synth: blackbox SYNTH_BLACKBOXES modules before hierarchy check

Adds a checkpoint-based code path for SYNTH_BLACKBOXES to synth.tcl
and aligns the existing source-based path in synth_preamble.tcl so
both flows behave identically.

Use case: external orchestrators that drive parallel partition
synthesis.  Each partition loads the same canonical RTLIL checkpoint
and blackboxes the modules that belong to other partitions, so it
only synthesises its own subhierarchy.  The same SYNTH_BLACKBOXES
list is reused across partitions, so unknown names are skipped
silently with `catch { blackbox $m }` instead of erroring.

In synth_preamble.tcl the verilog-frontend path is reordered to
blackbox before `hierarchy -check -top` (matching the new synth.tcl
path) and to use `catch` (matching the slang and synth.tcl paths).
SYNTH_BLACKBOXES is already documented in variables.yaml.

No behaviour change when the env var is unset.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

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/synth_preamble.tcl

@@ -127,10 +127,10 @@ proc read_design_sources { } {
     }
 
     if { [env_var_exists_and_non_empty SYNTH_BLACKBOXES] } {
-      hierarchy -check -top $::env(DESIGN_NAME)
       foreach m $::env(SYNTH_BLACKBOXES) {
-        blackbox $m
+        catch { blackbox $m }
       }
+      hierarchy -check -top $::env(DESIGN_NAME)
     }
   } else {
     error "Unrecognized HDL frontend: $::env(SYNTH_HDL_FRONTEND)"