ThinLTO: workaround LLVM bug related to -nostdlib.

chrome/test/data/nacl/BUILD.gn

 # Copyright 2015 The Chromium 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/features.gni")
 import("//build/config/nacl/config.gni")
+import("//build/toolchain/toolchain.gni")
 import("//ppapi/native_client/nacl_test_data.gni")
 
 group("nacl") {
   newlib = "//build/toolchain/nacl:clang_newlib_${target_cpu}"
   pnacl = "//build/toolchain/nacl:newlib_pnacl"
   data_deps = [
     ":exit_status_test($newlib)",
     ":exit_status_test($pnacl)",
     ":extension_validation_cache($newlib)",
     ":irt_exception_test($newlib)",
@@ skipping 529 matching lines @@
       # Optimizers may translate the original code to code which
       # requires builtin functions and/or relocations. Specifically,
       # the LLVM's optimizer translates for-loop based zero
       # clear to memset.
       "-O0",
 
       # ARM GCC emits symbols like __aeabi_unwind_cpp_pr0 in
       # .exidx sections without this flag.
       "-fno-unwind-tables",
     ]
+
+    # Workaround against LLVM bug.
+    # In short, an LTO optimization pass might recognize
+    # naive implementations of builtins (such as memset)
+    # and replace them with references to the real builtins,
+    # which, in the case of -nostdlib, might cause the binary
+    # to get undefined references to those symbols.
+    #
+    # See more details:
+    # https://llvm.org/bugs/show_bug.cgi?id=30403
+    if (use_thin_lto) {
+      ldflags = [ "-Wl,-plugin-opt,O0" ]
+    }
   }
 
   executable("nonsfi_libc_free_nexe") {
     output_name = "libc_free_$arch"
     output_extension = "nexe"
     sources = [
       "nonsfi/libc_free.c",
     ]
 
     # Here, we would like to link a relocatable, libc-free executable.
@@ skipping 39 matching lines @@
       "nonsfi/libc_free.nmf",
     ]
     outputs = [
       "${root_build_dir}/nacl_test_data/libc-free/{{source_file_part}}",
     ]
     deps = [
       ":nonsfi_libc_free_nexe",
     ]
   }
 }