slang: add SYNTH_SLANG_ARGS

fixes #3644

Signed-off-by: Øyvind Harboe <[email protected]>

Author: oharboe

flow/scripts/synth_preamble.tcl

@@ -47,7 +47,7 @@ proc read_design_sources { } {
     # slang requires all files at once
     plugin -i slang
     yosys read_slang -D SYNTHESIS --keep-hierarchy --compat=vcs \
-      --ignore-assertions --no-implicit-memories --top $::env(DESIGN_NAME) \
+      --ignore-assertions {*}$::env(SYNTH_SLANG_ARGS) --top $::env(DESIGN_NAME) \
       {*}$vIdirsArgs {*}$::env(VERILOG_FILES) {*}[env_var_or_empty VERILOG_DEFINES]
     # Workaround for yosys-slang#119
     setattr -unset init

flow/scripts/variables.yaml

@@ -184,6 +184,12 @@ SYNTH_HDL_FRONTEND:
     command.
   stages:
     - synth
+SYNTH_SLANG_ARGS:
+  description: >
+    Additional arguments passed to the slang frontend during synthesis.
+  stages:
+    - synth
+  default: ""
 SYNTH_BLACKBOXES:
   description: >
     List of cells treated as a black box by Yosys. With Bazel, this can be used