ar linker: generate thin archives for uninstalled static libraries

Since they will never be used outside of the build directory, they do
not need to literally contain the .o files, and references will be
sufficient.

This covers a major use of object libraries, which is that the static
library would potentially take up a lot of space by including another
copy of every .o file.

Fixes #9292
Fixes #8057
Fixes #2129

docs/markdown/snippets/uninstalled_static_linker.md

@@ -0,0 +1,11 @@
+## More efficient static linking of uninstalled libraries
+
+A somewhat common use case of [[static_library]] is to create uninstalled
+internal convenience libraries which are solely meant to be linked to other
+targets. Some build systems call these "object libraries". Meson's
+implementation does always create a static archive.
+
+This will now check to see if the static linker supports "thin archives"
+(archives which do not contain the actual object code, only references to their
+location on disk) and if so, use them to minimize space usage and speed up
+linking.

mesonbuild/backend/ninjabackend.py

@@ -2790,7 +2790,7 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
             if target.import_filename:
                 commands += linker.gen_import_library_args(self.get_import_filename(target))
         elif isinstance(target, build.StaticLibrary):
-            commands += linker.get_std_link_args()
+            commands += linker.get_std_link_args(not target.should_install())
         else:
             raise RuntimeError('Unknown build target type.')
         return commands

mesonbuild/linkers/linkers.py

@@ -59,7 +59,7 @@ class StaticLinker:
     def get_exelist(self) -> T.List[str]:
         return self.exelist.copy()
 
-    def get_std_link_args(self) -> T.List[str]:
+    def get_std_link_args(self, is_thin: bool) -> T.List[str]:
         return []
 
     def get_buildtype_linker_args(self, buildtype: str) -> T.List[str]:
@@ -176,7 +176,7 @@ class ArLikeLinker(StaticLinker):
         # in fact, only the 'ar' id can
         return False
 
-    def get_std_link_args(self) -> T.List[str]:
+    def get_std_link_args(self, is_thin: bool) -> T.List[str]:
         return self.std_args
 
     def get_output_args(self, target: str) -> T.List[str]:
@@ -193,17 +193,28 @@ class ArLinker(ArLikeLinker):
         super().__init__(exelist)
         stdo = mesonlib.Popen_safe(self.exelist + ['-h'])[1]
         # Enable deterministic builds if they are available.
+        stdargs = 'csr'
+        thinargs = ''
         if '[D]' in stdo:
-            self.std_args = ['csrD']
-        else:
-            self.std_args = ['csrD']
+            stdargs += 'D'
+        if '[T]' in stdo:
+            thinargs = 'T'
+        self.std_args = [stdargs]
+        self.std_thin_args = [stdargs + thinargs]
         self.can_rsp = '@<' in stdo
 
     def can_linker_accept_rsp(self) -> bool:
         return self.can_rsp
 
+    def get_std_link_args(self, is_thin: bool) -> T.List[str]:
+        # FIXME: osx ld rejects this: "file built for unknown-unsupported file format"
+        if is_thin and not mesonlib.is_osx():
+            return self.std_thin_args
+        else:
+            return self.std_args
+
 
-class ArmarLinker(ArLikeLinker):  # lgtm [py/missing-call-to-init]
+class ArmarLinker(ArLikeLinker):
     id = 'armar'
 
 
@@ -214,7 +225,7 @@ class DLinker(StaticLinker):
         self.arch = arch
         self.__rsp_syntax = rsp_syntax
 
-    def get_std_link_args(self) -> T.List[str]:
+    def get_std_link_args(self, is_thin: bool) -> T.List[str]:
         return ['-lib']
 
     def get_output_args(self, target: str) -> T.List[str]:
@@ -1114,7 +1125,7 @@ class PGIStaticLinker(StaticLinker):
         self.id = 'ar'
         self.std_args = ['-r']
 
-    def get_std_link_args(self) -> T.List[str]:
+    def get_std_link_args(self, is_thin: bool) -> T.List[str]:
         return self.std_args
 
     def get_output_args(self, target: str) -> T.List[str]:

unittests/allplatformstests.py

@@ -1447,7 +1447,7 @@ class AllPlatformTests(BasePlatformTests):
             link_cmd = ['ar', 'csr', outfile, objectfile]
         link_cmd = linker.get_exelist()
         link_cmd += linker.get_always_args()
-        link_cmd += linker.get_std_link_args()
+        link_cmd += linker.get_std_link_args(False)
         link_cmd += linker.get_output_args(outfile)
         link_cmd += [objectfile]
         self.pbcompile(compiler, source, objectfile, extra_args=extra_args)