Use llvm-objcpy to strip the producers section, so we strip it by default even in -O0. Followup to #11996, fixes #12071

Author: kripken

ChangeLog.md

@@ -31,13 +31,6 @@ Current Trunk
   `EXPORTED_FUNCTIONS` is not relevant in the deciding the type of application
   to build.
 - Allow polymorphic types to be used without RTTI when using embind. (#10914)
-- Only strip the LLVM producer's section in release builds. In `-O0` builds, we
-  try to leave the wasm from LLVM unmodified as much as possible, so if it
-  emitted the producers section, it will be there. Normally that only matters
-  in release builds, which is not changing here. If you want to not have a
-  producer's section in debug builds, you can remove it a tool like
-  `wasm-opt --strip-producers` (which is what Emscripten still does in release
-  builds, as always) or use `llvm-objcopy`.
 - Only strip debug info in release builds + when `-g` is not present. Previously
   even in an `-O0` build without `-g` we would strip it. This was not documented
   behavior, and has no effect on program behavior, but may be noticeable

emcc.py

@@ -553,8 +553,6 @@ def backend_binaryen_passes():
   if shared.Settings.OPT_LEVEL > 0:
     if shared.Settings.DEBUG_LEVEL < 3:
       passes += ['--strip-debug']
-    if not shared.Settings.EMIT_PRODUCERS_SECTION:
-      passes += ['--strip-producers']
   if shared.Settings.AUTODEBUG:
     # adding '--flatten' here may make these even more effective
     passes += ['--instrument-locals']
@@ -2607,12 +2605,23 @@ def do_binaryen(target, asm_target, options, memfile, wasm_binary_target,
       options.binaryen_passes += ['--pass-arg=emscripten-sbrk-ptr@%d' % shared.Settings.DYNAMICTOP_PTR]
       if shared.Settings.STANDALONE_WASM:
         options.binaryen_passes += ['--pass-arg=emscripten-sbrk-val@%d' % shared.Settings.DYNAMIC_BASE]
-    building.save_intermediate(wasm_binary_target, 'pre-byn.wasm')
-    args = options.binaryen_passes
-    building.run_wasm_opt(wasm_binary_target,
-                          wasm_binary_target,
-                          args=args,
-                          debug=intermediate_debug_info)
+    if options.binaryen_passes:
+      # if we need to strip the producers section, and we have wasm-opt passes
+      # to run, do it with them.
+      if not shared.Settings.EMIT_PRODUCERS_SECTION:
+        options.binaryen_passes += ['--strip-producers']
+      building.save_intermediate(wasm_binary_target, 'pre-byn.wasm')
+      building.run_wasm_opt(wasm_binary_target,
+                            wasm_binary_target,
+                            args=options.binaryen_passes,
+                            debug=intermediate_debug_info)
+    else:
+      # we are not running wasm-opt. if we need to strip the producers section
+      # then do so using llvm-objcpy which is faster and does not rewrite the
+      # code (which is better for debug info)
+      if not shared.Settings.EMIT_PRODUCERS_SECTION:
+        building.save_intermediate(wasm_binary_target, 'pre-noprosec.wasm')
+        building.strip_producers(wasm_binary_target, wasm_binary_target)
 
   if shared.Settings.BINARYEN_SCRIPTS:
     binaryen_scripts = os.path.join(shared.BINARYEN_ROOT, 'scripts')

src/settings.js

@@ -1147,16 +1147,12 @@ var WASM_ASYNC_COMPILATION = 1;
 var WASM_BIGINT = 0;
 
 // WebAssembly defines a "producers section" which compilers and tools can
-// annotate themselves in, and LLVM emits this by default. In release builds,
+// annotate themselves in, and LLVM emits this by default.
 // Emscripten will strip that out so that it is *not* emitted because it
 // increases code size, and also some users may not want information
 // about their tools to be included in their builds for privacy or security
 // reasons, see
 // https://github.com/WebAssembly/tool-conventions/issues/93.
-// (In debug builds (-O0) we leave the wasm file as it is from LLVM, in which
-// case it may contain this section, if you didn't tell LLVM to not emit it. You
-// can also run wasm-opt --strip-producers manually, which is what Emscripten
-// does in release builds for you automatically.)
 var EMIT_PRODUCERS_SECTION = 0;
 
 // If set then generated WASM files will contain a custom

tests/test_other.py

@@ -7898,17 +7898,14 @@ def test_separate_dwarf_with_filename_and_path(self):
       self.assertIn(b'somewhere.com/hosted.wasm', f.read())
 
   @parameterized({
-    'O0': (True, ['-O0']), # unoptimized builds try not to modify the LLVM wasm.
-    'O1': (False, ['-O1']), # optimized builds strip the producer's section
-    'O2': (False, ['-O2']), # by default.
+    'O0': (['-O0'],),
+    'O1': (['-O1'],),
+    'O2': (['-O2'],),
   })
-  def test_wasm_producers_section(self, expect_producers_by_default, args):
+  def test_wasm_producers_section(self, args):
     self.run_process([EMCC, path_from_root('tests', 'hello_world.c')] + args)
     with open('a.out.wasm', 'rb') as f:
       data = f.read()
-    if expect_producers_by_default:
-      self.assertIn('clang', str(data))
-      return
     # if there is no producers section expected by default, verify that, and
     # see that the flag works to add it.
     self.assertNotIn('clang', str(data))

tools/building.py

@@ -1423,6 +1423,14 @@ def wasm2js(js_file, wasm_file, opt_level, minify_whitespace, use_closure_compil
   return js_file
 
 
+def strip_debug(infile, outfile):
+  run_process([LLVM_OBJCOPY, '--remove-section=.debug*', infile, outfile])
+
+
+def strip_producers(infile, outfile):
+  run_process([LLVM_OBJCOPY, '--remove-section=producers', infile, outfile])
+
+
 # extract the DWARF info from the main file, and leave the wasm with
 # debug into as a file on the side
 # TODO: emit only debug sections in the side file, and not the entire
@@ -1435,7 +1443,7 @@ def emit_debug_on_side(wasm_file, wasm_file_with_dwarf):
   embedded_path = shared.Settings.SEPARATE_DWARF_URL or wasm_file_with_dwarf
 
   shutil.move(wasm_file, wasm_file_with_dwarf)
-  run_process([LLVM_OBJCOPY, '--remove-section=.debug*', wasm_file_with_dwarf, wasm_file])
+  strip_debug(wasm_file_with_dwarf, wasm_file)
 
   # embed a section in the main wasm to point to the file with external DWARF,
   # see https://yurydelendik.github.io/webassembly-dwarf/#external-DWARF