Merge pull request #3260 from antmicro/mgan/default-flatten-flag

Add `-flatten` to synth pass explicitly

Author: maliberty

docs/user/FlowVariables.md

@@ -218,7 +218,7 @@ 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

@@ -30,8 +30,7 @@ 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 $synth_args per default)
-  synth -run :fine {*}$synth_full_args
+  synth -flatten -run :fine {*}$synth_full_args
 } else {
   # Perform standard coarse-level synthesis script,
   # defer flattening until we have decided what hierarchy to keep
@@ -48,7 +47,7 @@ if {![env_var_equals SYNTH_HIERARCHICAL 1]} {
   }
 
   # Re-run coarse-level script, this time do pass -flatten
-  synth -run coarse:fine {*}$synth_full_args
+  synth -flatten -run coarse:fine {*}$synth_full_args
 }
 
 json -o $::env(RESULTS_DIR)/mem.json

flow/scripts/variables.yaml

@@ -683,7 +683,7 @@ SYNTH_KEEP_MODULES:
 SYNTH_ARGS:
   description: |
     Optional synthesis variables for yosys.
-  default: -flatten
+  default: ""
 SYNTH_HIER_SEPARATOR:
   description: |
     Separator used for the synthesis flatten stage.