Bug 1561146 - Enable PGO for Windows aarch64 builds; r=firefox-build-system-reviewers,chmanchester

This build re-uses the PGO profile from the win64 build in the
win64-aarch64-shippable-no-eme part of the aarch64 build. Even though
the profile isn't generated on the smae platform, we still get enough of
a performance win to make this worthwhile.

Note that the pgo_flags() in configure need to be tweaked slightly since
we don't supprt the -fprofile-generate flag for aarch64 (we don't build
the clang_rt.profile lib there). So we always want to return the flags
namespace to make sure we get the use_* versions of flags, which we do
need.

Differential Revision: https://phabricator.services.mozilla.com/D38928

--HG--
extra : moz-landing-system : lando

build/moz.configure/toolchain.configure

@@ -1442,16 +1442,15 @@ def pgo_flags(compiler, build_env, target):
         else:
             gen_ldflags = ['-fprofile-generate']
 
-        if gen_ldflags:
-            return namespace(
-                gen_cflags=[prefix + '-fprofile-generate'],
-                gen_ldflags=gen_ldflags,
-                use_cflags=[prefix + '-fprofile-use=%s' % profdata,
-                            # Some error messages about mismatched profile data
-                            # come in via -Wbackend-plugin, so disable those too.
-                            '-Wno-error=backend-plugin'],
-                use_ldflags=[],
-            )
+        return namespace(
+            gen_cflags=[prefix + '-fprofile-generate'],
+            gen_ldflags=gen_ldflags,
+            use_cflags=[prefix + '-fprofile-use=%s' % profdata,
+                        # Some error messages about mismatched profile data
+                        # come in via -Wbackend-plugin, so disable those too.
+                        '-Wno-error=backend-plugin'],
+            use_ldflags=[],
+        )
 
 
 set_config('PROFILE_GEN_CFLAGS', pgo_flags.gen_cflags)

taskcluster/ci/build/windows.yml

@@ -1119,6 +1119,7 @@ win64-aarch64-shippable/opt:
 
 win64-aarch64-shippable-no-eme/opt:
     description: "AArch64 Win64 Shippable w/o EME"
+    use-pgo: win64-shippable/opt
     index:
         product: firefox
         job-name: win64-aarch64-no-eme-opt