scons: Add an option specifying the path to mold linker binary

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 mold linker path allows users to use
built mold instances anywhere on the system, not just the default
locations in /usr where they may not have permission to install mold
(i.e., no sudo permissions).

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

Change-Id: Ifb2366a0c2342bf4e7207df8db6196e14184a9d4
Signed-off-by: Hoa Nguyen <hn@hnpl.org>

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 $PATH, you " \
+            "should do so and recompiling gem5.\n" \
+            "If that does not work, you can manually specify the path to " \
+            "the binary of mold using the --mold-path option. This will " \
+            "cause scons to look for ld or ld.mold in the same folder as " \
+            "the binary of mold."
         if linker:
             with gem5_scons.Configure(env) as conf:
                 if not conf.CheckLinkFlag(f'-fuse-ld={linker}'):
-                    # check mold support for gcc older than 12.1.0
-                    if linker == 'mold' and \
-                       (env['GCC'] and \
-                           compareVersions(env['CXXVERSION'],
-                                           "12.1.0") < 0) and \
-                       ((isdir('/usr/libexec/mold') and \
-                           conf.CheckLinkFlag('-B/usr/libexec/mold')) or \
-                       (isdir('/usr/local/libexec/mold') and \
-                           conf.CheckLinkFlag('-B/usr/local/libexec/mold'))):
-                        pass # support mold
+                    # If -fuse-ld=mold fails, we use "-B$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'):