cxxrtl: add $divfloor.

backends/cxxrtl/cxxrtl.h

@@ -1595,6 +1595,25 @@ value<BitsY> modfloor_ss(const value<BitsA> &a, const value<BitsB> &b) {
 	return r;
 }
 
+template<size_t BitsY, size_t BitsA, size_t BitsB>
+CXXRTL_ALWAYS_INLINE
+value<BitsY> divfloor_uu(const value<BitsA> &a, const value<BitsB> &b) {
+	return divmod_uu<BitsY>(a, b).first;
+}
+
+// Divfloor. Similar to above: returns q=a//b, where q has the sign of a*b and a=b*q+N.
+// In other words, returns (truncating) a/b, except if a and b have different signs
+// and there's non-zero remainder, subtract one more towards floor.
+template<size_t BitsY, size_t BitsA, size_t BitsB>
+CXXRTL_ALWAYS_INLINE
+value<BitsY> divfloor_ss(const value<BitsA> &a, const value<BitsB> &b) {
+	value<BitsY> q, r;
+	std::tie(q, r) = divmod_ss<BitsY>(a, b);
+	if ((b.is_neg() != a.is_neg()) && !r.is_zero())
+		return sub_uu<BitsY>(q, value<1> { 1u });
+	return q;
+
+}
 
 // Memory helper
 struct memory_index {

backends/cxxrtl/cxxrtl_backend.cc

@@ -185,7 +185,7 @@ bool is_binary_cell(RTLIL::IdString type)
 		ID($and), ID($or), ID($xor), ID($xnor), ID($logic_and), ID($logic_or),
 		ID($shl), ID($sshl), ID($shr), ID($sshr), ID($shift), ID($shiftx),
 		ID($eq), ID($ne), ID($eqx), ID($nex), ID($gt), ID($ge), ID($lt), ID($le),
-		ID($add), ID($sub), ID($mul), ID($div), ID($mod), ID($modfloor));
+		ID($add), ID($sub), ID($mul), ID($div), ID($mod), ID($modfloor), ID($divfloor));
 }
 
 bool is_extending_cell(RTLIL::IdString type)