Bug 1569709 - add --enable-profile-{generate,use}=cross option; r=mshal

This change gives us the ability to selectively turn on cross-language
PGO, just like we have the ability to selectively turn on cross-language LTO.

There is room for things to go wrong here: it's not guaranteed that
`--enable-profile-generate=cross` will always be used with
`--enable-profile-use=cross`.  Nothing bad will happen in the sense that
the build will succeed, but it's possible that we miss out on
optimizations on the Rust side.  Either we fail to generate profile data
for the Rust code, or the Rust compiler fails to use the profile data.

In the future, we may want to default to cross-language PGO to avoid
these kind of mismatches.

Differential Revision: https://phabricator.services.mozilla.com/D39727

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

build/moz.configure/lto-pgo.configure

@@ -58,6 +58,8 @@ llvm_profdata = check_prog('LLVM_PROFDATA', ['llvm-profdata'],
                            paths=toolchain_search_path)
 
 js_option('--enable-profile-generate',
+          nargs='?',
+          choices=('cross',),
           help='Build a PGO instrumented binary')
 
 imply_option('MOZ_PGO',
@@ -70,12 +72,17 @@ set_define('MOZ_PROFILE_GENERATE',
            depends_if('--enable-profile-generate')(lambda _: True))
 
 js_option('--enable-profile-use',
+          nargs='?',
+          choices=('cross',),
           help='Use a generated profile during the build')
 
 js_option('--with-pgo-profile-path',
           help='Path to the directory with unmerged profile data to use during the build',
           nargs=1)
 
+js_option('--enable-cross-pgo',
+          help='Enable PGO on Rust code')
+
 imply_option('MOZ_PGO',
              depends_if('--enable-profile-use')(lambda _: True))
 
@@ -108,6 +115,37 @@ option('--with-pgo-jarlog',
 
 set_config('PGO_JARLOG_PATH', depends_if('--with-pgo-jarlog')(lambda p: p))
 
+
+@depends('MOZ_PGO', '--enable-profile-use', '--enable-profile-generate',
+         c_compiler, rustc_info)
+def moz_pgo_rust(pgo, profile_use, profile_generate, c_compiler, rustc):
+    if not pgo:
+        return
+
+    # Enabling PGO through MOZ_PGO only and not --enable* flags.
+    if not profile_use and not profile_generate:
+        return
+
+    if profile_use and profile_generate:
+        die('Cannot build with --enable-profile-use and --enable-profile-generate.')
+
+    want_cross = (len(profile_use) and profile_use[0] == 'cross') \
+                 or (len(profile_generate) and profile_generate[0] == 'cross')
+
+    if not want_cross:
+        return
+
+    if c_compiler.type == 'gcc':
+        die('Cannot use cross-language PGO with GCC.')
+
+    # PGO is not stable prior to 1.37
+    if rustc.version < Version('1.37'):
+        die('Cannot use cross-language PGO with Rust version %s.' % rustc.version)
+
+    return True
+
+set_config('MOZ_PGO_RUST', moz_pgo_rust)
+
 # LTO
 # ==============================================================