synth: allow user to specify custom hier separator for flatten pass

Author: magancarz

docs/user/FlowVariables.md

@@ -182,7 +182,7 @@ configuration file.
 | <a name="SKIP_PIN_SWAP"></a>SKIP_PIN_SWAP| Do not use pin swapping as a transform to fix timing violations (default: use pin swapping).| | |
 | <a name="SKIP_REPORT_METRICS"></a>SKIP_REPORT_METRICS| If set to 1, then metrics, report_metrics does nothing. Useful to speed up builds.| | |
 | <a name="SLEW_MARGIN"></a>SLEW_MARGIN| Specifies a slew margin when fixing max slew violations. This option allows you to overfix.| | |
-| <a name="SYNTH_ARGS"></a>SYNTH_ARGS| Optional synthesis variables for yosys.| -flatten| |
+| <a name="SYNTH_ARGS"></a>SYNTH_ARGS| Optional synthesis variables for yosys.| | |
 | <a name="SYNTH_BLACKBOXES"></a>SYNTH_BLACKBOXES| List of cells treated as a black box by Yosys. With Bazel, this can be used to run synthesis in parallel for the large modules of the design.| | |
 | <a name="SYNTH_GUT"></a>SYNTH_GUT| Load design and remove all internal logic before doing synthesis. This is useful when creating a mock .lef abstract that has a smaller area than the amount of logic would allow. bazel-orfs uses this to mock SRAMs, for instance.| | |
 | <a name="SYNTH_HDL_FRONTEND"></a>SYNTH_HDL_FRONTEND| Select an alternative language frontend to ingest the design. Available option is "slang". If the variable is empty, design is read with the Yosys read_verilog command.| | |

flow/scripts/synth.tcl

@@ -17,6 +17,11 @@ if {[env_var_exists_and_non_empty SYNTH_KEEP_MODULES]} {
   }
 }
 
+set flatten_hier_separator .
+if {[env_var_exists_and_non_empty SYNTH_HIER_SEPARATOR]} {
+  set flatten_hier_separator $::env(SYNTH_HIER_SEPARATOR)
+}
+
 set synth_full_args $::env(SYNTH_ARGS)
 if {[env_var_exists_and_non_empty SYNTH_OPERATIONS_ARGS]} {
   set synth_full_args [concat $synth_full_args $::env(SYNTH_OPERATIONS_ARGS)]
@@ -26,7 +31,7 @@ if {[env_var_exists_and_non_empty SYNTH_OPERATIONS_ARGS]} {
 
 if {![env_var_equals SYNTH_HIERARCHICAL 1]} {
   # Perform standard coarse-level synthesis script, flatten right away
-  # (-flatten part of $synth_args per default)
+  flatten -separator $flatten_hier_separator
   synth -run :fine {*}$synth_full_args
 } else {
   # Perform standard coarse-level synthesis script,
@@ -43,7 +48,9 @@ if {![env_var_equals SYNTH_HIERARCHICAL 1]} {
     keep_hierarchy
   }
 
-  # Re-run coarse-level script, this time do pass -flatten
+  flatten -separator $flatten_hier_separator
+
+  # Re-run coarse-level script
   synth -run coarse:fine {*}$synth_full_args
 }
 

flow/scripts/variables.yaml

@@ -676,7 +676,7 @@ SYNTH_KEEP_MODULES:
 SYNTH_ARGS:
   description: |
     Optional synthesis variables for yosys.
-  default: -flatten
+  default: ""
 VERILOG_TOP_PARAMS:
   description: |
     Apply toplevel params (if exist).