Set Clang as the default compiler on macOS (#2225)

* Set OS-specific default compilers

* Fix flake8 warnings

etc/spack/defaults/darwin/packages.yaml

@@ -0,0 +1,18 @@
+# -------------------------------------------------------------------------
+# This file controls default concretization preferences for Spack.
+#
+# Settings here are versioned with Spack and are intended to provide
+# sensible defaults out of the box. Spack maintainers should edit this
+# file to keep it current.
+#
+# Users can override these settings by editing the following files.
+#
+# Per-spack-instance settings (overrides defaults):
+#   $SPACK_ROOT/etc/spack/packages.yaml
+#
+# Per-user settings (overrides default and site settings):
+#   ~/.spack/packages.yaml
+# -------------------------------------------------------------------------
+packages:
+  all:
+    compiler: [clang, gcc, intel]

etc/spack/defaults/packages.yaml

@@ -15,6 +15,7 @@
 # -------------------------------------------------------------------------
 packages:
   all:
+    compiler: [gcc, intel, pgi, clang, xl, nag]
     providers:
       mpi: [openmpi, mpich]
       blas: [openblas]

lib/spack/spack/compilers/__init__.py

@@ -26,7 +26,6 @@
 system and configuring Spack to use multiple compilers.
 """
 import imp
-import platform
 
 from llnl.util.lang import list_modules
 from llnl.util.filesystem import join_path
@@ -44,12 +43,6 @@
 _other_instance_vars = ['modules', 'operating_system']
 _cache_config_file = []
 
-# TODO: customize order in config file
-if platform.system() == 'Darwin':
-    _default_order = ['clang', 'gcc', 'intel']
-else:
-    _default_order = ['gcc', 'intel', 'pgi', 'clang', 'xlc', 'nag']
-
 
 def _auto_compiler_spec(function):
     def converter(cspec_like, *args, **kwargs):
@@ -169,18 +162,6 @@ def all_compilers(scope=None, init_config=True):
             for s in all_compilers_config(scope, init_config)]
 
 
-def default_compiler():
-    versions = []
-    for name in _default_order:
-        versions = find(name)
-        if versions:
-            break
-    else:
-        raise NoCompilersError()
-
-    return sorted(versions)[-1]
-
-
 def find_compilers(*paths):
     """Return a list of compilers found in the suppied paths.
        This invokes the find_compilers() method for each operating

lib/spack/spack/preferred_packages.py

@@ -28,9 +28,6 @@
 
 
 class PreferredPackages(object):
-    # Arbitrary, but consistent
-    _default_order = {'compiler': ['gcc', 'intel', 'clang', 'pgi', 'xlc']}
-
     def __init__(self):
         self.preferred = spack.config.get_config('packages')
         self._spec_for_pkgname_cache = {}
@@ -128,9 +125,6 @@ def _spec_for_pkgname(self, pkgname, component, second_key):
         key = (pkgname, component, second_key)
         if key not in self._spec_for_pkgname_cache:
             pkglist = self._order_for_package(pkgname, component, second_key)
-            if not pkglist:
-                if component in self._default_order:
-                    pkglist = self._default_order[component]
             if component == 'compiler':
                 self._spec_for_pkgname_cache[key] = \
                     [spack.spec.CompilerSpec(s) for s in pkglist]