Adjust interpretation of SYNTH_KEEP_MODULES

Only apply `SYNTH_KEEP_MODULES` when `SYNTH_HIERARCHICAL=1`. Also make
this variable override the list of kept modules selected by the `SYNTH_MINIMUM_KEEP_SIZE` threshold. This is done to reduce user
confusion.

Signed-off-by: Martin Povišer <[email protected]>

Author: povik

flow/designs/asap7/swerv_wrapper/config.mk

@@ -1,6 +1,8 @@
 export DESIGN_NAME = swerv_wrapper
 export PLATFORM    = asap7
 
+export SYNTH_HIERARCHICAL = 1
+
 # SYNTH_KEEP_MODULES below is a captured list of kept modules
 # with:
 #

flow/scripts/synth.tcl

@@ -9,14 +9,6 @@ if { [env_var_equals SYNTH_GUT 1] } {
   delete $::env(DESIGN_NAME)/c:*
 }
 
-if { [env_var_exists_and_non_empty SYNTH_KEEP_MODULES] } {
-  foreach module $::env(SYNTH_KEEP_MODULES) {
-    select -module $module
-    setattr -mod -set keep_hierarchy 1
-    select -clear
-  }
-}
-
 if { [env_var_exists_and_non_empty SYNTH_HIER_SEPARATOR] } {
   scratchpad -set flatten.separator $::env(SYNTH_HIER_SEPARATOR)
 }
@@ -37,7 +29,13 @@ if { ![env_var_equals SYNTH_HIERARCHICAL 1] } {
   # defer flattening until we have decided what hierarchy to keep
   synth -run :fine
 
-  if { [env_var_exists_and_non_empty SYNTH_MINIMUM_KEEP_SIZE] } {
+  if { [env_var_exists_and_non_empty SYNTH_KEEP_MODULES] } {
+    foreach module $::env(SYNTH_KEEP_MODULES) {
+      select -module $module
+      setattr -mod -set keep_hierarchy 1
+      select -clear
+    }
+  } elseif { [env_var_exists_and_non_empty SYNTH_MINIMUM_KEEP_SIZE] } {
     set ungroup_threshold $::env(SYNTH_MINIMUM_KEEP_SIZE)
     puts "Keep modules above estimated size of $ungroup_threshold gate equivalents"
 

flow/scripts/variables.yaml

@@ -702,7 +702,8 @@ DONT_USE_LIBS:
     Set liberty files as `dont_use`.
 SYNTH_KEEP_MODULES:
   description: |
-    Mark modules to keep from getting removed in flattening.
+    Mark modules to keep from getting removed in flattening. This option applies
+    when SYNTH_HIERARCHICAL is enabled.
   stages:
     - synth
 SYNTH_ARGS: