scons: Add a SourceLib method for adding libs to gem5.

Sometimes a library is needed to support particular functionality in
gem5, and that functionality is only used (or even desirable) in
certain binaries SCons can build. We can currently filter sources to
include in a particular executable using tags, but libraries have been
added to the environment globally using the LIBS variable which applies
to all Executables.

This change adds a SourceLib() mechanism which is a new category of
source which represents libraries. This is independent from classes
which inherit from SourceFile which represent actual files, as opposed
to more abstract libraries.

When gem5 builds an executable, the filters it provides are used to
select both Source()-es, aka c/c++ files, and libraries. If something
like a unit test does not need all the libraries gem5 proper does,
then those won't be picked up by its filter, and it won't include them.

Change-Id: I003e029eb82f7800a7ecff698c260e2d18ea2900
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58069
Reviewed-by: Yu-hsin Wang <yuhsingw@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

site_scons/gem5_scons/sources.py

@@ -188,13 +188,13 @@ class SourceMeta(type):
         super(SourceMeta, cls).__init__(name, bases, dict)
         cls.all = SourceList()
 
-class SourceFile(object, metaclass=SourceMeta):
-    '''Base object that encapsulates the notion of a source file.
-    This includes, the source node, target node, various manipulations
-    of those.  A source file also specifies a set of tags which
-    describing arbitrary properties of the source file.'''
-
+class SourceItem(object, metaclass=SourceMeta):
+    '''Base object that encapsulates the notion of a source component for
+    gem5. This specifies a set of tags which help group components into groups
+    based on arbitrary properties.'''
     def __init__(self, source, tags=None, add_tags=None, append=None):
+        self.source = source
+
         if tags is None:
             tags='gem5 lib'
         if isinstance(tags, str):
@@ -212,16 +212,24 @@ class SourceFile(object, metaclass=SourceMeta):
 
         self.append = append
 
+        for base in type(self).__mro__:
+            if issubclass(base, SourceItem):
+                base.all.append(self)
+
+class SourceFile(SourceItem):
+    '''Base object that encapsulates the notion of a source file.
+    This includes, the source node, target node, various manipulations
+    of those.'''
+
+    def __init__(self, source, tags=None, add_tags=None, append=None):
+        super().__init__(source, tags=tags, add_tags=add_tags, append=append)
+
         tnode = SCons.Script.File(source)
 
         self.tnode = tnode
         self.filename = str(self.tnode)
         self.snode = tnode.srcnode()
 
-        for base in type(self).__mro__:
-            if issubclass(base, SourceFile):
-                base.all.append(self)
-
     def static(self, env):
         if self.append:
             env = env.Clone()
@@ -234,6 +242,7 @@ class SourceFile(object, metaclass=SourceMeta):
             env.Append(**self.append)
         return env.SharedObject(self.tnode)
 
+
 __all__ = ['TagImpliesTool', 'SourceFilter', 'SourceList', 'SourceFile',
-           'with_any_tags', 'with_all_tags', 'with_tag', 'without_tags',
-           'without_tag']
+           'SourceItem', 'with_any_tags', 'with_all_tags', 'with_tag',
+           'without_tags', 'without_tag']

src/SConscript

@@ -78,6 +78,9 @@ def GdbXml(xml_id, symbol, tags=None, add_tags=None):
 class Source(SourceFile):
     pass
 
+class SourceLib(SourceItem):
+    pass
+
 build_tools = Dir('#build_tools')
 
 # Build a small helper that runs Python code using the same version of Python
@@ -318,11 +321,15 @@ class TopLevelBase(object, metaclass=TopLevelMeta):
         self.target = target
 
         isFilter = lambda arg: isinstance(arg, SourceFilter)
-        self.filters = filter(isFilter, srcs_and_filts)
-        sources = filter(lambda a: not isFilter(a), srcs_and_filts)
+        isLib = lambda arg: isinstance(arg, SourceLib)
+        # If something isn't a library or filter, assume it's a source file.
+        isSourceFile = lambda arg: not isFilter(arg) and not isLib(arg)
+        self.filters = list(filter(isFilter, srcs_and_filts))
+        self.sourceLibs = list(filter(isLib, srcs_and_filts))
+        source_files = list(filter(isSourceFile, srcs_and_filts))
 
         srcs = SourceList()
-        for src in sources:
+        for src in source_files:
             if not isinstance(src, SourceFile):
                 src = Source(src, tags=[])
             srcs.append(src)
@@ -336,6 +343,12 @@ class TopLevelBase(object, metaclass=TopLevelMeta):
             srcs += Source.all.apply_filter(env, f)
         return srcs
 
+    def libs(self, env):
+        libs = self.sourceLibs
+        for f in self.filters:
+            libs += SourceLib.all.apply_filter(env, f)
+        return libs
+
     def srcs_to_objs(self, env, sources):
         return list([ s.static(env) for s in sources ])
 
@@ -382,6 +395,10 @@ class Executable(TopLevelBase):
         env['BIN_RPATH_PREFIX'] = os.path.relpath(
                 env['BUILDDIR'], self.path(env).dir.abspath)
 
+        libs = self.libs(env)
+        if libs:
+            env.Append(LIBS=list(lib.source for lib in libs))
+
         executable = env.Program(self.path(env).abspath, objs)[0]
 
         if sys.platform == 'sunos5':
@@ -439,6 +456,7 @@ class GTest(Executable):
 # Children should have access
 Export('GdbXml')
 Export('Source')
+Export('SourceLib')
 Export('PySource')
 Export('SimObject')
 Export('ProtoBuf')