Fix extract_ir.py for macos. (#260)

* Fix extract_ir.py for macos.

For whatever reason, the section names for .llvmcmd and .llvmbc are different
on macos. One problem is that macos uses macho object files instead of elf, so
in addition to the section names there are segment names that need to be
specified. The other problem is that regardless of the segments, the section
names are different. This patch makes the segment name configurable by a flag,
and gives sensible defaults for linux + documentation for macos.

* Remove spurious changes.

* OS -> file format.

Author: jacob-hegna

compiler_opt/tools/extract_ir.py

@@ -76,6 +76,18 @@
     'passed in the local case.')
 flags.mark_flags_as_required(['output_dir'])
 
+flags.DEFINE_string(
+    'cmd_section_name', '.llvmcmd',
+    'The section name passed to llvm-objcopy. For ELF object files, the '
+    'default .llvmcmd is correct. For Mach-O object files, one should use '
+    'something like __LLVM,__cmdline')
+
+flags.DEFINE_string(
+    'bitcode_section_name', '.llvmbc',
+    'The section name passed to llvm-objcopy. For ELF object files, the '
+    'default .llvmbc is correct. For Mach-O object files, one should use '
+    '__LLVM,__bitcode')
+
 FLAGS = flags.FLAGS
 
 
@@ -98,10 +110,7 @@ def get_thinlto_index(cmdline: str, basedir: str) -> Optional[str]:
 
 
 class TrainingIRExtractor:
-  """IR and command line extraction from an object file.
-
-  The object file is assumed to have the .llvmbc and .llvmcmd sections.
-  """
+  """IR and command line extraction from an object file."""
 
   def __init__(self, obj_relative_path, output_base_dir, obj_base_dir=None):
     """Set up a TrainingIRExtractor.
@@ -156,16 +165,18 @@ def thinlto_index_file(self):
     return os.path.join(self.dest_dir(), self.module_name() + '.thinlto.bc')
 
   def _get_extraction_cmd_command(self, llvm_objcopy_path):
-    """Call llvm_objcopy to extract the .llvmcmd section in self._cmd_file."""
+    """Call llvm_objcopy to extract the llvmcmd section in self._cmd_file."""
     return [
-        llvm_objcopy_path, '--dump-section=.llvmcmd=' + self.cmd_file(),
+        llvm_objcopy_path,
+        '--dump-section=' + FLAGS.cmd_section_name + '=' + self.cmd_file(),
         self.input_obj(), '/dev/null'
     ]
 
   def _get_extraction_bc_command(self, llvm_objcopy_path):
-    """Call llvm_objcopy to extract the .llvmbc section in self._bc_file."""
+    """Call llvm_objcopy to extract the llvmbc section in self._bc_file."""
     return [
-        llvm_objcopy_path, '--dump-section=.llvmbc=' + self.bc_file(),
+        llvm_objcopy_path,
+        '--dump-section=' + FLAGS.bitcode_section_name + '=' + self.bc_file(),
         self.input_obj(), '/dev/null'
     ]
 
@@ -371,4 +382,5 @@ def main(argv):
 
 
 if __name__ == '__main__':
+  multiprocessing.set_start_method('fork')
   app.run(main)