diff --git a/techlibs/ice40/ice40_dsp.pmg b/techlibs/ice40/ice40_dsp.pmg
index 63bc8de4b..2cfb97b2a 100644
--- a/techlibs/ice40/ice40_dsp.pmg
+++ b/techlibs/ice40/ice40_dsp.pmg
@@ -32,8 +32,8 @@ code sigA sigB sigH
 		++i;
 		return sig.extract(0, i);
 	};
-	sigA = unextend(port(mul, \A));
-	sigB = unextend(port(mul, \B));
+	sigA = port(mul, \A).is_fully_const() ? port(mul, \A) : unextend(port(mul, \A));
+	sigB = port(mul, \B).is_fully_const() ? port(mul, \B) : unextend(port(mul, \B));
 
 	SigSpec O;
 	if (mul->type == $mul)