Add --host-arch and --target-arch, and 'host-arches' conditional

This command adds initial cross-compilation support to BuildStream.

It has been tested against a converted version of the Baserock
compiler bootstrap and used to cross build sysroots for armv8l64 and ppc64l
from an x86_64 host.

For example, to build a sysroot for ARM v8 64-bit you can do this:

    bst build --target-arch=armv8b64 gnu-toolchain/stage2.bst

This would cause the adapted Baserock definitions to produce a stage1 simple
cross compiler that runs on the native architecture and produces armv8b64
binaries, and then cross build a stage2 sysroot that executes on armv8b64.

Currently the --host-arch option does nothing of use. It will one day
enable host-incompatible builds using a QEMU-powered cross sandbox.

The `--arch=` option is now shorthand for `--host-arch= --target-arch=`.

Elements have 2 new variables available, %{bst-host-arch} and
%{bst-target-arch}. The 'arches' conditional now follows %{bst-target-arch},
while the new 'host-arches' conditional follows %{bst-host-arch}. All
of --arch, --host-arch and --target-arch default to the output of `uname -a`.

There's no magic here that would make all BuildStream elements suddenly
able to cross compile. It is up to an individual element to support this by
honouring %{bst-target-arch} in whatever way makes sense.

14 files changed, 232 insertions, 97 deletions

diff --git a/buildstream/_frontend/main.py b/buildstream/_frontend/main.py
index 0224ce50e..fa8ea12a6 100644
--- a/buildstream/_frontend/main.py
+++ b/buildstream/_frontend/main.py
@@ -98,15 +98,20 @@ def cli(context, **kwargs):
 @click.option('--track', default=False, is_flag=True,
               help="Track new source references before building (implies --all)")
 @click.option('--arch', '-a', default=host_machine,
-              help="The target architecture (default: %s)" % host_machine)
+              help="Architecture of the machine running the build (default: %s)" % host_machine)
+@click.option('--host-arch',
+              help="Run as a native build for the given architecture (defaults to --arch)")
+@click.option('--target-arch',
+              help="Produce elements that execute on this architecture (defaults to --arch)")
 @click.option('--variant',
               help='A variant of the specified target')
 @click.argument('target')
 @click.pass_obj
-def build(app, target, arch, variant, all, track):
+def build(app, target, arch, host_arch, target_arch, variant, all, track):
     """Build elements in a pipeline"""
 
-    app.initialize(target, arch, variant, rewritable=track, inconsistent=track)
+    app.initialize(target, host_arch or arch, target_arch or arch, variant,
+                   rewritable=track, inconsistent=track)
     app.print_heading()
     try:
         app.pipeline.build(app.scheduler, all, track)
@@ -128,12 +133,16 @@ def build(app, target, arch, variant, all, track):
 @click.option('--track', default=False, is_flag=True,
               help="Track new source references before fetching")
 @click.option('--arch', '-a', default=host_machine,
-              help="The target architecture (default: %s)" % host_machine)
+              help="Architecture of the machine running the build (default: %s)" % host_machine)
+@click.option('--host-arch',
+              help="Run as a native build for the given architecture (defaults to --arch)")
+@click.option('--target-arch',
+              help="Produce elements that execute on this architecture (defaults to --arch)")
 @click.option('--variant',
               help='A variant of the specified target')
 @click.argument('target')
 @click.pass_obj
-def fetch(app, target, arch, variant, deps, track, except_):
+def fetch(app, target, arch, host_arch, target_arch, variant, deps, track, except_):
     """Fetch sources required to build the pipeline
 
     By default this will only try to fetch sources which are
@@ -148,7 +157,8 @@ def fetch(app, target, arch, variant, deps, track, except_):
         plan:  Only dependencies required for the build plan
         all:   All dependencies
     """
-    app.initialize(target, arch, variant, rewritable=track, inconsistent=track)
+    app.initialize(target, host_arch or arch, target_arch or arch, variant,
+                   rewritable=track, inconsistent=track)
     try:
         dependencies = app.pipeline.deps_elements(deps, except_)
         app.print_heading(deps=dependencies)
@@ -169,12 +179,16 @@ def fetch(app, target, arch, variant, deps, track, except_):
               type=click.Choice(['none', 'all']),
               help='The dependencies to track (default: none)')
 @click.option('--arch', '-a', default=host_machine,
-              help="The target architecture (default: %s)" % host_machine)
+              help="Architecture of the machine running the build (default: %s)" % host_machine)
+@click.option('--host-arch',
+              help="Run as a native build for the given architecture (defaults to --arch)")
+@click.option('--target-arch',
+              help="Produce elements that execute on this architecture (defaults to --arch)")
 @click.option('--variant',
               help='A variant of the specified target')
 @click.argument('target')
 @click.pass_obj
-def track(app, target, arch, variant, deps, except_):
+def track(app, target, arch, host_arch, target_arch, variant, deps, except_):
     """Consults the specified tracking branches for new versions available
     to build and updates the project with any newly available references.
 
@@ -187,7 +201,8 @@ def track(app, target, arch, variant, deps, except_):
         none:  No dependencies, just the element itself
         all:   All dependencies
     """
-    app.initialize(target, arch, variant, rewritable=True, inconsistent=True)
+    app.initialize(target, host_arch or arch, target_arch or arch, variant,
+                   rewritable=True, inconsistent=True)
     try:
         dependencies = app.pipeline.deps_elements(deps, except_)
         app.print_heading(deps=dependencies)
@@ -214,12 +229,16 @@ def track(app, target, arch, variant, deps, except_):
               type=click.STRING,
               help='Format string for each element')
 @click.option('--arch', '-a', default=host_machine,
-              help="The target architecture (default: %s)" % host_machine)
+              help="Architecture of the machine running the build (default: %s)" % host_machine)
+@click.option('--host-arch',
+              help="Run as a native build for the given architecture (defaults to --arch)")
+@click.option('--target-arch',
+              help="Produce elements that execute on this architecture (defaults to --arch)")
 @click.option('--variant',
               help='A variant of the specified target')
 @click.argument('target')
 @click.pass_obj
-def show(app, target, arch, variant, deps, except_, order, format):
+def show(app, target, arch, host_arch, target_arch, variant, deps, except_, order, format):
     """Show elements in the pipeline
 
     By default this will show all of the dependencies of the
@@ -264,7 +283,7 @@ def show(app, target, arch, variant, deps, except_, order, format):
         bst show target.bst --format \\
             $'---------- %{name} ----------\\n%{vars}'
     """
-    app.initialize(target, arch, variant)
+    app.initialize(target, host_arch or arch, target_arch or arch, variant)
     try:
         dependencies = app.pipeline.deps_elements(deps, except_)
     except PipelineError as e:
@@ -289,12 +308,16 @@ def show(app, target, arch, variant, deps, except_, order, format):
               type=click.Choice(['build', 'run']),
               help='Specify element scope to stage')
 @click.option('--arch', '-a', default=host_machine,
-              help="The target architecture (default: %s)" % host_machine)
+              help="Architecture of the machine running the build (default: %s)" % host_machine)
+@click.option('--host-arch',
+              help="Run as a native build for the given architecture (defaults to --arch)")
+@click.option('--target-arch',
+              help="Produce elements that execute on this architecture (defaults to --arch)")
 @click.option('--variant',
               help='A variant of the specified target')
 @click.argument('target')
 @click.pass_obj
-def shell(app, target, arch, variant, builddir, scope):
+def shell(app, target, arch, host_arch, target_arch, variant, builddir, scope):
     """Shell into an element's sandbox environment
 
     This can be used either to debug building or to launch
@@ -313,7 +336,7 @@ def shell(app, target, arch, variant, builddir, scope):
     elif scope == "build":
         scope = Scope.BUILD
 
-    app.initialize(target, arch, variant)
+    app.initialize(target, host_arch or arch, target_arch or arch, variant)
 
     # Assert we have everything we need built.
     missing_deps = []
@@ -346,16 +369,20 @@ def shell(app, target, arch, variant, builddir, scope):
 @click.option('--force', '-f', default=False, is_flag=True,
               help="Overwrite files existing in checkout directory")
 @click.option('--arch', '-a', default=host_machine,
-              help="The target architecture (default: %s)" % host_machine)
+              help="Architecture of the machine running the build (default: %s)" % host_machine)
+@click.option('--host-arch',
+              help="Run as a native build for the given architecture (defaults to --arch)")
+@click.option('--target-arch',
+              help="Produce elements that execute on this architecture (defaults to --arch)")
 @click.option('--variant',
               help='A variant of the specified target')
 @click.argument('target')
 @click.argument('directory')
 @click.pass_obj
-def checkout(app, target, arch, variant, directory, force):
+def checkout(app, target, arch, host_arch, target_arch, variant, directory, force):
     """Checkout a built artifact to the specified directory
     """
-    app.initialize(target, arch, variant)
+    app.initialize(target, host_arch or arch, target_arch or arch, variant)
     try:
         app.pipeline.checkout(directory, force)
         click.echo("")
@@ -380,7 +407,11 @@ def checkout(app, target, arch, variant, directory, force):
 @click.option('--track', default=False, is_flag=True,
               help="Track new source references before building")
 @click.option('--arch', '-a', default=host_machine,
-              help="The target architecture (default: %s)" % host_machine)
+              help="Architecture of the machine running the build (default: %s)" % host_machine)
+@click.option('--host-arch',
+              help="Run as a native build for the given architecture (defaults to --arch)")
+@click.option('--target-arch',
+              help="Produce elements that execute on this architecture (defaults to --arch)")
 @click.option('--variant',
               help='A variant of the specified target')
 @click.option('--force', '-f', default=False, is_flag=True,
@@ -389,7 +420,7 @@ def checkout(app, target, arch, variant, directory, force):
               help="The directory to write the tarball to")
 @click.argument('target')
 @click.pass_obj
-def source_bundle(app, target, arch, variant, force, directory,
+def source_bundle(app, target, arch, host_arch, target_arch, variant, force, directory,
                   track, deps, compression, except_):
     """Produce a build bundle to be manually executed
 
@@ -400,7 +431,8 @@ def source_bundle(app, target, arch, variant, force, directory,
         run:   Runtime dependencies, including the element itself
         build: Build time dependencies, excluding the element itself
     """
-    app.initialize(target, arch, variant, rewritable=track, inconsistent=track)
+    app.initialize(target, host_arch or arch, target_arch or arch, variant,
+                   rewritable=track, inconsistent=track)
     try:
         dependencies = app.pipeline.deps_elements(deps, except_)
         app.print_heading(dependencies)
@@ -425,7 +457,8 @@ class App():
         self.logger = None
         self.status = None
         self.target = None
-        self.arch = None
+        self.host_arch = None
+        self.target_arch = None
         self.variant = None
 
         # Main asset handles
@@ -473,18 +506,20 @@ class App():
     #
     # Initialize the main pipeline
     #
-    def initialize(self, target, arch, variant, rewritable=False, inconsistent=False):
+    def initialize(self, target, host_arch, target_arch, variant,
+                   rewritable=False, inconsistent=False):
         self.target = target
-        self.arch = arch
+        self.host_arch = host_arch
+        self.target_arch = target_arch
         self.variant = variant
 
-        profile_start(Topics.LOAD_PIPELINE, target.replace(os.sep, '-') + '-' + arch)
+        profile_start(Topics.LOAD_PIPELINE, target.replace(os.sep, '-') + '-' + host_arch + '-' + target_arch)
 
         directory = self.main_options['directory']
         config = self.main_options['config']
 
         try:
-            self.context = Context(arch)
+            self.context = Context(host_arch, target_arch)
             self.context.load(config)
         except _BstError as e:
             click.echo("Error loading user configuration: %s" % str(e))
@@ -534,7 +569,7 @@ class App():
         self.context._set_message_handler(self.message_handler)
 
         try:
-            self.project = Project(directory, arch)
+            self.project = Project(directory, host_arch, target_arch)
         except _BstError as e:
             click.echo("Error loading project: %s" % str(e))
             sys.exit(1)
@@ -559,7 +594,7 @@ class App():
         self.logger.size_request(self.pipeline)
         self.messaging_enabled = True
 
-        profile_end(Topics.LOAD_PIPELINE, target.replace(os.sep, '-') + '-' + arch)
+        profile_end(Topics.LOAD_PIPELINE, target.replace(os.sep, '-') + '-' + host_arch + '-' + target_arch)
 
     #
     # Render the status area, conditional on some internal state
diff --git a/buildstream/_frontend/widget.py b/buildstream/_frontend/widget.py
index a9f574d32..760a40b6a 100644
--- a/buildstream/_frontend/widget.py
+++ b/buildstream/_frontend/widget.py
@@ -426,7 +426,8 @@ class LogLine(Widget):
         values["Session Start"] = starttime.strftime('%A, %d-%m-%Y at %H:%M:%S')
         values["Project"] = "{} ({})".format(project.name, project.directory)
         values["Target"] = pipeline.target.name
-        values["Machine Architecture"] = context.arch
+        values["Host Architecture"] = context.host_arch
+        values["Target Architecture"] = context.target_arch
         values["Variant"] = variant
         text += self.format_values(values)
 
diff --git a/buildstream/_loader.py b/buildstream/_loader.py
index f8196ed8e..fd2eef655 100644
--- a/buildstream/_loader.py
+++ b/buildstream/_loader.py
@@ -45,6 +45,7 @@ class Symbol():
     VARIANT = "variant"
     VARIANTS = "variants"
     ARCHES = "arches"
+    HOST_ARCHES = "host-arches"
     SOURCES = "sources"
     CONFIG = "config"
     VARIABLES = "variables"
@@ -117,9 +118,11 @@ class VariantError(Exception):
 # the arches dict from the data node afterwards, this is shared
 # with project.py
 #
-def resolve_arch(data, active_arch):
+def resolve_arch(data, host_arch, target_arch=None):
+
+    def resolve_single_arch_conditional(symbol, active_arch):
+        arches = _yaml.node_get(data, Mapping, symbol, default_value={})
 
-        arches = _yaml.node_get(data, Mapping, Symbol.ARCHES, default_value={})
         arch = {}
         if arches:
             arch = _yaml.node_get(arches, Mapping, active_arch, default_value={})
@@ -133,12 +136,16 @@ def resolve_arch(data, active_arch):
                 provenance = _yaml.node_get_provenance(arch, key=active_arch)
                 raise LoadError(LoadErrorReason.ILLEGAL_COMPOSITE,
                                 "%s: Arch %s specifies type '%s' for path '%s', expected '%s'" %
-                                (str(provenance), active_arch,
+                                (str(provenance),
+                                 active_arch,
                                  e.actual_type.__name__,
                                  e.path,
                                  e.expected_type.__name__)) from e
 
-        del data[Symbol.ARCHES]
+        del data[symbol]
+
+    resolve_single_arch_conditional(Symbol.HOST_ARCHES, active_arch=host_arch)
+    resolve_single_arch_conditional(Symbol.ARCHES, active_arch=target_arch or host_arch)
 
 
 # A transient object breaking down what is loaded
@@ -147,11 +154,12 @@ def resolve_arch(data, active_arch):
 #
 class LoadElement():
 
-    def __init__(self, data, filename, basedir, arch, elements):
+    def __init__(self, data, filename, basedir, host_arch, target_arch, elements):
 
         self.filename = filename
         self.data = data
-        self.arch = arch
+        self.host_arch = host_arch
+        self.target_arch = target_arch
         self.name = filename
         self.elements = elements
 
@@ -159,7 +167,7 @@ class LoadElement():
         self.basedir = basedir
 
         # Process arch conditionals
-        resolve_arch(self.data, self.arch)
+        resolve_arch(self.data, self.host_arch, self.target_arch)
 
         # Dependency objects after resolving variants
         self.variant_name = None
@@ -178,7 +186,7 @@ class LoadElement():
             variant = Variant(self.name, variant_node)
 
             # Process arch conditionals on individual variants
-            resolve_arch(variant.data, self.arch)
+            resolve_arch(variant.data, self.host_arch, self.target_arch)
             self.variants.append(variant)
 
         if len(self.variants) == 1:
@@ -356,7 +364,7 @@ def extract_depends_from_node(owner, data):
 #
 class Loader():
 
-    def __init__(self, basedir, filename, variant, arch):
+    def __init__(self, basedir, filename, variant, host_arch, target_arch):
 
         # Ensure we have an absolute path for the base directory
         #
@@ -381,8 +389,8 @@ class Loader():
         # Optional variant
         self.target_variant = variant
 
-        # Build architecture
-        self.arch = arch
+        self.host_arch = host_arch
+        self.target_arch = target_arch
 
         self.loaded_files = {}   # Table of files we've already loaded
         self.meta_elements = {}  # Dict of resolved meta elements by name
@@ -465,7 +473,7 @@ class Loader():
 
         # Load the element and track it in our elements table
         data = _yaml.load(fullpath, shortname=filename, copy_tree=rewritable)
-        element = LoadElement(data, filename, self.basedir, self.arch, self.elements)
+        element = LoadElement(data, filename, self.basedir, self.host_arch, self.target_arch, self.elements)
 
         self.elements[filename] = element
 
diff --git a/buildstream/_pipeline.py b/buildstream/_pipeline.py
index f931a0e83..c1eb3a863 100644
--- a/buildstream/_pipeline.py
+++ b/buildstream/_pipeline.py
@@ -140,7 +140,8 @@ class Pipeline():
         self.element_factory = ElementFactory(pluginbase, project._plugin_element_paths)
         self.source_factory = SourceFactory(pluginbase, project._plugin_source_paths)
 
-        loader = Loader(self.project.element_path, target, target_variant, context.arch)
+        loader = Loader(self.project.element_path, target, target_variant,
+                        context.host_arch, context.target_arch)
         meta_element = loader.load(rewritable, load_ticker)
         if load_ticker:
             load_ticker(None)
diff --git a/buildstream/context.py b/buildstream/context.py
index 33aae86ae..0ad894d96 100644
--- a/buildstream/context.py
+++ b/buildstream/context.py
@@ -45,15 +45,19 @@ class Context():
     """Context of how BuildStream was invoked
 
     Args:
-       arch (string): The target architecture to build for
+       host_arch (string): The desired architecture on which to run the build
+       target_arch (string): The machine on which the results of the build should execute
     """
-    def __init__(self, arch):
+    def __init__(self, host_arch, target_arch=None):
 
         self.config_origin = None
         """Filename indicating which configuration file was used, or None for the defaults"""
 
-        self.arch = arch
-        """The target architecture to build for"""
+        self.host_arch = host_arch
+        """The desired architecture on which to run the build"""
+
+        self.target_arch = target_arch or host_arch
+        """The machine on which the results of the build should execute"""
 
         self.sourcedir = None
         """The directory where various sources are stored"""
@@ -205,7 +209,8 @@ class Context():
 
             # Anything that alters the build goes into the unique key
             self._cache_key = utils._generate_key({
-                'arch': self.arch
+                'host-arch': self.host_arch,
+                'target-arch': self.target_arch
             })
 
         return self._cache_key
diff --git a/buildstream/project.py b/buildstream/project.py
index 6af6657f5..6ec270857 100644
--- a/buildstream/project.py
+++ b/buildstream/project.py
@@ -44,12 +44,16 @@ class Project():
 
     Args:
        directory (str): The project directory
-       arch (str): Symbolic machine architecture name
+       host_arch (str): Symbolic host machine architecture name
+       target_arch (str): Symbolic target machine architecture name
 
     Raises:
        :class:`.LoadError`
     """
-    def __init__(self, directory, arch):
+    def __init__(self, directory, host_arch, target_arch=None):
+
+        host_arch = host_arch
+        target_arch = target_arch or host_arch
 
         self.name = None
         """str: The project name"""
@@ -69,7 +73,7 @@ class Project():
         self._cache_key = None
 
         profile_start(Topics.LOAD_PROJECT, self.directory.replace(os.sep, '-'))
-        self._load(arch)
+        self._load(host_arch, target_arch)
         profile_end(Topics.LOAD_PROJECT, self.directory.replace(os.sep, '-'))
 
     def translate_url(self, url):
@@ -101,7 +105,7 @@ class Project():
     #
     # Raises: LoadError if there was a problem with the project.conf
     #
-    def _load(self, arch):
+    def _load(self, host_arch, target_arch):
 
         # Load builtin default
         projectfile = os.path.join(self.directory, "project.conf")
@@ -111,14 +115,20 @@ class Project():
         # be processed here before compositing any overrides
         variables = _yaml.node_get(config, Mapping, 'variables')
         variables['max-jobs'] = multiprocessing.cpu_count()
-        variables['bst-arch'] = arch
+
+        variables['bst-host-arch'] = host_arch
+        variables['bst-target-arch'] = target_arch
+
+        # This is kept around for compatibility with existing definitions,
+        # but we should probably remove it due to being ambiguous.
+        variables['bst-arch'] = host_arch
 
         # Load project local config and override the builtin
         project_conf = _yaml.load(projectfile)
         _yaml.composite(config, project_conf, typesafe=True)
 
         # Resolve arches keyword, project may have arch conditionals
-        _loader.resolve_arch(config, arch)
+        _loader.resolve_arch(config, host_arch, target_arch)
 
         # The project name
         self.name = _yaml.node_get(config, str, 'name')
diff --git a/doc/source/format.rst b/doc/source/format.rst
index f77f3a368..ebe8e2636 100644
--- a/doc/source/format.rst
+++ b/doc/source/format.rst
@@ -453,6 +453,9 @@ The ``arches`` attribute, if provided, overrides the element for a given archite
 name. It is not considered an error if the element does not provide an architecture
 clause for the specific architecture BuildStream was launched to build for.
 
+There is also a ``host-arches`` attribute, which operates in the same manner but
+follows the *host* architecture rather than the *target* architecture.
+
 In the above example we demonstrate that a given ``config`` attribute can be overridden
 by an architecture conditional, this can however be done for any segment of the
 element such as ``depends``, ``sources`` and ``public`` as well. It is however illegal
diff --git a/tests/context/context.py b/tests/context/context.py
index c09643b75..65f6f51fd 100644
--- a/tests/context/context.py
+++ b/tests/context/context.py
@@ -30,7 +30,7 @@ def context_fixture():
 def test_context_create(context_fixture):
     context = context_fixture['context']
     assert(isinstance(context, Context))
-    assert(context.arch == 'x86_64')
+    assert(context.host_arch == 'x86_64')
 
 
 #######################################
diff --git a/tests/loader/arches.py b/tests/loader/arches.py
index 99602d181..bc69d5b8f 100644
--- a/tests/loader/arches.py
+++ b/tests/loader/arches.py
@@ -19,7 +19,7 @@ def test_simple_conditional_nomatch(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/simple-conditional.bst', None, 'arm')
+        basedir, 'elements/simple-conditional.bst', None, 'arm', None)
 
     element = loader.load()
     assert(isinstance(element, MetaElement))
@@ -34,7 +34,7 @@ def test_simple_conditional_x86_64(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/simple-conditional.bst', None, 'x86_64')
+        basedir, 'elements/simple-conditional.bst', None, 'x86_64', None)
 
     element = loader.load()
     assert(isinstance(element, MetaElement))
@@ -49,7 +49,7 @@ def test_simple_conditional_x86_32(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/simple-conditional.bst', None, 'x86_32')
+        basedir, 'elements/simple-conditional.bst', None, 'x86_32', None)
 
     element = loader.load()
     assert(isinstance(element, MetaElement))
@@ -60,6 +60,58 @@ def test_simple_conditional_x86_32(datafiles):
 
 
 ##############################################################
+#            Test Arch and Host-Arch Conditionals            #
+##############################################################
+
+
+@pytest.mark.datafiles(DATA_DIR)
+def test_host_arch_conditional_armv8(datafiles):
+
+    basedir = os.path.join(datafiles.dirname, datafiles.basename)
+    loader = Loader(
+        basedir, 'elements/host-arch-conditional.bst', None, 'armv8', None)
+
+    element = loader.load()
+    assert(isinstance(element, MetaElement))
+    number = element.config.get('number')
+
+    # armv8 host-arch overrides the number to 88
+    assert(number == 88)
+
+
+@pytest.mark.datafiles(DATA_DIR)
+def test_host_arch_conditional_ignores_target_arch(datafiles):
+
+    basedir = os.path.join(datafiles.dirname, datafiles.basename)
+    loader = Loader(
+        basedir, 'elements/host-arch-conditional.bst', None, 'armv8', 'x86_32')
+
+    element = loader.load()
+    assert(isinstance(element, MetaElement))
+    number = element.config.get('number')
+
+    # The setting a target-arch has no effect on host-arches: the number is
+    # still 88
+    assert(number == 88)
+
+
+@pytest.mark.datafiles(DATA_DIR)
+def test_host_arch_conditional_overridden(datafiles):
+
+    basedir = os.path.join(datafiles.dirname, datafiles.basename)
+    loader = Loader(
+        basedir, 'elements/host-arch-conditional.bst', None, 'armv8', 'x86_64')
+
+    element = loader.load()
+    assert(isinstance(element, MetaElement))
+    number = element.config.get('number')
+
+    # The 'arches' conditional follows the target architecture, and overrides
+    # anything specified in 'host-arches'.
+    assert(number == 6)
+
+
+##############################################################
 #            Test Arch Conditionals inside Variants          #
 ##############################################################
 @pytest.mark.datafiles(DATA_DIR)
@@ -67,7 +119,7 @@ def test_variant_arch_default(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/variant-arch-conditional.bst', None, 'arm')
+        basedir, 'elements/variant-arch-conditional.bst', None, 'arm', None)
 
     element = loader.load()
     assert(isinstance(element, MetaElement))
@@ -82,7 +134,7 @@ def test_variant_arch_default_x86_64(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/variant-arch-conditional.bst', None, 'x86_64')
+        basedir, 'elements/variant-arch-conditional.bst', None, 'x86_64', None)
 
     element = loader.load()
     assert(isinstance(element, MetaElement))
@@ -96,7 +148,7 @@ def test_variant_arch_default_x86_32(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/variant-arch-conditional.bst', None, 'x86_32')
+        basedir, 'elements/variant-arch-conditional.bst', None, 'x86_32', None)
 
     element = loader.load()
     assert(isinstance(element, MetaElement))
@@ -110,7 +162,7 @@ def test_variant_arch_pink_pony(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/variant-arch-conditional.bst', 'pink', 'arm')
+        basedir, 'elements/variant-arch-conditional.bst', 'pink', 'arm', None)
 
     element = loader.load()
     assert(isinstance(element, MetaElement))
@@ -125,7 +177,7 @@ def test_variant_arch_pink_pony_x86_64(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/variant-arch-conditional.bst', 'pink', 'x86_64')
+        basedir, 'elements/variant-arch-conditional.bst', 'pink', 'x86_64', None)
 
     element = loader.load()
     assert(isinstance(element, MetaElement))
@@ -139,7 +191,7 @@ def test_variant_arch_pink_pony_x86_32(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/variant-arch-conditional.bst', 'pink', 'x86_32')
+        basedir, 'elements/variant-arch-conditional.bst', 'pink', 'x86_32', None)
 
     element = loader.load()
     assert(isinstance(element, MetaElement))
@@ -153,7 +205,7 @@ def test_variant_arch_blue_pony(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/variant-arch-conditional.bst', 'blue', 'arm')
+        basedir, 'elements/variant-arch-conditional.bst', 'blue', 'arm', None)
 
     element = loader.load()
     assert(isinstance(element, MetaElement))
@@ -168,7 +220,7 @@ def test_variant_arch_blue_pony_x86_64(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/variant-arch-conditional.bst', 'blue', 'x86_64')
+        basedir, 'elements/variant-arch-conditional.bst', 'blue', 'x86_64', None)
 
     element = loader.load()
     assert(isinstance(element, MetaElement))
@@ -182,7 +234,7 @@ def test_variant_arch_blue_pony_x86_32(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/variant-arch-conditional.bst', 'blue', 'x86_32')
+        basedir, 'elements/variant-arch-conditional.bst', 'blue', 'x86_32', None)
 
     element = loader.load()
     assert(isinstance(element, MetaElement))
diff --git a/tests/loader/arches/elements/host-arch-conditional.bst b/tests/loader/arches/elements/host-arch-conditional.bst
new file mode 100644
index 000000000..4c9272feb
--- /dev/null
+++ b/tests/loader/arches/elements/host-arch-conditional.bst
@@ -0,0 +1,20 @@
+kind: pony
+description: |
+  An element with host-arches and arches conditionals.
+
+  The 'arches' conditional follows --target-arch if specified, while the
+  'host-arches' conditional does not. The 'arches' conditional overrides
+  'host-arches' if both for some reason try to override the same thing.
+config:
+  number: 5
+arches:
+  x86_64:
+    config:
+      number: 6
+host-arches:
+  x86_64:
+    config:
+      number: 66
+  armv8:
+    config:
+      number: 88
diff --git a/tests/loader/basics.py b/tests/loader/basics.py
index f706d06ec..d7f85b0fe 100644
--- a/tests/loader/basics.py
+++ b/tests/loader/basics.py
@@ -18,7 +18,7 @@ DATA_DIR = os.path.join(
 def test_one_file(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/onefile.bst', None, None)
+    loader = Loader(basedir, 'elements/onefile.bst', None, None, None)
 
     element = loader.load()
 
@@ -30,7 +30,7 @@ def test_one_file(datafiles):
 def test_missing_file(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/missing.bst', None, None)
+    loader = Loader(basedir, 'elements/missing.bst', None, None, None)
 
     with pytest.raises(LoadError) as exc:
         element = loader.load()
@@ -42,7 +42,7 @@ def test_missing_file(datafiles):
 def test_invalid_reference(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/badreference.bst', None, None)
+    loader = Loader(basedir, 'elements/badreference.bst', None, None, None)
 
     with pytest.raises(LoadError) as exc:
         element = loader.load()
@@ -54,7 +54,7 @@ def test_invalid_reference(datafiles):
 def test_invalid_yaml(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/badfile.bst', None, None)
+    loader = Loader(basedir, 'elements/badfile.bst', None, None, None)
 
     with pytest.raises(LoadError) as exc:
         element = loader.load()
@@ -69,6 +69,6 @@ def test_fail_fullpath_target(datafiles):
     fullpath = os.path.join(basedir, 'elements', 'onefile.bst')
 
     with pytest.raises(LoadError) as exc:
-        loader = Loader(basedir, fullpath, None, None)
+        loader = Loader(basedir, fullpath, None, None, None)
 
     assert (exc.value.reason == LoadErrorReason.INVALID_DATA)
diff --git a/tests/loader/dependencies.py b/tests/loader/dependencies.py
index 8a88bcaf8..a701d4723 100644
--- a/tests/loader/dependencies.py
+++ b/tests/loader/dependencies.py
@@ -18,7 +18,7 @@ DATA_DIR = os.path.join(
 def test_two_files(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/target.bst', None, None)
+    loader = Loader(basedir, 'elements/target.bst', None, None, None)
     element = loader.load()
 
     assert(isinstance(element, MetaElement))
@@ -34,7 +34,7 @@ def test_two_files(datafiles):
 def test_shared_dependency(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/shareddeptarget.bst', None, None)
+    loader = Loader(basedir, 'elements/shareddeptarget.bst', None, None, None)
     element = loader.load()
 
     # Toplevel is 'pony' with 2 dependencies
@@ -77,7 +77,7 @@ def test_shared_dependency(datafiles):
 def test_dependency_dict(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/target-depdict.bst', None, None)
+    loader = Loader(basedir, 'elements/target-depdict.bst', None, None, None)
     element = loader.load()
 
     assert(isinstance(element, MetaElement))
@@ -92,7 +92,7 @@ def test_dependency_dict(datafiles):
 @pytest.mark.datafiles(DATA_DIR)
 def test_invalid_dependency_declaration(datafiles):
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/invaliddep.bst', None, None)
+    loader = Loader(basedir, 'elements/invaliddep.bst', None, None, None)
 
     with pytest.raises(LoadError) as exc:
         element = loader.load()
@@ -103,7 +103,7 @@ def test_invalid_dependency_declaration(datafiles):
 @pytest.mark.datafiles(DATA_DIR)
 def test_circular_dependency(datafiles):
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/circulartarget.bst', None, None)
+    loader = Loader(basedir, 'elements/circulartarget.bst', None, None, None)
 
     with pytest.raises(LoadError) as exc:
         element = loader.load()
@@ -114,7 +114,7 @@ def test_circular_dependency(datafiles):
 @pytest.mark.datafiles(DATA_DIR)
 def test_invalid_dependency_type(datafiles):
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/invaliddeptype.bst', None, None)
+    loader = Loader(basedir, 'elements/invaliddeptype.bst', None, None, None)
 
     with pytest.raises(LoadError) as exc:
         element = loader.load()
@@ -125,7 +125,7 @@ def test_invalid_dependency_type(datafiles):
 @pytest.mark.datafiles(DATA_DIR)
 def test_build_dependency(datafiles):
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/builddep.bst', None, None)
+    loader = Loader(basedir, 'elements/builddep.bst', None, None, None)
     element = loader.load()
 
     assert(isinstance(element, MetaElement))
@@ -141,7 +141,7 @@ def test_build_dependency(datafiles):
 @pytest.mark.datafiles(DATA_DIR)
 def test_runtime_dependency(datafiles):
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/runtimedep.bst', None, None)
+    loader = Loader(basedir, 'elements/runtimedep.bst', None, None, None)
     element = loader.load()
 
     assert(isinstance(element, MetaElement))
@@ -157,7 +157,7 @@ def test_runtime_dependency(datafiles):
 @pytest.mark.datafiles(DATA_DIR)
 def test_build_runtime_dependency(datafiles):
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/target.bst', None, None)
+    loader = Loader(basedir, 'elements/target.bst', None, None, None)
     element = loader.load()
 
     assert(isinstance(element, MetaElement))
diff --git a/tests/loader/variants.py b/tests/loader/variants.py
index 0e20faf93..865ffa268 100644
--- a/tests/loader/variants.py
+++ b/tests/loader/variants.py
@@ -18,7 +18,7 @@ DATA_DIR = os.path.join(
 def test_variant_not_list(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/variants-not-list.bst', None, None)
+    loader = Loader(basedir, 'elements/variants-not-list.bst', None, None, None)
 
     with pytest.raises(LoadError) as exc:
         element = loader.load()
@@ -30,7 +30,7 @@ def test_variant_not_list(datafiles):
 def test_variant_unnamed(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/unnamed-variant.bst', None, None)
+    loader = Loader(basedir, 'elements/unnamed-variant.bst', None, None, None)
 
     with pytest.raises(LoadError) as exc:
         element = loader.load()
@@ -42,7 +42,7 @@ def test_variant_unnamed(datafiles):
 def test_variant_bad_name(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/variant-bad-name.bst', None, None)
+    loader = Loader(basedir, 'elements/variant-bad-name.bst', None, None, None)
 
     with pytest.raises(LoadError) as exc:
         element = loader.load()
@@ -54,7 +54,7 @@ def test_variant_bad_name(datafiles):
 def test_variant_only_one(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/only-one-variant.bst', None, None)
+    loader = Loader(basedir, 'elements/only-one-variant.bst', None, None, None)
 
     with pytest.raises(LoadError) as exc:
         element = loader.load()
@@ -67,7 +67,7 @@ def test_variant_illegal_composite(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/variant-illegal-composite.bst', None, None)
+        basedir, 'elements/variant-illegal-composite.bst', None, None, None)
 
     with pytest.raises(LoadError) as exc:
         element = loader.load()
@@ -83,7 +83,7 @@ def test_variant_simple_composite_default(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/simple-variant-compositing.bst', None, None)
+        basedir, 'elements/simple-variant-compositing.bst', None, None, None)
 
     element = loader.load()
 
@@ -100,7 +100,7 @@ def test_variant_simple_composite_pink_pony(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/simple-variant-compositing.bst', 'pink', None)
+        basedir, 'elements/simple-variant-compositing.bst', 'pink', None, None)
 
     element = loader.load()
 
@@ -117,7 +117,7 @@ def test_variant_simple_composite_blue_pony(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/simple-variant-compositing.bst', 'blue', None)
+        basedir, 'elements/simple-variant-compositing.bst', 'blue', None, None)
 
     element = loader.load()
 
@@ -155,7 +155,7 @@ def test_variant_simple_dependency_default(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/simple-dependency-variants.bst', None, None)
+        basedir, 'elements/simple-dependency-variants.bst', None, None, None)
     element = loader.load()
 
     assert(isinstance(element, MetaElement))
@@ -170,7 +170,7 @@ def test_variant_simple_dependency_pink_pony(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/simple-dependency-variants.bst', 'pink', None)
+        basedir, 'elements/simple-dependency-variants.bst', 'pink', None, None)
     element = loader.load()
 
     assert(isinstance(element, MetaElement))
@@ -185,7 +185,7 @@ def test_variant_simple_dependency_blue_pony(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/simple-dependency-variants.bst', 'blue', None)
+        basedir, 'elements/simple-dependency-variants.bst', 'blue', None, None)
     element = loader.load()
 
     assert(isinstance(element, MetaElement))
@@ -200,7 +200,7 @@ def test_variant_indirect_dependency_default(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/indirect-dependency-variants.bst', None, None)
+        basedir, 'elements/indirect-dependency-variants.bst', None, None, None)
     element = loader.load()
 
     assert(isinstance(element, MetaElement))
@@ -219,7 +219,7 @@ def test_variant_indirect_dependency_blue_pony(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/indirect-dependency-variants.bst', 'blue', None)
+        basedir, 'elements/indirect-dependency-variants.bst', 'blue', None, None)
     element = loader.load()
 
     assert(isinstance(element, MetaElement))
@@ -238,7 +238,7 @@ def test_variant_indirect_dependency_pink_pony(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
     loader = Loader(
-        basedir, 'elements/indirect-dependency-variants.bst', 'pink', None)
+        basedir, 'elements/indirect-dependency-variants.bst', 'pink', None, None)
     element = loader.load()
 
     assert(isinstance(element, MetaElement))
@@ -256,7 +256,7 @@ def test_variant_indirect_dependency_pink_pony(datafiles):
 def test_engine_resolve_agreement(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/tricky.bst', None, None)
+    loader = Loader(basedir, 'elements/tricky.bst', None, None, None)
     element = loader.load()
 
     assert(isinstance(element, MetaElement))
@@ -271,7 +271,7 @@ def test_engine_resolve_agreement(datafiles):
 def test_engine_disagreement(datafiles):
 
     basedir = os.path.join(datafiles.dirname, datafiles.basename)
-    loader = Loader(basedir, 'elements/disagreement.bst', None, None)
+    loader = Loader(basedir, 'elements/disagreement.bst', None, None, None)
 
     with pytest.raises(LoadError) as exc:
         element = loader.load()
diff --git a/tests/sources/fixture.py b/tests/sources/fixture.py
index a9ba94e1d..c7b3a8fe6 100644
--- a/tests/sources/fixture.py
+++ b/tests/sources/fixture.py
@@ -39,7 +39,7 @@ class Setup():
         if not os.path.exists(self.context.builddir):
             os.mkdir(self.context.builddir)
 
-        loader = Loader(directory, target, None, None)
+        loader = Loader(directory, target, None, None, None)
         element = loader.load()
         assert(len(element.sources) == 1)
         self.meta_source = element.sources[0]