From c4a7958f70be956130119bbb3c682df8e0953b9c Mon Sep 17 00:00:00 2001
From: Clifford Wolf <clifford@clifford.at>
Date: Sun, 6 Aug 2017 13:27:18 +0200
Subject: [PATCH] Add handling of constant reset signals to opt_rmdff

---
 passes/opt/opt_rmdff.cc | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/passes/opt/opt_rmdff.cc b/passes/opt/opt_rmdff.cc
index 0eefd6a86..80217464c 100644
--- a/passes/opt/opt_rmdff.cc
+++ b/passes/opt/opt_rmdff.cc
@@ -186,7 +186,7 @@ bool handle_dff(RTLIL::Module *mod, RTLIL::Cell *dff)
 		goto delete_dff;
 	}
 
-	if (sig_d == sig_q && (!sig_r.size() || !has_init || val_init == val_rv)) {
+	if (sig_d == sig_q && (sig_r.empty() || !has_init || val_init == val_rv)) {
 		if (sig_r.size())
 			mod->connect(sig_q, val_rv);
 		if (has_init)
@@ -194,6 +194,28 @@ bool handle_dff(RTLIL::Module *mod, RTLIL::Cell *dff)
 		goto delete_dff;
 	}
 
+	if (!sig_r.empty() && sig_r.is_fully_const())
+	{
+		if (sig_r == val_rp || sig_r.is_fully_undef()) {
+			mod->connect(sig_q, val_rv);
+			goto delete_dff;
+		}
+
+		log("Removing unused reset from %s (%s) from module %s.\n", log_id(dff), log_id(dff->type), log_id(mod));
+
+		if (dff->type == "$adff") {
+			dff->type = "$dff";
+			dff->unsetPort("\\ARST");
+			dff->unsetParam("\\ARST_POLARITY");
+			dff->unsetParam("\\ARST_VALUE");
+			return true;
+		}
+
+		log_assert(dff->type.substr(0,6) == "$_DFF_");
+		dff->type = stringf("$_DFF_%c_", + dff->type[6]);
+		dff->unsetPort("\\R");
+	}
+
 	return false;
 
 delete_dff:
-- 
2.30.2