Make the GN build flag 'use_lld' usable on Linux.

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 3ee4412ed255fbde3e0b94f81b767bd353851eac..31b27dc5e810aa71429dec2fa540d53c34fa9fb1 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -45,10 +45,14 @@ declare_args() {
   # Requires profiling to be set to true.
   enable_full_stack_frames_for_profiling = false
 
+  # Set to true to use lld, the LLVM linker.

[brettw, 2016/04/27 22:55:46]

It seems this is Posixonly. If so, can you wrap this in an is_posix block so
it's not listed as available for other platforms? Put the comment inside the
block (the args help looks at the comment immediately above the variable and an
if statement in between will confuse it). If not or if this is going to change,
the comment should talk about when this flag is applicable.

[pcc1, 2016/04/27 23:21:05]

It's also available on Windows. I've added more information to the comment.

+  use_lld = false
+
   # TODO(GYP): We should be using 64-bit gold for linking on both 64-bit Linux
   # and 32-bit linux; 32-bit Gold runs out of address-space on 32-bit builds.
   # However, something isn't quite working right on the 32-bit builds.
-  use_gold = is_linux && (current_cpu == "x64" || current_cpu == "arm")
+  use_gold =
+      is_linux && (current_cpu == "x64" || current_cpu == "arm") && !use_lld

[brettw, 2016/04/27 22:55:46]

This won't actually do what you want. The build overrides are applied at the end
of the declare_args block so this won't see any user overrides of the use_lld
variable.

To fix this, make a new declare_args block after the current one and move
use_gold into it.

[pcc1, 2016/04/27 23:21:05]

Done.

 
   # When we are going to use gold we need to find it.
   # This is initialized below, after use_gold might have been overridden.
@@ -292,7 +296,9 @@ config("compiler") {
 
   # Linux-specific compiler flags setup.
   # ------------------------------------
-  if (use_gold) {
+  if (is_posix && use_lld) {
+    ldflags += [ "-fuse-ld=lld" ]
+  } else if (use_gold) {
     ldflags += [
       "-B$gold_path",
 
@@ -316,11 +322,6 @@ config("compiler") {
       ldflags += [ "-Wl,--gdb-index" ]
     }
 
-    if (!using_sanitizer && !(is_android && use_order_profiling)) {
-      # TODO(brettw) common.gypi has this only for target toolset.
-      ldflags += [ "-Wl,--icf=all" ]
-    }
-
     # TODO(thestig): Make this flag work with GN.
     #if (!is_official_build && !is_chromeos && !(is_asan || is_lsan || is_tsan || is_msan)) {
     #  ldflags += [
@@ -333,6 +334,12 @@ config("compiler") {
     ldflags += [ "-fuse-ld=bfd" ]
   }
 
+  if (is_posix && (use_gold || use_lld) && !using_sanitizer &&
+      !(is_android && use_order_profiling)) {
+    # TODO(brettw) common.gypi has this only for target toolset.

[brettw, 2016/04/27 22:55:46]

I think you can delete this comment.

[pcc1, 2016/04/27 23:21:05]

Done.

+    ldflags += [ "-Wl,--icf=all" ]
+  }
+
   if (linux_use_bundled_binutils) {
     cflags += [ "-B$binutils_path" ]
   }
@@ -413,7 +420,11 @@ config("compiler") {
 
     # Apply a lower LTO optimization level as the default is too slow.
     if (is_linux) {
-      ldflags += [ "-Wl,-plugin-opt,O1" ]
+      if (use_lld) {
+        ldflags += [ "-Wl,--lto-O1" ]
+      } else {
+        ldflags += [ "-Wl,-plugin-opt,O1" ]
+      }
     } else if (is_mac) {
       ldflags += [ "-Wl,-mllvm,-O1" ]
     }