GN: Fix NaCl IRT build options

build/config/nacl/BUILD.gn

Index: build/config/nacl/BUILD.gn
diff --git a/build/config/nacl/BUILD.gn b/build/config/nacl/BUILD.gn
index d2643b85e41bfe48415436c510c359d64e098d19..dcd736fe3346a6cd272dd9cd18337693d20b60f4 100644
--- a/build/config/nacl/BUILD.gn
+++ b/build/config/nacl/BUILD.gn
@@ -2,6 +2,8 @@
 # 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 = [
@@ -32,3 +34,62 @@ source_set("nacl_base") {
     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" ]
+}