Implement DWARF Fission support in Chrome Android.

- Modify some compiler build definitions to allow symbol_level=2 with
Chrome Android 32bit, and to make the build respect use_debug_fission in
those cases.
- Modify "link" and "solink" tools in Android toolchain so that DWP
files can be generated for builds with use_debug_fission.
- Modify partitioned_shared_library template/script so that DWP files
can be generated for partitioned libs when use_debug_fission is set.

Change-Id: I85b755302976d9675ef00c5a79f5184ecc8dbd4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1975081
Commit-Queue: Nelson Billing <nbilling@google.com>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729858}

build/config/compiler/BUILD.gn

@@ -2321,18 +2321,6 @@ config("symbols") {
       # version 7 also produces debug data that is incompatible with Breakpad
       # dump_syms, so this is still required (https://crbug.com/622406).
       cflags += [ "-fno-standalone-debug" ]
-    }
-    if (!use_debug_fission && current_cpu == "arm") {
-      # dump_syms has issues with dwarf4 on arm, https://crbug.com/744956
-      # TODO(thakis): Remove this again once dump_syms is fixed.
-      #
-      # debug fission needs DWARF DIEs to be emitted at version 4.
-      # Chrome OS emits Debug Frame in DWARF2's .debug_frame v1 to make breakpad
-      # happy [1].
-      # Unless Android needs debug fission, DWARF3 is the simplest solution.
-      #
-      # [1] crrev.com/a81d5ade0b043208e06ad71a38bcf9c348a1a52f
-      cflags += [ "-gdwarf-3" ]
     } else if (is_mac) {
       # clang defaults to DWARF2 on macOS unless mac_deployment_target is
       # at least 10.11.
@@ -2350,17 +2338,22 @@ config("symbols") {
       cflags += [ "-g2" ]
     }
 
-    if (use_debug_fission && !is_nacl && !is_android) {
+    ldflags = []
+    if (use_debug_fission && !is_nacl) {
       # NOTE: Some Chrome OS builds globally set |use_debug_fission| to true,
-      # but they also build some targets against Android toolchains which aren't
+      # but they also build some targets whose toolchains aren't
       # compatible with it.
       #
       # TODO(https://crbug.com/837032): See if we can clean this up by e.g. not
       # setting use_debug_fission globally.
       cflags += [ "-gsplit-dwarf" ]
+
+      # NOTE: This flag is for use by the link wrapper scripts. They are also
+      # expected to remove it before sending the rest of ldflags to the actual
+      # linker.
+      ldflags += [ "--generate-dwp" ]
     }
     asmflags = cflags
-    ldflags = []
 
     # TODO(thakis): Figure out if there's a way to make this go for 32-bit,
     # currently we get "warning:

build/config/compiler/compiler.gni

@@ -225,7 +225,8 @@ if (strip_absolute_paths_from_debug_symbols_default) {
 # If it wasn't manually set, set to an appropriate default.
 assert(symbol_level >= -1 && symbol_level <= 2, "Invalid symbol_level")
 if (symbol_level == -1) {
-  if (is_android && !is_component_build) {
+  if (is_android && !is_component_build &&
+      !(use_debug_fission != "default" && use_debug_fission)) {
     # Reduce symbol level when it will cause invalid elf files to be created
     # (due to file size). https://crbug.com/648948.
     symbol_level = 1
@@ -280,12 +281,23 @@ if (forbid_non_component_debug_builds) {
 # An exception is made when target_os == "chromeos" as we only use the Android
 # toolchain there to build relatively small binaries.
 assert(ignore_elf32_limitations || !is_android || target_os == "chromeos" ||
-           is_component_build || symbol_level < 2,
+           is_component_build || symbol_level < 2 ||
+           (use_debug_fission != "default" && use_debug_fission),
        "Android 32-bit non-component builds cannot have symbol_level=2 " +
            "due to 4GiB file size limit, see https://crbug.com/648948. " +
            "If you really want to try this out, " +
            "set ignore_elf32_limitations=true.")
 
+# Assert that we aren't simultaneously expecting ThinLTO and DWARF Fission to
+# work. The way that ThinLTO is implemented in Android build will silently
+# disable DWARF Fission, causing build steps that expect Fission outputs to
+# break. This work should clear a path to having both enabled for Android:
+# https://crbug.com/877722
+assert(!is_android || !use_thin_lto || use_debug_fission == "default" ||
+           !use_debug_fission,
+       "Android builds do not yet support ThinLTO and DWARF Fission " +
+           "simultaneously.")
+
 # This variable is true if the current toolchain is either the default
 # toolchain, or, on Android, the secondary ABI toolchain. In other words, it's
 # a toolchain that generates targets for the product, as opposed to the host

build/extract_partition.py

@@ -30,6 +30,7 @@ def main():
       required=True,
       help='Stripped output file',
       metavar='FILE')
+  parser.add_argument('--dwp', help='Path to dwp binary', metavar='FILE')
   parser.add_argument('input', help='Input file')
   args = parser.parse_args()
 
@@ -46,6 +47,13 @@ def main():
   ]
   subprocess.check_call(objcopy_args)
 
+  if args.dwp:
+    dwp_args = [
+        args.dwp, '-e', args.unstripped_output, '-o',
+        args.stripped_output + '.dwp'
+    ]
+    subprocess.check_call(dwp_args)
+
 
 if __name__ == '__main__':
   sys.exit(main())

build/partitioned_shared_library.gni

@@ -3,7 +3,9 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/android/config.gni")
 import("//build/config/clang/clang.gni")
+import("//build/config/compiler/compiler.gni")
 
 # This template creates a set of shared libraries, by linking a single
 # "partitioned" shared library, then splitting it into multiple pieces.
@@ -102,6 +104,13 @@ template("partitioned_shared_library") {
           "${invoker.partition}",
         ]
       }
+
+      if (use_debug_fission != "default" && use_debug_fission &&
+          symbol_level == 2) {
+        dwp = rebase_path("${android_tool_prefix}dwp", root_build_dir)
+        args += [ "--dwp=${dwp}" ]
+        outputs += [ invoker.stripped_output + ".dwp" ]
+      }
       args += [ rebase_path(sources[0], root_build_dir) ]
     }
   }

build/toolchain/android/BUILD.gn

@@ -29,7 +29,8 @@ template("android_clang_toolchain") {
     # Output linker map files for binary size analysis.
     enable_linker_map = true
 
-    _android_tool_prefix = "$android_toolchain_root/bin/${invoker.binary_prefix}-"
+    _android_tool_prefix =
+        "$android_toolchain_root/bin/${invoker.binary_prefix}-"
 
     # The tools should be run relative to the build dir.
     _tool_prefix = rebase_path("$_android_tool_prefix", root_build_dir)
@@ -41,6 +42,7 @@ template("android_clang_toolchain") {
     ld = cxx
     readelf = _tool_prefix + "readelf"
     nm = _tool_prefix + "nm"
+    dwp = _tool_prefix + "dwp"
     strip = rebase_path("//buildtools/third_party/eu-strip/bin/eu-strip",
                         root_build_dir)
     use_unstripped_as_runtime_outputs = android_unstripped_runtime_outputs

build/toolchain/gcc_link_wrapper.py

@@ -45,6 +45,7 @@ def main():
                       help=('Use --Wl,-Map to generate a map file. Will be '
                             'gzipped if extension ends with .gz'),
                       metavar='FILE')
+  parser.add_argument('--dwp', help=('The dwp binary to run'), metavar='FILE')
   parser.add_argument('--output',
                       required=True,
                       help='Final output executable file',
@@ -53,6 +54,10 @@ def main():
                       help='Linking command')
   args = parser.parse_args()
 
+  generate_dwp = '--generate-dwp' in args.command
+  if generate_dwp:
+    args.command.remove('--generate-dwp')
+
   # Work-around for gold being slow-by-default. http://crbug.com/632230
   fast_env = dict(os.environ)
   fast_env['LC_ALL'] = 'C'
@@ -61,12 +66,29 @@ def main():
   if result != 0:
     return result
 
+  # If dwp is set, then package debug info for this exe.
+  dwp_proc = None
+  if generate_dwp:
+    if not args.dwp:
+      parser.error('--generate-dwp requireds --dwp')
+    exe_file = args.output
+    if args.unstripped_file:
+      exe_file = args.unstripped_file
+    dwp_proc = subprocess.Popen(
+        wrapper_utils.CommandToRun(
+            [args.dwp, '-e', exe_file, '-o', args.output + '.dwp']))
+
   # Finally, strip the linked executable (if desired).
   if args.strip:
     result = subprocess.call(CommandToRun([
         args.strip, '-o', args.output, args.unstripped_file
         ]))
 
+  if dwp_proc:
+    dwp_result = dwp_proc.wait()
+    if dwp_result != 0:
+      return dwp_result
+
   return result
 
 

build/toolchain/gcc_solink_wrapper.py

@@ -70,6 +70,7 @@ def main():
   parser.add_argument('--strip',
                       help='The strip binary to run',
                       metavar='PATH')
+  parser.add_argument('--dwp', help='The dwp binary to run', metavar='PATH')
   parser.add_argument('--sofile',
                       required=True,
                       help='Shared object file produced by linking command',
@@ -102,6 +103,10 @@ def main():
   if link_only:
     args.command.remove('--link-only')
 
+  generate_dwp = '--generate-dwp' in args.command
+  if generate_dwp:
+    args.command.remove('--generate-dwp')
+
   # First, run the actual link.
   command = wrapper_utils.CommandToRun(args.command)
   result = wrapper_utils.RunLinkWithOptionalMapFile(command, env=fast_env,
@@ -130,6 +135,15 @@ def main():
       pass
     return 0
 
+  # If dwp is set, then package debug info for this SO.
+  dwp_proc = None
+  if generate_dwp:
+    if not args.dwp:
+      parser.error('--generate-dwp requireds --dwp')
+    dwp_proc = subprocess.Popen(
+        wrapper_utils.CommandToRun(
+            [args.dwp, '-e', args.sofile, '-o', args.output + '.dwp']))
+
   # Next, generate the contents of the TOC file.
   result, toc = CollectTOC(args)
   if result != 0:
@@ -144,6 +158,11 @@ def main():
     result = subprocess.call(wrapper_utils.CommandToRun(
         [args.strip, '-o', args.output, args.sofile]))
 
+  if dwp_proc:
+    dwp_result = dwp_proc.wait()
+    if dwp_result != 0:
+      return dwp_result
+
   return result
 
 

build/toolchain/gcc_toolchain.gni

@@ -225,6 +225,11 @@ template("gcc_toolchain") {
     } else {
       nm = "nm"
     }
+    if (defined(invoker.dwp)) {
+      dwp = invoker.dwp
+    } else {
+      dwp = "dwp"
+    }
 
     if (defined(invoker.shlib_extension)) {
       default_shlib_extension = invoker.shlib_extension
@@ -394,7 +399,7 @@ template("gcc_toolchain") {
       # The host might not have a POSIX shell and utilities (e.g. Windows).
       solink_wrapper =
           rebase_path("//build/toolchain/gcc_solink_wrapper.py", root_build_dir)
-      command = "$python_path \"$solink_wrapper\" --readelf=\"$readelf\" --nm=\"$nm\" $strip_switch--sofile=\"$unstripped_sofile\" --tocfile=\"$tocfile\"$map_switch --output=\"$sofile\" -- $link_command"
+      command = "$python_path \"$solink_wrapper\" --readelf=\"$readelf\" --nm=\"$nm\" $strip_switch--dwp=\"${dwp}\" --sofile=\"$unstripped_sofile\" --tocfile=\"$tocfile\"$map_switch --output=\"$sofile\" -- $link_command"
 
       if (target_cpu == "mipsel" && is_component_build && is_android) {
         rspfile_content = "-Wl,--start-group -Wl,--whole-archive {{inputs}} {{solibs}} -Wl,--no-whole-archive {{libs}} -Wl,--end-group"
@@ -526,13 +531,9 @@ template("gcc_toolchain") {
         strip_switch = " --strip=\"${invoker.strip}\" --unstripped-file=\"$unstripped_outfile\""
       }
 
-      if (strip_switch != "" || map_switch != "") {
-        link_wrapper =
-            rebase_path("//build/toolchain/gcc_link_wrapper.py", root_build_dir)
-        command = "$python_path \"$link_wrapper\" --output=\"$outfile\"$strip_switch$map_switch -- $link_command"
-      } else {
-        command = link_command
-      }
+      link_wrapper =
+          rebase_path("//build/toolchain/gcc_link_wrapper.py", root_build_dir)
+      command = "$python_path \"$link_wrapper\" --output=\"$outfile\"$strip_switch$map_switch --dwp=\"${dwp}\" -- $link_command"
 
       description = "LINK $outfile"
       rspfile_content = "{{inputs}}"
@@ -596,6 +597,7 @@ template("clang_toolchain") {
     readelf = "${toolprefix}readelf"
     ar = "${prefix}/llvm-ar"
     nm = "${toolprefix}nm"
+    dwp = "${toolprefix}dwp"
 
     forward_variables_from(invoker,
                            [