GN: Fix NaCl IRT build options

build/config/nacl/BUILD.gn

 # Copyright (c) 2014 The Native Client Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/nacl/config.gni")
+
 # Native Client Definitions
 config("nacl_defines") {
   defines = [
     "_DEFAULT_SOURCE=1",
     "_BSD_SOURCE=1",
     "_POSIX_C_SOURCE=199506",
     "_XOPEN_SOURCE=600",
     "_GNU_SOURCE=1",
     "__STDC_LIMIT_MACROS=1",
   ]
@@ skipping 10 matching lines @@
 # This allows configs to be modified for everything in the NaCl build, even when
 # the NaCl build is composed into the Chrome build.  (GN has no functionality to
 # add flags to everythin in //native_client, having a base target works around
 # that limitation.)
 source_set("nacl_base") {
   public_configs = [ ":nacl_defines" ]
   if (current_os == "nacl") {
     public_configs += [ ":nexe_defines" ]
   }
 }
+
+config("compiler") {
+  configs = []
+  cflags = []
+
+  if (is_clang) {
+    # -no-integrated-as is the default in nacl-clang for historical
+    # compatibility with inline assembly code and so forth.  But there
+    # are no such cases in Chromium code, and -integrated-as is nicer in
+    # general.  Moreover, the IRT must be built using LLVM's assembler
+    # on x86-64 to preserve sandbox base address hiding.  Use it
+    # everywhere for consistency (and possibly quicker builds).
+    cflags += [ "-integrated-as" ]
+  }
+
+  asmflags = cflags
+}
+
+config("compiler_codegen") {
+  cflags = []
+
+  if (is_nacl_irt) {
+    cflags += [
+      # A debugger should be able to unwind IRT call frames.  This is
+      # the default behavior on x86-64 and when compiling C++ with
+      # exceptions enabled; the change is for the benefit of x86-32 C.
+      # The frame pointer is unnecessary when unwind tables are used.
+      "-fasynchronous-unwind-tables",
+      "-fomit-frame-pointer",
+    ]
+
+    if (current_cpu == "x86") {
+      # The x86-32 IRT needs to be callable with an under-aligned
+      # stack; so we disable SSE instructions, which can fault on
+      # misaligned addresses.  See
+      # https://code.google.com/p/nativeclient/issues/detail?id=3935
+      cflags += [
+        "-mstackrealign",
+        "-mno-sse",
+      ]
+    }
+  }
+
+  asmflags = cflags
+}
+
+config("irt_optimize") {
+  cflags = [
+    # Optimize for space, keep the IRT nexe small.
+    "-Os",
+
+    # These are omitted from non-IRT libraries to keep the libraries
+    # themselves small.
+    "-ffunction-sections",
+    "-fdata-sections",
+  ]
+
+  ldflags = [ "-Wl,--gc-sections" ]
+}