synth: blackbox SYNTH_BLACKBOXES modules before hierarchy check

Adds an optional SYNTH_BLACKBOXES env var that, when set, blackboxes
the listed modules before `hierarchy -check`.  Intended for external
orchestrators that drive parallel partition synthesis: every partition
loads the same canonical RTLIL checkpoint, then blackboxes the modules
that belong to other partitions so it only synthesises its own
subhierarchy.

Names not present in the loaded design are skipped silently, so the
same list can be reused across partitions without per-partition
filtering.

No behaviour change when the env var is unset.

Author: oharboe

flow/scripts/synth.tcl

@@ -38,6 +38,18 @@ if { [env_var_exists_and_non_empty SYNTH_CHECKPOINT] } {
   read_checkpoint $::env(RESULTS_DIR)/1_1_yosys_canonicalize.rtlil
 }
 
+# When this synthesis run is one partition of a parallel split (driven by
+# an external orchestrator), `SYNTH_BLACKBOXES` lists modules outside this
+# partition.  Blackboxing them before the hierarchy check lets each
+# partition load the same canonical RTLIL checkpoint while only synthesising
+# its own subhierarchy.  Names not present in the loaded design are skipped
+# silently so the same list can be passed to every partition.
+if { [env_var_exists_and_non_empty SYNTH_BLACKBOXES] } {
+  foreach m $::env(SYNTH_BLACKBOXES) {
+    catch {blackbox $m}
+  }
+}
+
 hierarchy -check -top $::env(DESIGN_NAME)
 
 if { $::env(SYNTH_GUT) } {

flow/scripts/variables.yaml

@@ -983,6 +983,17 @@ SYNTH_KEEP_MODULES:
     Mark modules to keep from getting removed in flattening.
   stages:
     - synth
+SYNTH_BLACKBOXES:
+  description: |
+    Space-separated list of module names to blackbox before the hierarchy
+    check. Used by external orchestrators that drive parallel partition
+    synthesis: each partition loads the shared canonical RTLIL checkpoint
+    and blackboxes the modules that belong to other partitions, so it only
+    synthesises its own subhierarchy. Names not present in the loaded
+    design are skipped silently, so the same list can be reused across
+    partitions.
+  stages:
+    - synth
 SYNTH_ARGS:
   description: |
     Optional synthesis variables for yosys.