Bug 1915571 - Move mac and windows clang stage 2 build to cross-compiles. r=firefox-build-system-reviewers,sergesanspaille

By doing so, we don't really need a stage 1 clang for mac and windows
(with the downside that the final PGOed clang becomes the
clang-toolchain artifact on those platforms), and cross-compiling stage
2 is much faster as a cross-compile, leaving only stage 3 to be native.

While here, for the one build type that still does stage 2 and 3 in one
task, we actually prefer taking the llvm-profdata from stage 1, as it
doesn't contain profile instrumentation (it should actually have been
this way since the beginning).

Differential Revision: https://phabricator.services.mozilla.com/D220654

build/build-clang/build-clang.py

@@ -799,12 +799,16 @@ def main():
             assertions,
             target,
             targets,
-            is_final_stage=(stages == 2),
+            is_final_stage=(stages == 2 and not pgo),
             profile="gen" if pgo else None,
         )
 
     if stages >= 3 and skip_stages < 3:
         stage3_dir = build_dir + "/stage3"
+        if pgo:
+            profiles_dir = build_dir + "/profiles"
+            mkdir_p(profiles_dir)
+            os.environ["LLVM_PROFILE_FILE"] = profiles_dir + "/%m.profraw"
         stage3_inst_dir = stage3_dir + "/" + package_name
         final_stage_dir = stage3_dir
         if skip_stages < 2:
@@ -826,14 +830,16 @@ def main():
             assertions,
             target,
             targets,
-            (stages == 3),
+            is_final_stage=(stages == 3 and not pgo),
         )
         if pgo:
-            llvm_profdata = stage2_inst_dir + "/bin/llvm-profdata%s" % exe_ext
+            del os.environ["LLVM_PROFILE_FILE"]
+            if skip_stages < 1:
+                llvm_profdata = stage1_inst_dir + "/bin/llvm-profdata%s" % exe_ext
+            else:
+                llvm_profdata = get_tool(config, "llvm-profdata")
             merge_cmd = [llvm_profdata, "merge", "-o", "merged.profdata"]
-            profraw_files = glob.glob(
-                os.path.join(stage2_dir, "build", "profiles", "*.profraw")
-            )
+            profraw_files = glob.glob(os.path.join(profiles_dir, "*.profraw"))
             run_in(stage3_dir, merge_cmd + profraw_files)
             if stages == 3:
                 mkdir_p(upload_dir)
@@ -869,7 +875,7 @@ def main():
             assertions,
             target,
             targets,
-            (stages == 4),
+            is_final_stage=(stages == 4),
             profile=profile,
         )