tc_build: Enable '--conservative-instrumentation' for BOLT on Apple Silicon

When performing BOLT on Apple Silicon (such as when building in a UTM
virtual machine), running clang hangs due to instrumentation between
exclusive load and store instructions (see the LLVM issue linked in the
comments for more information).

Detect when running on Apple Silicon and enable the conservative
instrumentation option to avoid hitting this issue.

Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Author: nathanchance

tc_build/llvm.py

@@ -78,6 +78,13 @@ def bolt_clang(self):
                 clang_inst,
                 clang,
             ]
+            # When running an instrumented binary on certain platforms (namely
+            # Apple Silicon), there may be hangs due to instrumentation in
+            # between exclusive load and store instructions:
+            # https://github.com/llvm/llvm-project/issues/153492
+            # Enable conservative instrumentation to avoid this.
+            if tc_build.utils.cpu_is_apple_silicon():
+                clang_inst_cmd.append('--conservative-instrumentation')
             self.run_cmd(clang_inst_cmd)
 
             self.bolt_builder.bolt_instrumentation = True

tc_build/utils.py

@@ -1,10 +1,21 @@
 #!/usr/bin/env python3
 
+from pathlib import Path
 import subprocess
 import sys
+import re
 import time
 
 
+def cpu_is_apple_silicon():
+    cpuinfo = Path('/proc/cpuinfo').read_text(encoding='utf-8')
+    if match := re.search(r"implementer\s+:\s+(\w+)", cpuinfo):
+        # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/include/asm/cputype.h?h=v6.17-rc4#n62
+        return match.groups()[0] == '0x61'
+    # If we cannot prove that it is Apple Silicon, we assume it is not
+    return False
+
+
 def create_gitignore(folder):
     folder.joinpath('.gitignore').write_text('*\n', encoding='utf-8')