From a946ed0206cda29ce1bfe7f5a09f1b5e3d21794d Mon Sep 17 00:00:00 2001
From: gatecat <gatecat@ds0.me>
Date: Tue, 14 Dec 2021 19:27:20 +0000
Subject: [PATCH] ice40: Pack LUT at start of carry chain if there is 1
 candidate

Signed-off-by: gatecat <gatecat@ds0.me>
---
 ice40/pack.cc | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/ice40/pack.cc b/ice40/pack.cc
index 0db78992..921e73e0 100644
--- a/ice40/pack.cc
+++ b/ice40/pack.cc
@@ -203,6 +203,8 @@ static void pack_carries(Context *ctx)
             CellInfo *carry_lc = nullptr;
             if (carry_ci_lc && carry_lcs.find(carry_ci_lc->name) != carry_lcs.end()) {
                 carry_lc = carry_ci_lc;
+            } else if (ci_const && carry_lcs.size() == 1) {
+                carry_lc = ctx->cells.at(*(carry_lcs.begin())).get();
             } else {
                 // No LC to pack into matching I0/I1, insert a new one
                 std::unique_ptr<CellInfo> created_lc =