Add yasm to the GN build.

This is forked off of https://codereview.chromium.org/266613002 The code to compile yasm itself is mostly from that patch, with a few updates for the other changes.

Adds a template for running compiled binaries. Compared to Albert's patch above, this assumes the binary is generated by the source tree so can have a cleaner interface (just specify the label of the tool you use).

The yasm rule is new compared to Albert's patch. It uses a special wrapper script instead of the new compiled_action templates so it can properly support depfiles. This also adds convenient support for defines and include dirs.

This adds some trivial ios changes to the content/public/browser. This should be a NOP now.

Fixes a bug in GN depfile creation. Previously, the .d file was emitted as an output of the script, but this confuses ninja. This change just removes that.

BUG=
R=ajwong@chromium.org

Review URL: https://codereview.chromium.org/321323004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@276772 0039d316-1c4b-4281-b951-d872f2087c98

3 files changed, 638 insertions, 0 deletions

diff --git a/third_party/yasm/BUILD.gn b/third_party/yasm/BUILD.gn
new file mode 100644
index 0000000..7fd3065
--- /dev/null
+++ b/third_party/yasm/BUILD.gn
@@ -0,0 +1,402 @@
+# Copyright 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# The yasm build process creates a slew of small C subprograms that
+# dynamically generate files at various point in the build process.  This makes
+# the build integration moderately complex.
+#
+# There are three classes of dynamically generated files:
+#   1) C source files that should be included in the build (eg., lc3bid.c)
+#   2) C source files that are #included by static C sources (eg., license.c)
+#   3) Intermediate files that are used as input by other subprograms to
+#      further generate files in category #1 or #2.  (eg., version.mac)
+#
+# This structure is represented with the following targets:
+#   1) yasm -- Sources, flags for the main yasm executable. Also has most of
+#              of the actions and rules that invoke the subprograms.
+#   2) yasm_config -- General build configuration including setting a
+#              source_prereqs listing the checked in version of files
+#              generated by manually running configure. These manually
+#              generated files are used by all binaries.
+#   3) yasm_utils -- Object files with memory management and hashing utilities
+#              shared between yasm and the genperf subprogram.
+#   4) genmacro, genmodule, etc. -- One executable target for each subprogram.
+#   5) generate_license, generate_module, etc. -- Actions that invoke programs
+#              built in #4 to generate .c files.
+#   6) compile_gperf, compile_re2c, etc. -- Actions that invoke programs that
+#              turn intermediate files into .c files.
+
+if (default_toolchain == host_toolchain) {
+
+  # Various files referenced by multiple targets.
+  yasm_gen_include_dir = "$target_gen_dir/include"
+  config_makefile = "source/config/$os/Makefile"
+  version_file = "version.mac"
+
+  import("//build/compiled_action.gni")
+
+  config("yasm_config") {
+    include_dirs = [
+      "source/config/$os",
+      "source/patched-yasm",
+    ]
+    source_prereqs = [
+      config_makefile,
+      "source/config/$os/config.h",
+      "source/config/$os/libyasm-stdint.h",
+    ]
+    defines = [ "HAVE_CONFIG_H" ]
+    cflags = [ "-std=gnu99" ]
+  }
+
+  executable("genmacro") {
+    sources = [ "source/patched-yasm/tools/genmacro/genmacro.c" ]
+    configs -= [ "//build/config/compiler:chromium_code" ]
+    configs += [ ":yasm_config",
+                 "//build/config/compiler:no_chromium_code" ]
+  }
+
+  executable("genmodule") {
+    sources = [ "source/patched-yasm/libyasm/genmodule.c" ]
+    configs -= [ "//build/config/compiler:chromium_code" ]
+    configs += [ ":yasm_config",
+                 "//build/config/compiler:no_chromium_code" ]
+  }
+
+  executable("genperf") {
+    sources = [
+      "source/patched-yasm/tools/genperf/genperf.c",
+      "source/patched-yasm/tools/genperf/perfect.c",
+    ]
+
+    configs -= [ "//build/config/compiler:chromium_code" ]
+    configs += [ ":yasm_config",
+                 "//build/config/compiler:no_chromium_code" ]
+
+    deps = [ ":yasm_utils" ]
+  }
+
+  # Used by both yasm and genperf binaries.
+  source_set("yasm_utils") {
+    sources = [
+      "source/patched-yasm/libyasm/phash.c",
+      "source/patched-yasm/libyasm/xmalloc.c",
+      "source/patched-yasm/libyasm/xstrdup.c",
+    ]
+
+    configs -= [ "//build/config/compiler:chromium_code" ]
+    configs += [ ":yasm_config",
+                 "//build/config/compiler:no_chromium_code" ]
+  }
+
+  executable("genstring") {
+    sources = [ "source/patched-yasm/genstring.c", ]
+    configs -= [ "//build/config/compiler:chromium_code" ]
+    configs += [ ":yasm_config",
+                 "//build/config/compiler:no_chromium_code" ]
+  }
+
+  executable("genversion") {
+    sources = [ "source/patched-yasm/modules/preprocs/nasm/genversion.c" ]
+    configs -= [ "//build/config/compiler:chromium_code" ]
+    configs += [ ":yasm_config",
+                 "//build/config/compiler:no_chromium_code" ]
+  }
+
+  executable("re2c") {
+    sources = [
+       "source/patched-yasm/tools/re2c/main.c",
+       "source/patched-yasm/tools/re2c/code.c",
+       "source/patched-yasm/tools/re2c/dfa.c",
+       "source/patched-yasm/tools/re2c/parser.c",
+       "source/patched-yasm/tools/re2c/actions.c",
+       "source/patched-yasm/tools/re2c/scanner.c",
+       "source/patched-yasm/tools/re2c/mbo_getopt.c",
+       "source/patched-yasm/tools/re2c/substr.c",
+       "source/patched-yasm/tools/re2c/translate.c",
+    ]
+
+    configs -= [ "//build/config/compiler:chromium_code" ]
+    configs += [ ":yasm_config",
+                 "//build/config/compiler:no_chromium_code" ]
+
+    # re2c is missing CLOSEVOP from one switch.
+    cflags = [ "-Wno-switch" ]
+  }
+
+  executable("yasm") {
+    sources = [
+      "source/patched-yasm/frontends/yasm/yasm-options.c",
+      "source/patched-yasm/frontends/yasm/yasm.c",
+      "source/patched-yasm/libyasm/assocdat.c",
+      "source/patched-yasm/libyasm/bc-align.c",
+      "source/patched-yasm/libyasm/bc-data.c",
+      "source/patched-yasm/libyasm/bc-incbin.c",
+      "source/patched-yasm/libyasm/bc-org.c",
+      "source/patched-yasm/libyasm/bc-reserve.c",
+      "source/patched-yasm/libyasm/bitvect.c",
+      "source/patched-yasm/libyasm/bytecode.c",
+      "source/patched-yasm/libyasm/errwarn.c",
+      "source/patched-yasm/libyasm/expr.c",
+      "source/patched-yasm/libyasm/file.c",
+      "source/patched-yasm/libyasm/floatnum.c",
+      "source/patched-yasm/libyasm/hamt.c",
+      "source/patched-yasm/libyasm/insn.c",
+      "source/patched-yasm/libyasm/intnum.c",
+      "source/patched-yasm/libyasm/inttree.c",
+      "source/patched-yasm/libyasm/linemap.c",
+      "source/patched-yasm/libyasm/md5.c",
+      "source/patched-yasm/libyasm/mergesort.c",
+      "source/patched-yasm/libyasm/section.c",
+      "source/patched-yasm/libyasm/strcasecmp.c",
+      "source/patched-yasm/libyasm/strsep.c",
+      "source/patched-yasm/libyasm/symrec.c",
+      "source/patched-yasm/libyasm/valparam.c",
+      "source/patched-yasm/libyasm/value.c",
+      "source/patched-yasm/modules/arch/lc3b/lc3barch.c",
+      "source/patched-yasm/modules/arch/lc3b/lc3bbc.c",
+      "source/patched-yasm/modules/arch/x86/x86arch.c",
+      "source/patched-yasm/modules/arch/x86/x86bc.c",
+      "source/patched-yasm/modules/arch/x86/x86expr.c",
+      "source/patched-yasm/modules/arch/x86/x86id.c",
+      "source/patched-yasm/modules/dbgfmts/codeview/cv-dbgfmt.c",
+      "source/patched-yasm/modules/dbgfmts/codeview/cv-symline.c",
+      "source/patched-yasm/modules/dbgfmts/codeview/cv-type.c",
+      "source/patched-yasm/modules/dbgfmts/dwarf2/dwarf2-aranges.c",
+      "source/patched-yasm/modules/dbgfmts/dwarf2/dwarf2-dbgfmt.c",
+      "source/patched-yasm/modules/dbgfmts/dwarf2/dwarf2-info.c",
+      "source/patched-yasm/modules/dbgfmts/dwarf2/dwarf2-line.c",
+      "source/patched-yasm/modules/dbgfmts/null/null-dbgfmt.c",
+      "source/patched-yasm/modules/dbgfmts/stabs/stabs-dbgfmt.c",
+      "source/patched-yasm/modules/listfmts/nasm/nasm-listfmt.c",
+      "source/patched-yasm/modules/objfmts/bin/bin-objfmt.c",
+      "source/patched-yasm/modules/objfmts/coff/coff-objfmt.c",
+      "source/patched-yasm/modules/objfmts/coff/win64-except.c",
+      "source/patched-yasm/modules/objfmts/dbg/dbg-objfmt.c",
+      "source/patched-yasm/modules/objfmts/elf/elf-objfmt.c",
+      "source/patched-yasm/modules/objfmts/elf/elf-x86-amd64.c",
+      "source/patched-yasm/modules/objfmts/elf/elf-x86-x86.c",
+      "source/patched-yasm/modules/objfmts/elf/elf.c",
+      "source/patched-yasm/modules/objfmts/macho/macho-objfmt.c",
+      "source/patched-yasm/modules/objfmts/rdf/rdf-objfmt.c",
+      "source/patched-yasm/modules/objfmts/xdf/xdf-objfmt.c",
+      "source/patched-yasm/modules/parsers/gas/gas-parse.c",
+      "source/patched-yasm/modules/parsers/gas/gas-parse-intel.c",
+      "source/patched-yasm/modules/parsers/gas/gas-parser.c",
+      "source/patched-yasm/modules/parsers/nasm/nasm-parse.c",
+      "source/patched-yasm/modules/parsers/nasm/nasm-parser.c",
+      "source/patched-yasm/modules/preprocs/cpp/cpp-preproc.c",
+      "source/patched-yasm/modules/preprocs/nasm/nasm-eval.c",
+      "source/patched-yasm/modules/preprocs/nasm/nasm-pp.c",
+      "source/patched-yasm/modules/preprocs/nasm/nasm-preproc.c",
+      "source/patched-yasm/modules/preprocs/nasm/nasmlib.c",
+      "source/patched-yasm/modules/preprocs/raw/raw-preproc.c",
+
+      # Files generated by compile_gperf
+      "$target_gen_dir/x86cpu.c",
+      "$target_gen_dir/x86regtmod.c",
+
+      # Files generated by compile_re2c
+      "$target_gen_dir/gas-token.c",
+      "$target_gen_dir/nasm-token.c",
+
+      # File generated by compile_re2c_lc3b
+      "$target_gen_dir/lc3bid.c",
+
+      # File generated by generate_module
+      "$target_gen_dir/module.c"
+    ]
+
+    configs -= [ "//build/config/compiler:chromium_code" ]
+    configs += [ ":yasm_config",
+                 "//build/config/compiler:no_chromium_code" ]
+
+    # Yasm generates a bunch of .c files which its source file #include.
+    # Add the |target_gen_dir| into the include path so it can find them.
+    # Ideally, these generated .c files would be placed into a separate
+    # directory, but the gen_x86_insn.py script does not make this easy.
+    include_dirs = [ yasm_gen_include_dir ]
+
+    cflags = [ "-ansi", "-pedantic" ]
+    if (is_clang) {
+      cflags += [ "-Wno-incompatible-pointer-types" ]
+    }
+
+    # TODO(ajwong): This should take most of the generated output as
+    # source_prereqs.
+    deps = [
+      ":compile_gperf",
+      ":compile_gperf_for_include",
+      ":compile_nasm_macros",
+      ":compile_nasm_version",
+      ":compile_win64_gas",
+      ":compile_win64_nasm",
+      ":compile_re2c",
+      ":generate_license",
+      ":generate_module",
+      ":generate_version",
+      ":yasm_utils",
+    ]
+  }
+
+  compiled_action_foreach("compile_gperf") {
+    tool = ":genperf"
+    sources = [
+      "source/patched-yasm/modules/arch/x86/x86cpu.gperf",
+      "source/patched-yasm/modules/arch/x86/x86regtmod.gperf",
+    ]
+
+    outputs = [ "$target_gen_dir/{{source_name_part}}.c" ]
+    args = [
+      "{{source}}",
+      rebase_path(target_gen_dir, ".") + "/{{source_name_part}}.c",
+    ]
+    deps = [
+      ":generate_x86_insn"
+    ]
+  }
+
+  # This differs from |compile_gperf| in where it places it output files.
+  compiled_action_foreach("compile_gperf_for_include") {
+    tool = ":genperf"
+    sources = [
+      # Make sure the generated gperf files in $target_gen_dir are synced with
+      # the outputs for the related generate_*_insn actions in the
+      # generate_files target below.
+      #
+      # The output for these two are #included by
+      #   source/patched-yasm/modules/arch/x86/x86id.c
+      "$yasm_gen_include_dir/x86insn_gas.gperf",
+      "$yasm_gen_include_dir/x86insn_nasm.gperf",
+    ]
+
+    outputs = [ "$yasm_gen_include_dir/{{source_name_part}}.c" ]
+    args = [
+      "{{source}}",
+      rebase_path(yasm_gen_include_dir, ".") + "/{{source_name_part}}.c",
+    ]
+    deps = [
+      ":generate_x86_insn"
+    ]
+  }
+
+  template("compile_macro") {
+    compiled_action(target_name) {
+      tool = ":genmacro"
+      # Output #included by source/patched-yasm/frontends/yasm/yasm.c.
+      sources = invoker.sources
+      outputs = invoker.outputs
+      args = [
+        rebase_path(outputs[0], root_build_dir),
+        invoker.macro_varname,
+        rebase_path(sources[0], root_build_dir),
+      ]
+      if (defined(invoker.deps)) {
+        deps = invoker.deps
+      }
+    }
+  }
+
+  compile_macro("compile_nasm_macros") {
+    # Output #included by
+    #   source/patched-yasm/modules/preprocs/nasm/nasm-parser.c
+    sources = [ "source/patched-yasm/modules/parsers/nasm/nasm-std.mac" ]
+    outputs = [ "$yasm_gen_include_dir/nasm-macros.c" ]
+    macro_varname = "nasm_standard_mac"
+  }
+
+  compile_macro("compile_nasm_version") {
+    # Output #included by
+    #   source/patched-yasm/modules/preprocs/nasm/nasm-preproc.c
+    sources = [ "$target_gen_dir/$version_file" ]
+    outputs = [ "$yasm_gen_include_dir/nasm-version.c" ]
+    macro_varname = "nasm_version_mac"
+    deps = [ ":generate_version" ]
+  }
+
+  compile_macro("compile_win64_gas") {
+    # Output #included by source/patched-yasm/frontends/yasm/yasm.c.
+    sources = [ "source/patched-yasm/modules/objfmts/coff/win64-gas.mac" ]
+    outputs = [ "$yasm_gen_include_dir/win64-gas.c" ]
+    macro_varname = "win64_gas_stdmac"
+  }
+
+  compile_macro("compile_win64_nasm") {
+    # Output #included by source/patched-yasm/frontends/yasm/yasm.c.
+    sources = [ "source/patched-yasm/modules/objfmts/coff/win64-nasm.mac" ]
+    outputs = [ "$yasm_gen_include_dir/win64-nasm.c" ]
+    macro_varname = "win64_nasm_stdmac"
+  }
+
+  compiled_action_foreach("compile_re2c") {
+    tool = ":re2c"
+    sources = [
+      "source/patched-yasm/modules/parsers/gas/gas-token.re",
+      "source/patched-yasm/modules/parsers/nasm/nasm-token.re",
+    ]
+    outputs = [ "$target_gen_dir/{{source_name_part}}.c" ]
+    args = [
+      "-b",
+      "-o",
+      rebase_path(target_gen_dir, ".") + "/{{source_name_part}}.c",
+      "{{source}}",
+    ]
+  }
+
+  # This call doesn't fit into the re2c template above.
+  compiled_action("compile_re2c_lc3b") {
+    tool = ":re2c"
+    sources = [ "source/patched-yasm/modules/arch/lc3b/lc3bid.re" ]
+    outputs = [ "$target_gen_dir/lc3bid.c" ]
+    args = [
+      "-s",
+      "-o",
+      rebase_path(outputs[0], root_build_dir),
+      rebase_path(sources[0], root_build_dir),
+    ]
+  }
+
+  compiled_action("generate_license") {
+    tool = ":genstring"
+    # Output #included by source/patched-yasm/frontends/yasm/yasm.c.
+    sources = [ "source/patched-yasm/COPYING" ]
+    outputs = [ "$yasm_gen_include_dir/license.c" ]
+    args = [
+      "license_msg",
+      rebase_path(outputs[0], root_build_dir),
+      rebase_path(sources[0], root_build_dir),
+    ]
+  }
+
+  compiled_action("generate_module") {
+    tool = ":genmodule"
+    sources = [ "source/patched-yasm/libyasm/module.in" ]
+    outputs = [ "$target_gen_dir/module.c" ]
+    args = [
+      rebase_path(sources[0], root_build_dir),
+      rebase_path(config_makefile, root_build_dir),
+      rebase_path(outputs[0], root_build_dir),
+    ]
+  }
+
+  compiled_action("generate_version") {
+    tool = ":genversion"
+    outputs = [ "$target_gen_dir/$version_file" ]
+    args = [
+      rebase_path(outputs[0],
+      root_build_dir)
+    ]
+  }
+
+  action("generate_x86_insn") {
+    script = "source/patched-yasm/modules/arch/x86/gen_x86_insn.py"
+    # Output eventually #included by source/patched-yasm/frontends/yasm/x86id.c
+    outputs = [
+      "$yasm_gen_include_dir/x86insns.c",
+      "$yasm_gen_include_dir/x86insn_gas.gperf",
+      "$yasm_gen_include_dir/x86insn_nasm.gperf",
+    ]
+    args = [ rebase_path(yasm_gen_include_dir, root_build_dir) ]
+  }
+}
diff --git a/third_party/yasm/run_yasm.py b/third_party/yasm/run_yasm.py
new file mode 100644
index 0000000..cbd79cc
--- /dev/null
+++ b/third_party/yasm/run_yasm.py
@@ -0,0 +1,51 @@
+# Copyright 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""A wrapper to run yasm.
+
+Its main job is to provide a Python wrapper for GN integration, and to write
+the makefile-style output yasm generates in stdout to a .d file for dependency
+management of .inc files.
+
+Run with:
+  python run_yasm.py <yasm_binary_path> <all other yasm args>
+
+Note that <all other yasm args> must include an explicit output file (-o). This
+script will append a ".d" to this and write the dependencies there. This script
+will add "-M" to cause yasm to write the deps to stdout, so you don't need to
+specify that.
+"""
+
+import argparse
+import sys
+import subprocess
+
+# Extract the output file name from the yasm command line so we can generate a
+# .d file with the same base name.
+parser = argparse.ArgumentParser()
+parser.add_argument("-o", dest="objfile")
+options, _ = parser.parse_known_args()
+
+objfile = options.objfile
+depfile = objfile + '.d'
+
+# Assemble.
+result_code = subprocess.call(sys.argv[1:])
+if result_code != 0:
+  sys.exit(result_code)
+
+# Now generate the .d file listing the dependencies. The -M option makes yasm
+# write the Makefile-style dependencies to stdout, but it seems that inhibits
+# generating any compiled output so we need to do this in a separate pass.
+# However, outputting deps seems faster than actually assembling, and yasm is
+# so fast anyway this is not a big deal.
+#
+# This guarantees proper dependency management for assembly files. Otherwise,
+# we would have to require people to manually specify the .inc files they
+# depend on in the build file, which will surely be wrong or out-of-date in
+# some cases.
+deps = subprocess.check_output(sys.argv[1:] + ['-M'])
+with open(depfile, "wb") as f:
+  f.write(deps)
+
diff --git a/third_party/yasm/yasm_assemble.gni b/third_party/yasm/yasm_assemble.gni
new file mode 100644
index 0000000..9044efa
--- /dev/null
+++ b/third_party/yasm/yasm_assemble.gni
@@ -0,0 +1,185 @@
+# Copyright 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# This provides the yasm_assemble() template which uses YASM to assemble
+# assembly files.
+#
+# Files to be assembled with YASM should have an extension of .asm.
+#
+# Parameters
+#
+#   yasm_flags (optional)
+#       [list of strings] Pass additional flags into YASM. These are appended
+#       to the command line. Note that the target machine type and system is
+#       already set up based on the current toolchain so you don't need to
+#       specify these things (see below).
+#
+#       Example: yasm_flags = [ "--force-strict" ]
+#
+#   include_dirs (optional)
+#       [list of dir names] List of additional include dirs. Note that the
+#       source root and the root generated file dir is always added, just like
+#       our C++ build sets up.
+#
+#       Example: include_dirs = [ "//some/other/path", target_gen_dir ]
+#
+#   defines (optional)
+#       [list of strings] List of defines, as with the native code defines.
+#
+#       Example: defines = [ "FOO", "BAR=1" ]
+#
+#   source_prereqs, deps, visibility  (optional)
+#       These have the same meaning as in an action.
+#
+# Example
+#
+#   yasm_assemble("my_yasm_target") {
+#     sources = [
+#       "ultra_optimized_awesome.asm",
+#     ]
+#     include_dirs = [ "assembly_include" ]
+#   }
+
+if (is_mac || is_ios) {
+  if (cpu_arch == "x86") {
+    _yasm_flags = [
+      "-fmacho32",
+      "-m", "x86",
+    ]
+  } else if (cpu_arch == "x64") {
+    _yasm_flags = [
+      "-fmacho64",
+      "-m", "amd64",
+    ]
+  }
+} else if (is_posix) {
+  if (cpu_arch == "x86") {
+    _yasm_flags = [
+      "-felf32",
+      "-m", "x86",
+    ]
+  } else if (cpu_arch == "x64") {
+    _yasm_flags = [
+      "-DPIC",
+      "-felf64",
+      "-m", "amd64",
+    ]
+  }
+} else if (is_win) {
+  if (cpu_arch == "x86") {
+    _yasm_flags = [
+      "-DPREFIX",
+      "-fwin32",
+      "-m", "x86",
+    ]
+  } else if (cpu_arch == "x64") {
+    _yasm_flags = [
+      "-fwin64",
+      "-m", "amd64",
+    ]
+  }
+}
+
+# Default yasm include dirs. Make it match the native build (source root and
+# root generated code directory).
+_yasm_flags += [
+  # Using "//." will produce a relative path "../.." which looks better than
+  # "../../" which will result from using "//" as the base (although both
+  # work). This is because rebase_path will terminate the result in a slash if
+  # the input ends in a slash.
+  "-I" + rebase_path("//.", root_build_dir),
+  "-I" + rebase_path(root_gen_dir, root_build_dir),
+]
+
+if (is_win) {
+  asm_obj_extension = "obj"
+} else {
+  asm_obj_extension = "o"
+}
+
+template("yasm_assemble") {
+  # TODO(ajwong): Support use_system_yasm.
+  assert(defined(invoker.sources), "Need sources defined for $target_name")
+
+  # Only depend on YASM on x86 systems. Force compilation of .asm files for
+  # ARM to fail.
+  assert(cpu_arch == "x86" || cpu_arch == "x64")
+
+  action_name = "${target_name}_action"
+  source_set_name = target_name
+
+  action_foreach(action_name) {
+    visibility = ":$source_set_name"  # Only the source set can depend on this.
+
+    script = "//third_party/yasm/run_yasm.py"
+    sources = invoker.sources
+
+    if (defined(invoker.source_prereqs)) {
+      source_prereqs = invoker.source_prereqs
+    }
+
+    # Executable (first in the args). The binary might be in the root build dir
+    # (no cross-compiling) or in a toolchain-specific subdirectory of that
+    # (when cross-compiling).
+    yasm_label = "//third_party/yasm($host_toolchain)"
+    args = [ "./" +  # Force current dir.
+      rebase_path(get_label_info(yasm_label, "root_out_dir") + "/yasm",
+                  root_build_dir)
+    ]
+
+    # Deps.
+    deps = [ yasm_label ]
+    if (defined(invoker.deps)) {
+      deps += invoker.deps
+    }
+
+    # Flags.
+    args += _yasm_flags
+    if (defined(invoker.yasm_flags)) {
+      args += invoker.yasm_flags
+    }
+
+    # Extra include directories.
+    if (defined(invoker.include_dirs)) {
+      foreach(include, invoker.include_dirs) {
+        args += [ "-I" + rebase_path(include, root_build_dir) ]
+      }
+    }
+
+    # Extra defines.
+    if (defined(invoker.defines)) {
+      foreach(def, invoker.defines) {
+        args += [ "-D$def" ]
+      }
+    }
+
+    # Output file.
+    #
+    # TODO(brettw) it might be nice if there was a source expansion for the
+    # path of the source file relative to the source root. Then we could
+    # exactly duplicate the naming and location of object files from the
+    # native build, which would be:
+    # "$root_out_dir/${target_name}.{{source_dir_part}}.$asm_obj_extension"
+    outputs = [ "$target_out_dir/{{source_name_part}}.o" ]
+    args += [
+      "-o", rebase_path(outputs[0], root_build_dir),
+      "{{source}}"
+    ]
+
+    # The wrapper script run_yasm will write the depfile to the same name as
+    # the output but with .d appended (like gcc will).
+    depfile = outputs[0] + ".d"
+  }
+
+  # Gather the .o files into a linkable thing. This doesn't actually link
+  # anything (a source set just compiles files to link later), but will pass
+  # the object files generated by the action up the dependency chain.
+  source_set(source_set_name) {
+    if (defined(invoker.visibility)) {
+      visibility = invoker.visibility
+    }
+
+    sources = get_target_outputs(":$action_name")
+  }
+}