scons: Add an option specifying the path to mold linker binary (#244)

To use mold linker with gcc of version older than 12.1.0, the user has
to pass the -B option to specify where the linker is. [1]

Currently in gem5, scons only looks for the mold binary at conventional
places, such as /usr/libexec/mold and /usr/local/libexec/mold. There's
no option to manually specify the path to the linker.

gcc-12 and mold are not widely available on older systems. Having an
option to manually input the linker path allows user to build and use
mold without sudo permission.

[1] https://github.com/rui314/mold#how-to-use

SConstruct

@@ -84,6 +84,7 @@ from os import mkdir, remove, environ
 from os.path import abspath, dirname, expanduser
 from os.path import isdir, isfile
 from os.path import join, split
+from pathlib import Path
 
 import logging
 logging.basicConfig()
@@ -121,6 +122,8 @@ AddOption('--ignore-style', action='store_true',
           help='Disable style checking hooks')
 AddOption('--linker', action='store', default=None, choices=linker_options,
           help=f'Select which linker to use ({", ".join(linker_options)})')
+AddOption('--mold-path', action='store', default='/usr/libexec/mold/',
+          help='Path to the binary of mold linker.')
 AddOption('--gold-linker', action='store_const', const='gold', dest='linker',
           help='Use the gold linker. Deprecated: Use --linker=gold')
 AddOption('--no-compress-debug', action='store_true',
@@ -425,19 +428,36 @@ for variant_path in variant_paths:
             conf.CheckLinkFlag('-Wl,--as-needed')
 
         linker = GetOption('linker')
+        # check for mold linker when -fuse-ld=mold fails
+        # this checks whether mold_dir has ld or ld.mold and
+        # whether -B{mold_dir} works
+        mold_check = lambda mold_dir: \
+            isdir(mold_dir) and \
+            (isfile(mold_dir/'ld') or isfile(mold_dir/'ld.mold')) and \
+            conf.CheckLinkFlag('-B' + str(mold_dir))
+        mold_check_message = \
+            "You are seeing this error because -fuse-ld=mold failed.\n" \
+            "If you did not add the folder containing mold to $PATH, you " \
+            "should do so and recompile gem5.\n" \
+            "If that does not work, you can manually specify the path to " \
+            "the binary of mold using the --mold-path option. This will " \
+            "cause scons to look for ld or ld.mold in the same folder as " \
+            "the binary of mold."
         if linker:
             with gem5_scons.Configure(env) as conf:
                 if not conf.CheckLinkFlag(f'-fuse-ld={linker}'):
-                    # check mold support for gcc older than 12.1.0
-                    if linker == 'mold' and \
-                       (env['GCC'] and \
-                           compareVersions(env['CXXVERSION'],
-                                           "12.1.0") < 0) and \
-                       ((isdir('/usr/libexec/mold') and \
-                           conf.CheckLinkFlag('-B/usr/libexec/mold')) or \
-                       (isdir('/usr/local/libexec/mold') and \
-                           conf.CheckLinkFlag('-B/usr/local/libexec/mold'))):
-                        pass # support mold
+                    # If -fuse-ld=mold fails, we use "-B$mold_path" instead.
+                    mold_path = Path(GetOption('mold_path')).parent
+                    if linker == 'mold':
+                        if any(map(mold_check, [mold_path,
+                            Path("/usr/libexec/mold"),
+                            Path("/usr/libexec/mold")])):
+                            pass # support mold
+                        else:
+                            error(
+                                f'Linker "{linker}" is not supported. '
+                                f'{mold_check_message}'
+                            )
                     else:
                         error(f'Linker "{linker}" is not supported')
                 if linker == 'gold' and not GetOption('with_lto'):