synth: match slang \$-suffixed module names in SYNTH_KEEP_MODULES

flow/scripts/synth.tcl

@@ -44,7 +44,20 @@ if { $::env(SYNTH_GUT) } {
 
 if { [env_var_exists_and_non_empty SYNTH_KEEP_MODULES] } {
   foreach module $::env(SYNTH_KEEP_MODULES) {
-    select -module $module
+    # Two patterns so both frontends work:
+    #  - `$module` matches the bare name produced by verilog.
+    #  - `$module\$*` matches the `$`-suffixed canonical names the
+    #    slang frontend generates for parameterized instances
+    #    (e.g. `\foo$1`); yosys's match_ids retries the pattern with
+    #    the id's leading `\` stripped, so no `\`-prefix is needed
+    #    here -- see tools/yosys/passes/cmds/select.cc match_ids().
+    # Multiple patterns on one `select` are unioned at the end of the
+    # command (select.cc: `while (work_stack.size() > 1) union`), so
+    # when a pattern matches nothing it degrades to a warning and the
+    # other pattern still applies -- no regression for non-slang.
+    # `-module <name>` would error if the module doesn't exist, which
+    # is why we use bare patterns instead.
+    select "$module" "$module\\\$*"
     setattr -mod -set keep_hierarchy 1
     select -clear
   }