back.verilog: use `proc -nomux` if it is available.

Yosys offers no stability guarantees for individual `proc_*` passes,
though so far it worked out fine. This commit changes the Verilog
backend to use `proc -nomux` instead, which is guaranteed to have
backwards-compatible behavior.

Fixes #479.

diff --git a/nmigen/back/verilog.py b/nmigen/back/verilog.py
index 9f813d0de061380bcae42dc1e2aba6792abad774..37e4158e4f314fa7e05e62901997f537437fbaa0 100644 (file)
--- a/nmigen/back/verilog.py
+++ b/nmigen/back/verilog.py
@@ -13,15 +13,19 @@ def _convert_rtlil_text(rtlil_text, *, strip_internal_attrs=False, write_verilog
     script = []
     script.append("read_ilang <<rtlil\n{}\nrtlil".format(rtlil_text))
 
-    if yosys_version >= (0, 9, 3468):
-        # Yosys >=0.9+3468 (since commit f3d7e9a1) emits Verilog without a possible sim/synth
-        # mismatch, making $verilog_initial_trigger unnecessary.
+    if yosys_version >= (0, 9, 3527):
+        # Yosys >=0.9+3527 (since commit 656ee70f) supports the `-nomux` option for the `proc`
+        # script pass. Because the individual `proc_*` passes are not a stable interface,
+        # `proc -nomux` is used instead, if available.
         script.append("delete w:$verilog_initial_trigger")
-        script.append("proc_prune")
-    script.append("proc_init")
-    script.append("proc_arst")
-    script.append("proc_dff")
-    script.append("proc_clean")
+        script.append("proc -nomux")
+    else:
+        # On earlier versions, use individual `proc_*` passes; this is a known range of Yosys
+        # versions and we know it's compatible with what nMigen does.
+        script.append("proc_init")
+        script.append("proc_arst")
+        script.append("proc_dff")
+        script.append("proc_clean")
     script.append("memory_collect")
 
     if strip_internal_attrs: