Add gn arg use_goma_thin_lto to enable ThinLTO code generation on Goma

This adds a gn arg, use_goma_thin_lto. When true, this enables a
linker wrapper which causes ThinLTO code generation to be distributed
using Goma. This can be used to reduce build time compared to a
non-distributed ThinLTO build. The new gn arg allows this feature
to be controlled by the build configuration, allowing distributed
ThinLTO to be rolled out gradually. The arg defaults to false.

R=akhuang,gbiv,hans,rnk,thakis

Bug: 877722
Change-Id: Ibe4dac1669b297e8cb865dcfd3fe512f8f0cdf60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2067858
Commit-Queue: Bob Haarman <inglorion@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744473}

build/config/compiler/compiler.gni

@@ -74,6 +74,9 @@ declare_args() {
                                        (target_os == "chromeos" &&
                                         is_chromeos_device && !is_android)))
 
+  # Performs ThinLTO code generation on Goma.
+  use_goma_thin_lto = false
+
   # Limit the number of jobs (threads/processes) the linker is allowed
   # to use (for linkers that support this).
   max_jobs_per_link = 8
@@ -101,6 +104,9 @@ declare_args() {
 }
 
 assert(!is_cfi || use_thin_lto, "CFI requires ThinLTO")
+assert(
+    !use_goma_thin_lto || (use_goma && use_thin_lto),
+    "use_goma_thin_lto requires both use_goma and use_thin_lto to be enabled")
 
 # If true, optimize for size. Does not affect windows builds.
 # Linux & Mac favor speed over size.

build/toolchain/gcc_toolchain.gni

@@ -157,14 +157,25 @@ template("gcc_toolchain") {
     # But if needs_gomacc_path_arg is set in a Chrome OS build, the toolchain
     # wrapper will have picked up gomacc via cmd-line arg. So need to prepend
     # gomacc in that case.
+    goma_path = "$goma_dir/gomacc"
     if (toolchain_uses_goma &&
         (!defined(invoker_toolchain_args.needs_gomacc_path_arg) ||
          !invoker_toolchain_args.needs_gomacc_path_arg)) {
-      goma_path = "$goma_dir/gomacc"
       compiler_prefix = "${goma_path} "
     } else {
       compiler_prefix = "${toolchain_cc_wrapper} "
     }
+    if (use_goma_thin_lto && toolchain_uses_goma) {
+      # goma_ld.py uses autoninja in an attempt to set a reasonable
+      # number of jobs, but this results in too low a value on
+      # Chrome OS builders. So we pass in an explicit value.
+      goma_ld =
+          rebase_path("//tools/clang/scripts/goma_ld.py", root_build_dir) +
+          " --gomacc ${goma_path} --jobs 100 -- "
+    } else {
+      goma_ld = ""
+      not_needed([ "goma_path" ])
+    }
 
     # Create a distinct variable for "asm", since coverage runs pass a bunch of
     # flags to clang/clang++ that are nonsensical on assembler runs.
@@ -215,7 +226,7 @@ template("gcc_toolchain") {
     cxx = compiler_prefix + invoker.cxx
     asm = asm_prefix + invoker.cc
     ar = invoker.ar
-    ld = invoker.ld
+    ld = "$goma_ld${invoker.ld}"
     if (defined(invoker.readelf)) {
       readelf = invoker.readelf
     } else {