build/toolchain/variant_toolchain_suite.gni - fuchsia - Git at Google

 # Copyright 2020 The Fuchsia Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.

 # NOTE: Event though this file does not directly reference clang_toolchain()
 # nor zircon_toolchain(), invoking it through toolchain_template below, GN will
 # refuse to compile if the template is not previously defined.
 import("//build/toolchain/clang_toolchain.gni")
 import("//build/toolchain/toolchain_tags.gni")
 import("//build/toolchain/variant_tags.gni")
 import("//build/toolchain/zircon/zircon_toolchain.gni")

 # List of keys in toolchain_variant scope related to configs.
 _config_keys = [
   "configs",
   "executable_configs",
   "prefix_configs",
   "remove_common_configs",
   "remove_executable_configs",
   "remove_shared_configs",
 ]

 # Private subroutine of variant_toolchain_suite(), below, used to instantiate
 # a single variant toolchain() instance.
 #
 # Parameters:
 #
 #   toolchain_template: Name of GN template that is invoked to instantiate
 #     a GN toolchain() instance.
 #
 #   toolchain_template_args: Scope of arguments passed to the
 #     $toolchain_template() call. Note that the value of 'toolchain_variant' in
 #     this scope is ignored. Instead, these values should appear in this_variant.
 #
 #   this_variant: Scope of variables that will appear in this variant
 #     toolchain's toolchain_variant global variable.
 #
 #   this_invoker: The variant_toolchain_suite() call's parent invoker.
 #     Only its with_shared and toolchain_args keys will be forwarded,
 #     to its toolchain_variant and toolchain_args scope, respectively.
 #
 #   is_pic_default: Required boolean flag that is forwarded to the
 #     toolchain()'s toolchain_variant scope, overriding any value from
 #     this_variant, if any.
 #
 #   with_shared: Required boolean flag that is forwarded to the toolchain()'s
 #   toolchain_variant scope, overriding any value from this_variant, if any.
 #
 template("_variant_toolchain") {
   this_variant = invoker.this_variant
   this_invoker = invoker.this_invoker
   not_needed([ "this_invoker" ])

   # Every toolchain needs the global select_variant_canonical and
   # all_toolchain_variants passed down explicitly via toolchain_args.
   # See BUILDCONFIG.gn.
   pass_through_select_variant_canonical = select_variant_canonical
   pass_through_all_toolchain_variants = all_toolchain_variants

   _target_type = invoker.toolchain_template

   # Build the toolchain_variant scope used for this toolchain() instance.
   _toolchain_variant = {
     # First forward configs from the variant.
     forward_variables_from(this_variant, "*")

     # Then add toolchain-specific configs.
     if (defined(this_invoker.configs)) {
       if (!defined(configs)) {
         configs = []
       }
       configs += this_invoker.configs
     }
     if (defined(this_invoker.executable_configs)) {
       if (!defined(executable_configs)) {
         executable_configs = []
       }
       executable_configs += this_invoker.executable_configs
     }
     if (defined(this_invoker.prefix_configs)) {
       if (!defined(prefix_configs)) {
         prefix_configs = []
       }
       prefix_configs += this_invoker.prefix_configs
     }
     if (defined(this_invoker.remove_common_configs)) {
       if (!defined(remove_common_configs)) {
         remove_common_configs = []
       }
       remove_common_configs += this_invoker.remove_common_configs
     }
     if (defined(this_invoker.remove_executable_configs)) {
       if (!defined(remove_executable_configs)) {
         remove_executable_configs = []
       }
       remove_executable_configs += this_invoker.remove_executable_configs
     }
     if (defined(this_invoker.remove_shared_configs)) {
       if (!defined(remove_shared_configs)) {
         remove_shared_configs = []
       }
       remove_shared_configs += this_invoker.remove_shared_configs
     }

     is_pic_default = invoker.is_pic_default
     with_shared = invoker.with_shared
   }

   target(_target_type, target_name) {
     forward_variables_from(invoker.toolchain_template_args, "*")
     toolchain_args = {
       # GN's "can't overwrite a nonempty scope/list" rule also applies
       # to shadowing bindings in inner scopes.  Since there is always
       # a global binding of `toolchain_variant` that is being shadowed
       # here, we have to set it to the empty scope first.
       toolchain_variant = {
       }
       toolchain_variant = _toolchain_variant
       if (defined(this_variant.toolchain_args)) {
         forward_variables_from(this_variant.toolchain_args, "*")
       }
       if (defined(this_invoker.toolchain_args)) {
         forward_variables_from(this_invoker.toolchain_args, "*")
       }

       # See comment above.
       select_variant_canonical = []
       select_variant_canonical = pass_through_select_variant_canonical

       all_toolchain_variants = []
       all_toolchain_variants = pass_through_all_toolchain_variants
     }
   }
 }

 # Defines a standard suite of toolchain variants.
 #
 # This creates one or more GN toolchain() instances based on the values of
 # $known_variants, $universal_variants and $select_variant_canonical as
 # defined in BUILDCONFIG.gn.
 #
 # $target_name corresponds to the name of the base toolchain, and all
 # variant toolchains will have a name like ${target_name}-${variant_suffix}
 #
 # Parameters
 #
 #   toolchain_template:
 #     Required: A GN template name that will be invoked to instantiate the
 #     final toolchain() instance.
 #
 #   with_shared:
 #     Optional: If these toolchains will have a "-shared" companion.
 #     (Each variant toolchain has its own "-shared" companion.)
 #
 #   is_pic_default:
 #     Optional: If these toolchains will generate PIC-friendly code by
 #     default on ELF platforms. If true, this is incompatible with
 #     `with_shared == true`.
 #
 #   toolchain_tags:
 #     Optional: A list of tags applied to each toolchain instantiated in this
 #     suite. See //build/toolchain/toolchain_tags.gni for the list of valid
 #     values and their meaning.
 #
 #   exclude_variant_tags:
 #     Optional: A list of variant tags that this toolchain suite should
 #     *not* instantiate.
 #
 #   configs:
 #     Optional: A list of configs added to all targets built by toolchain()
 #     instances in this suite. Note that these are added after the default
 #     configs set in //build/config/BUILDCONFIG.gn.
 #
 #   executable_configs:
 #     Optional: Additional configs only for executable() targets.
 #
 #   prefix_configs:
 #     Optional: Like configs, but these configs are added before the default
 #     configs. Should only be used for kernel toolchains that need to
 #     override the system include directories to provide their own
 #     standard C and C++ headers.
 #
 #   remove_common_configs:
 #     Optional: A list of configs that are removed for all targets built by
 #     toolchain() instances in this suite. Useful to remove specific default
 #     configs when needed.
 #
 #   remove_executable_configs:
 #     Optional: Same as remove_common_configs, but will only apply to
 #     executable() targets.
 #
 #   remove_shared_configs:
 #     Optional: Same as remove_common_configs, but will only apply to
 #     shared_library() targets.
 #
 #   toolchain_args:
 #     Optional: Scope of common variables to be set in the variant toolchains'
 #     toolchain_args scope. Note that values in each selected variant's
 #     toolchain_args scope will override these values.
 #
 #   enable_variants:
 #     Optional: A list of variant names that should always be enabled,
 #     independent of the content of the global `select_variant` list. This
 #     should only be used for specific targets that need it, e.g. the C
 #     library needs to be built with the regular as well as ASan-enabled
 #     toolchains for the Fuchsia SDK.
 #
 # All other parameters are passed to ${toolchain_template}(). See
 # clang_toolchain_instance() and zircon_toolchain_instance() for their values.
 #
 template("variant_toolchain_suite") {
   # Expand all_toolchain_variants into a canonicalized, list of variant specs
   # that don't have `host_only`/`target_only` but instead take the body of one
   # or the other., and have .base.
   is_host = defined(invoker.toolchain_os) && invoker.toolchain_os == host_os
   not_needed([ "is_host" ])

   is_pic_default = defined(invoker.is_pic_default) && invoker.is_pic_default
   with_shared = defined(invoker.with_shared) && invoker.with_shared
   assert(!is_pic_default || !with_shared,
          "is_pic_default and with_shared cannot be true at the same!")

   exclude_tags = []
   if (defined(invoker.exclude_variant_tags)) {
     exclude_tags = invoker.exclude_variant_tags
     _unknown_tags = exclude_tags + all_variant_tags - all_variant_tags
     assert(
         _unknown_tags == [],
         "Unknown exclude_variant_tags ${_unknown_tags} in toolchain suite definition ${target_name}")
   }

   _toolchain_tags = []
   if (defined(invoker.toolchain_tags)) {
     _toolchain_tags = invoker.toolchain_tags
     _unknown_tags = []
     _unknown_tags = _toolchain_tags + all_toolchain_tags - all_toolchain_tags
     assert(
         _unknown_tags == [],
         "Unknown toolchain_tags ${_unknown_tags} in toolchain suite definition ${target_name}")
   }

   # As a special case, always ensure that 'kernel-excluded' or 'kernel-only' is part of
   # exclude_tags, based on whether 'kernel' appears in invoker.toolchain_tags.
   _is_kernel = _toolchain_tags + [ "kernel" ] - [ "kernel" ] != _toolchain_tags
   if (_is_kernel) {
     _kernel_exclude_tags = [ "kernel-excluded" ]
   } else {
     _kernel_exclude_tags = [ "kernel-only" ]
   }
   exclude_tags += _kernel_exclude_tags
   exclude_tags -= _kernel_exclude_tags
   exclude_tags += _kernel_exclude_tags

   # Ensure that `fuchsia-only` xor `excluded` are part or exclude_tags based on whether
   # the `toolchain_os` is fuchsia.
   if (defined(invoker.toolchain_os)) {
     _is_fuchsia = invoker.toolchain_os == "fuchsia"
   } else if (defined(invoker.os)) {
     # Note that the zircon toolchain doesn't define `toolchain_os`. Instead they use `os`.
     _is_fuchsia = invoker.os == "fuchsia"
   } else {
     assert(false,
            "Unable to determine if toolchain $target_name targets fuchsia")
   }
   if (_is_fuchsia) {
     _fuchsia_exclude_tags = [ "fuchsia-excluded" ]
   } else {
     _fuchsia_exclude_tags = [ "fuchsia-only" ]
   }
   exclude_tags += _fuchsia_exclude_tags
   exclude_tags -= _fuchsia_exclude_tags
   exclude_tags += _fuchsia_exclude_tags

   variants = []
   foreach(variant, all_toolchain_variants) {
     variants += [
       {
         base = get_label_info(":$target_name", "label_no_toolchain")

         forward_variables_from(variant,
                                "*",
                                [
                                  "host_only",
                                  "target_only",
                                ])

         if (is_host) {
           if (defined(variant.host_only)) {
             forward_variables_from(variant.host_only, "*")
           }
         } else {
           if (defined(variant.target_only)) {
             forward_variables_from(variant.target_only, "*")
           }
         }

         # NOTE: foreach() is used here to ensure that `unknown_tags` is not
         # recorded in the scope.
         foreach(unknown_tags, [ [] ]) {
           unknown_tags = tags + all_variant_tags - all_variant_tags
           assert(
               unknown_tags == [],
               "Unknown tags ${unknown_tags} in variant definition: ${variant}")
         }

         if (_toolchain_tags != []) {
           tags += _toolchain_tags
           tags -= _toolchain_tags
           tags += _toolchain_tags
         }

         # Recompute `instrumented` flag based on tags.
         instrumented = tags + [ "instrumented" ] - [ "instrumented" ] != tags
       },
     ]
   }

   # Exclude the arguments that are for us or are verboten.
   invoker_toolchain_template_args = {
     forward_variables_from(invoker,
                            "*",
                            [
                                  "enable_variants",
                                  "is_pic_default",
                                  "toolchain_args",
                                  "toolchain_template",
                                  "with_shared",
                                ] + _config_keys)
   }
   not_needed([ "invoker_toolchain_template_args" ])

   # Build up an array of boolean that's parallel to `select_variant`.
   # Below, we'll set `selectors_used[i]` to true when `select_variant[i]`
   # matches the name of some variant.
   selectors_used = []
   i = 0
   foreach(selector, select_variant_canonical) {
     selectors_used += [ false ]
     i = i + 1
   }

   # Handle `enable_variants` here. First ensure that the list of names corresponds
   # to known variant names.
   enabled_variant_names = []
   if (defined(invoker.enable_variants) && invoker.enable_variants != []) {
     _all_variant_names = []
     foreach(variant, variants) {
       _all_variant_names += [ variant.name ]
     }

     enabled_variant_names = invoker.enable_variants
     unknown_enabled_variants =
         invoker.enable_variants + _all_variant_names - _all_variant_names
     assert(
         unknown_enabled_variants == [],
         "Unknown variant names in enable_variants argument: $unknown_enabled_variants")
   }

   foreach(variant, variants) {
     # Figure out if this variant is actually used.  Don't bother to define
     # toolchains for variants that are never used.  This cuts down on extra
     # GN processing so `gn gen` is faster.  More importantly, it ensures
     # that there isn't a toolchain defined using toolchain_args build
     # arguments whose declare_args() block hasn't been imported because
     # it's defined in a BUILD.gn file containing only configs that are
     # never referenced by any target if the variant is never selected.
     variant_used = variant.name == ""
     if (!variant_used) {
       i = 0
       foreach(selector, select_variant_canonical) {
         assert(defined(selector.variant),
                "`select_variant` clause missing `variant` field: $selector")
         if (selector.variant == variant.name) {
           selectors_used[i] = true

           # Only use the variant if its tags allow it.
           if (variant.tags + exclude_tags - exclude_tags == variant.tags) {
             variant_used = true
           }
         } else if (selector.variant == false) {
           selectors_used[i] = true
         }
         i = i + 1
       }
     }

     # Ensure that any variant listed in invoker.enable_variants is used too.
     if (enabled_variant_names != []) {
       if (enabled_variant_names + [ variant.name ] - [ variant.name ] !=
           enabled_variant_names) {
         variant_used = true
       }
     }

     if (variant_used) {
       # Ensure that the exclude tags list is passed to toolchain_variant,
       # as it will be used in BUILDCONFIG.gn by variant_target()
       if (exclude_tags != []) {
         variant.exclude_variant_tags = exclude_tags
       }

       _variant_toolchain("${target_name}${variant.suffix}") {
         toolchain_template = invoker.toolchain_template
         toolchain_template_args = invoker_toolchain_template_args
         this_variant = variant
         this_invoker = invoker
         is_pic_default = is_pic_default
         with_shared = with_shared
       }
       if (with_shared) {
         _variant_toolchain("${target_name}${variant.suffix}-shared") {
           toolchain_template = invoker.toolchain_template
           toolchain_template_args = {
             forward_variables_from(invoker_toolchain_template_args, "*")
             propagates_configs = true
           }
           this_variant = variant
           this_invoker = invoker
           is_pic_default = true
           with_shared = true
         }
       }
     }
   }

   i = 0
   foreach(selector, select_variant_canonical) {
     assert(selectors_used[i],
            "unknown variant in `select_variant` clause: ${selector}")
     i = i + 1
   }
   not_needed([ "selectors_used" ])
 }
	# Copyright 2020 The Fuchsia Authors. All rights reserved.
	# Use of this source code is governed by a BSD-style license that can be
	# found in the LICENSE file.

	# NOTE: Event though this file does not directly reference clang_toolchain()
	# nor zircon_toolchain(), invoking it through toolchain_template below, GN will
	# refuse to compile if the template is not previously defined.
	import("//build/toolchain/clang_toolchain.gni")
	import("//build/toolchain/toolchain_tags.gni")
	import("//build/toolchain/variant_tags.gni")
	import("//build/toolchain/zircon/zircon_toolchain.gni")

	# List of keys in toolchain_variant scope related to configs.
	_config_keys = [
	"configs",
	"executable_configs",
	"prefix_configs",
	"remove_common_configs",
	"remove_executable_configs",
	"remove_shared_configs",
	]

	# Private subroutine of variant_toolchain_suite(), below, used to instantiate
	# a single variant toolchain() instance.
	#
	# Parameters:
	#
	# toolchain_template: Name of GN template that is invoked to instantiate
	# a GN toolchain() instance.
	#
	# toolchain_template_args: Scope of arguments passed to the
	# $toolchain_template() call. Note that the value of 'toolchain_variant' in
	# this scope is ignored. Instead, these values should appear in this_variant.
	#
	# this_variant: Scope of variables that will appear in this variant
	# toolchain's toolchain_variant global variable.
	#
	# this_invoker: The variant_toolchain_suite() call's parent invoker.
	# Only its with_shared and toolchain_args keys will be forwarded,
	# to its toolchain_variant and toolchain_args scope, respectively.
	#
	# is_pic_default: Required boolean flag that is forwarded to the
	# toolchain()'s toolchain_variant scope, overriding any value from
	# this_variant, if any.
	#
	# with_shared: Required boolean flag that is forwarded to the toolchain()'s
	# toolchain_variant scope, overriding any value from this_variant, if any.
	#
	template("_variant_toolchain") {
	this_variant = invoker.this_variant
	this_invoker = invoker.this_invoker
	not_needed([ "this_invoker" ])

	# Every toolchain needs the global select_variant_canonical and
	# all_toolchain_variants passed down explicitly via toolchain_args.
	# See BUILDCONFIG.gn.
	pass_through_select_variant_canonical = select_variant_canonical
	pass_through_all_toolchain_variants = all_toolchain_variants

	_target_type = invoker.toolchain_template

	# Build the toolchain_variant scope used for this toolchain() instance.
	_toolchain_variant = {
	# First forward configs from the variant.
	forward_variables_from(this_variant, "*")

	# Then add toolchain-specific configs.
	if (defined(this_invoker.configs)) {
	if (!defined(configs)) {
	configs = []
	}
	configs += this_invoker.configs
	}
	if (defined(this_invoker.executable_configs)) {
	if (!defined(executable_configs)) {
	executable_configs = []
	}
	executable_configs += this_invoker.executable_configs
	}
	if (defined(this_invoker.prefix_configs)) {
	if (!defined(prefix_configs)) {
	prefix_configs = []
	}
	prefix_configs += this_invoker.prefix_configs
	}
	if (defined(this_invoker.remove_common_configs)) {
	if (!defined(remove_common_configs)) {
	remove_common_configs = []
	}
	remove_common_configs += this_invoker.remove_common_configs
	}
	if (defined(this_invoker.remove_executable_configs)) {
	if (!defined(remove_executable_configs)) {
	remove_executable_configs = []
	}
	remove_executable_configs += this_invoker.remove_executable_configs
	}
	if (defined(this_invoker.remove_shared_configs)) {
	if (!defined(remove_shared_configs)) {
	remove_shared_configs = []
	}
	remove_shared_configs += this_invoker.remove_shared_configs
	}

	is_pic_default = invoker.is_pic_default
	with_shared = invoker.with_shared
	}

	target(_target_type, target_name) {
	forward_variables_from(invoker.toolchain_template_args, "*")
	toolchain_args = {
	# GN's "can't overwrite a nonempty scope/list" rule also applies
	# to shadowing bindings in inner scopes. Since there is always
	# a global binding of `toolchain_variant` that is being shadowed
	# here, we have to set it to the empty scope first.
	toolchain_variant = {
	}
	toolchain_variant = _toolchain_variant
	if (defined(this_variant.toolchain_args)) {
	forward_variables_from(this_variant.toolchain_args, "*")
	}
	if (defined(this_invoker.toolchain_args)) {
	forward_variables_from(this_invoker.toolchain_args, "*")
	}

	# See comment above.
	select_variant_canonical = []
	select_variant_canonical = pass_through_select_variant_canonical

	all_toolchain_variants = []
	all_toolchain_variants = pass_through_all_toolchain_variants
	}
	}
	}

	# Defines a standard suite of toolchain variants.
	#
	# This creates one or more GN toolchain() instances based on the values of
	# $known_variants, $universal_variants and $select_variant_canonical as
	# defined in BUILDCONFIG.gn.
	#
	# $target_name corresponds to the name of the base toolchain, and all
	# variant toolchains will have a name like ${target_name}-${variant_suffix}
	#
	# Parameters
	#
	# toolchain_template:
	# Required: A GN template name that will be invoked to instantiate the
	# final toolchain() instance.
	#
	# with_shared:
	# Optional: If these toolchains will have a "-shared" companion.
	# (Each variant toolchain has its own "-shared" companion.)
	#
	# is_pic_default:
	# Optional: If these toolchains will generate PIC-friendly code by
	# default on ELF platforms. If true, this is incompatible with
	# `with_shared == true`.
	#
	# toolchain_tags:
	# Optional: A list of tags applied to each toolchain instantiated in this
	# suite. See //build/toolchain/toolchain_tags.gni for the list of valid
	# values and their meaning.
	#
	# exclude_variant_tags:
	# Optional: A list of variant tags that this toolchain suite should
	# not instantiate.
	#
	# configs:
	# Optional: A list of configs added to all targets built by toolchain()
	# instances in this suite. Note that these are added after the default
	# configs set in //build/config/BUILDCONFIG.gn.
	#
	# executable_configs:
	# Optional: Additional configs only for executable() targets.
	#
	# prefix_configs:
	# Optional: Like configs, but these configs are added before the default
	# configs. Should only be used for kernel toolchains that need to
	# override the system include directories to provide their own
	# standard C and C++ headers.
	#
	# remove_common_configs:
	# Optional: A list of configs that are removed for all targets built by
	# toolchain() instances in this suite. Useful to remove specific default
	# configs when needed.
	#
	# remove_executable_configs:
	# Optional: Same as remove_common_configs, but will only apply to
	# executable() targets.
	#
	# remove_shared_configs:
	# Optional: Same as remove_common_configs, but will only apply to
	# shared_library() targets.
	#
	# toolchain_args:
	# Optional: Scope of common variables to be set in the variant toolchains'
	# toolchain_args scope. Note that values in each selected variant's
	# toolchain_args scope will override these values.
	#
	# enable_variants:
	# Optional: A list of variant names that should always be enabled,
	# independent of the content of the global `select_variant` list. This
	# should only be used for specific targets that need it, e.g. the C
	# library needs to be built with the regular as well as ASan-enabled
	# toolchains for the Fuchsia SDK.
	#
	# All other parameters are passed to ${toolchain_template}(). See
	# clang_toolchain_instance() and zircon_toolchain_instance() for their values.
	#
	template("variant_toolchain_suite") {
	# Expand all_toolchain_variants into a canonicalized, list of variant specs
	# that don't have `host_only`/`target_only` but instead take the body of one
	# or the other., and have .base.
	is_host = defined(invoker.toolchain_os) && invoker.toolchain_os == host_os
	not_needed([ "is_host" ])

	is_pic_default = defined(invoker.is_pic_default) && invoker.is_pic_default
	with_shared = defined(invoker.with_shared) && invoker.with_shared
	assert(!is_pic_default \|\| !with_shared,
	"is_pic_default and with_shared cannot be true at the same!")

	exclude_tags = []
	if (defined(invoker.exclude_variant_tags)) {
	exclude_tags = invoker.exclude_variant_tags
	_unknown_tags = exclude_tags + all_variant_tags - all_variant_tags
	assert(
	_unknown_tags == [],
	"Unknown exclude_variant_tags ${_unknown_tags} in toolchain suite definition ${target_name}")
	}

	_toolchain_tags = []
	if (defined(invoker.toolchain_tags)) {
	_toolchain_tags = invoker.toolchain_tags
	_unknown_tags = []
	_unknown_tags = _toolchain_tags + all_toolchain_tags - all_toolchain_tags
	assert(
	_unknown_tags == [],
	"Unknown toolchain_tags ${_unknown_tags} in toolchain suite definition ${target_name}")
	}

	# As a special case, always ensure that 'kernel-excluded' or 'kernel-only' is part of
	# exclude_tags, based on whether 'kernel' appears in invoker.toolchain_tags.
	_is_kernel = _toolchain_tags + [ "kernel" ] - [ "kernel" ] != _toolchain_tags
	if (_is_kernel) {
	_kernel_exclude_tags = [ "kernel-excluded" ]
	} else {
	_kernel_exclude_tags = [ "kernel-only" ]
	}
	exclude_tags += _kernel_exclude_tags
	exclude_tags -= _kernel_exclude_tags
	exclude_tags += _kernel_exclude_tags

	# Ensure that `fuchsia-only` xor `excluded` are part or exclude_tags based on whether
	# the `toolchain_os` is fuchsia.
	if (defined(invoker.toolchain_os)) {
	_is_fuchsia = invoker.toolchain_os == "fuchsia"
	} else if (defined(invoker.os)) {
	# Note that the zircon toolchain doesn't define `toolchain_os`. Instead they use `os`.
	_is_fuchsia = invoker.os == "fuchsia"
	} else {
	assert(false,
	"Unable to determine if toolchain $target_name targets fuchsia")
	}
	if (_is_fuchsia) {
	_fuchsia_exclude_tags = [ "fuchsia-excluded" ]
	} else {
	_fuchsia_exclude_tags = [ "fuchsia-only" ]
	}
	exclude_tags += _fuchsia_exclude_tags
	exclude_tags -= _fuchsia_exclude_tags
	exclude_tags += _fuchsia_exclude_tags

	variants = []
	foreach(variant, all_toolchain_variants) {
	variants += [
	{
	base = get_label_info(":$target_name", "label_no_toolchain")

	forward_variables_from(variant,
	"*",
	[
	"host_only",
	"target_only",
	])

	if (is_host) {
	if (defined(variant.host_only)) {
	forward_variables_from(variant.host_only, "*")
	}
	} else {
	if (defined(variant.target_only)) {
	forward_variables_from(variant.target_only, "*")
	}
	}

	# NOTE: foreach() is used here to ensure that `unknown_tags` is not
	# recorded in the scope.
	foreach(unknown_tags, [ [] ]) {
	unknown_tags = tags + all_variant_tags - all_variant_tags
	assert(
	unknown_tags == [],
	"Unknown tags ${unknown_tags} in variant definition: ${variant}")
	}

	if (_toolchain_tags != []) {
	tags += _toolchain_tags
	tags -= _toolchain_tags
	tags += _toolchain_tags
	}

	# Recompute `instrumented` flag based on tags.
	instrumented = tags + [ "instrumented" ] - [ "instrumented" ] != tags
	},
	]
	}

	# Exclude the arguments that are for us or are verboten.
	invoker_toolchain_template_args = {
	forward_variables_from(invoker,
	"*",
	[
	"enable_variants",
	"is_pic_default",
	"toolchain_args",
	"toolchain_template",
	"with_shared",
	] + _config_keys)
	}
	not_needed([ "invoker_toolchain_template_args" ])

	# Build up an array of boolean that's parallel to `select_variant`.
	# Below, we'll set `selectors_used[i]` to true when `select_variant[i]`
	# matches the name of some variant.
	selectors_used = []
	i = 0
	foreach(selector, select_variant_canonical) {
	selectors_used += [ false ]
	i = i + 1
	}

	# Handle `enable_variants` here. First ensure that the list of names corresponds
	# to known variant names.
	enabled_variant_names = []
	if (defined(invoker.enable_variants) && invoker.enable_variants != []) {
	_all_variant_names = []
	foreach(variant, variants) {
	_all_variant_names += [ variant.name ]
	}

	enabled_variant_names = invoker.enable_variants
	unknown_enabled_variants =
	invoker.enable_variants + _all_variant_names - _all_variant_names
	assert(
	unknown_enabled_variants == [],
	"Unknown variant names in enable_variants argument: $unknown_enabled_variants")
	}

	foreach(variant, variants) {
	# Figure out if this variant is actually used. Don't bother to define
	# toolchains for variants that are never used. This cuts down on extra
	# GN processing so `gn gen` is faster. More importantly, it ensures
	# that there isn't a toolchain defined using toolchain_args build
	# arguments whose declare_args() block hasn't been imported because
	# it's defined in a BUILD.gn file containing only configs that are
	# never referenced by any target if the variant is never selected.
	variant_used = variant.name == ""
	if (!variant_used) {
	i = 0
	foreach(selector, select_variant_canonical) {
	assert(defined(selector.variant),
	"`select_variant` clause missing `variant` field: $selector")
	if (selector.variant == variant.name) {
	selectors_used[i] = true

	# Only use the variant if its tags allow it.
	if (variant.tags + exclude_tags - exclude_tags == variant.tags) {
	variant_used = true
	}
	} else if (selector.variant == false) {
	selectors_used[i] = true
	}
	i = i + 1
	}
	}

	# Ensure that any variant listed in invoker.enable_variants is used too.
	if (enabled_variant_names != []) {
	if (enabled_variant_names + [ variant.name ] - [ variant.name ] !=
	enabled_variant_names) {
	variant_used = true
	}
	}

	if (variant_used) {
	# Ensure that the exclude tags list is passed to toolchain_variant,
	# as it will be used in BUILDCONFIG.gn by variant_target()
	if (exclude_tags != []) {
	variant.exclude_variant_tags = exclude_tags
	}

	_variant_toolchain("${target_name}${variant.suffix}") {
	toolchain_template = invoker.toolchain_template
	toolchain_template_args = invoker_toolchain_template_args
	this_variant = variant
	this_invoker = invoker
	is_pic_default = is_pic_default
	with_shared = with_shared
	}
	if (with_shared) {
	_variant_toolchain("${target_name}${variant.suffix}-shared") {
	toolchain_template = invoker.toolchain_template
	toolchain_template_args = {
	forward_variables_from(invoker_toolchain_template_args, "*")
	propagates_configs = true
	}
	this_variant = variant
	this_invoker = invoker
	is_pic_default = true
	with_shared = true
	}
	}
	}
	}

	i = 0
	foreach(selector, select_variant_canonical) {
	assert(selectors_used[i],
	"unknown variant in `select_variant` clause: ${selector}")
	i = i + 1
	}
	not_needed([ "selectors_used" ])
	}