for the sake of is_debug && is_optimized builds. The flags normally default to true except that if you pass /DEBUG they default to false.

[thakis]

This is fine for now, but I think eventually we'll want to either make this per-toolchain or put it in a config, for e.g. optimize-tablegen-in-otherwise-non-opt builds.

llvm/trunk/utils/gn/build/BUILD.gn

cflags += [
       "-isysroot",
       mac_sdk_path,
     ]
   }
 
   if (host_os != "win") {
     if (is_debug) {
       cflags += [ "-g" ]
-    } else {
+    }
+    if (is_optimized) {
       cflags += [ "-O3" ]
     }
     cflags += [ "-fdiagnostics-color" ]
     cflags_cc = [
       "-std=c++11",
       "-fno-exceptions",
       "-fvisibility-inlines-hidden",
     ]
   } else {
     if (is_debug) {
       cflags += [
         "/Zi",
         "/FS",
       ]
       ldflags += [ "/DEBUG" ]
-    } else {
+    }
+    if (is_optimized) {
       cflags += [
         "/O2",
         "/Zc:inline",
       ]
+      ldflags += [
+        "/OPT:REF",
+        "/OPT:ICF",
+      ]
     }
     defines += [
       "_CRT_SECURE_NO_DEPRECATE",
       "_CRT_SECURE_NO_WARNINGS",
       "_CRT_NONSTDC_NO_DEPRECATE",
       "_CRT_NONSTDC_NO_WARNINGS",
       "_SCL_SECURE_NO_DEPRECATE",
       "_SCL_SECURE_NO_WARNINGS",

llvm/trunk/utils/gn/build/buildflags.gni

 declare_args() {
-  # Whether to build with debug information and without optimizations.
+  # Whether to build with debug information.
   is_debug = false
 }
 
 # args that depend on other args must live in a later declare_args() block.
 declare_args() {
+  # Whether to build with optimizations.
+  is_optimized = !is_debug
+
   # Whether to enable assertions.
   llvm_enable_assertions = true
 }