optimize has_sign_bit and mod2k to not compute powers of two

this is very useful for bitvectors of large bitwidths

src/util/mpz.cpp

@@ -633,6 +633,79 @@ void mpz_manager<SYNCH>::mod(mpz const & a, mpz const & b, mpz & c) {
     STRACE(mpz, tout << to_string(c) << "\n";);
 }
 
+template<bool SYNCH>
+mpz mpz_manager<SYNCH>::mod2k(mpz const & a, unsigned k) {
+    if (is_zero(a))
+        return 0;
+
+    mpz result;
+
+    if (is_small(a) && k < 64) {
+        uint64_t mask = ((1ULL << k) - 1);
+        uint64_t uval = static_cast<uint64_t>(i64(a));
+        set_i64(result, static_cast<int64_t>(uval & mask));
+        return result;
+    }
+
+    if (is_nonneg(a) && bitsize(a) <= k) {
+        return dup(a);
+    }
+
+#ifndef _MP_GMP
+    sign_cell ca(*this, a);
+    unsigned digit_size = sizeof(digit_t) * 8;
+    unsigned digit_count = k / digit_size;
+    unsigned rem_bits = k % digit_size;
+    unsigned total_digits = digit_count + (rem_bits > 0);
+    digit_t mask = (1ULL << rem_bits) - 1;
+    bool is_zero = true;
+
+    allocate_if_needed(result, total_digits);
+
+    // compute |a| mod 2^k-
+    for (unsigned i = 0, e = std::min(digit_count, ca.cell()->m_size); i < e; ++i) {
+        is_zero &= (digits(result)[i] = ca.cell()->m_digits[i]) == 0;
+    }
+    for (unsigned i = ca.cell()->m_size; i < total_digits; ++i) {
+        digits(result)[i] = 0;
+    }
+
+    if (rem_bits > 0 && digit_count < ca.cell()->m_size) {
+        is_zero &= (digits(result)[digit_count] = ca.cell()->m_digits[digit_count] & mask) == 0;
+    }
+    result.m_ptr->m_size = total_digits;
+
+    if (ca.sign() < 0 && !is_zero) {
+        // Negative case: if non-zero, result = 2^k - (|a| mod 2^k)
+        // which boils down to computing ~result + 1
+        for (unsigned i = 0; i < total_digits; ++i) {
+            digits(result)[i] = ~digits(result)[i];
+        }
+
+        // Increment result
+        digit_t carry = 1;
+        for (unsigned i = 0; i < total_digits && carry; ++i) {
+            digit_t sum = digits(result)[i] + carry;
+            carry = sum < digits(result)[i];
+            digits(result)[i] = sum;
+        }
+
+        // Clamp to k bits
+        if (rem_bits != 0) {
+            digits(result)[digit_count] &= mask;
+        }
+    }
+    normalize(result);
+#else
+    ensure_mpz_t a1(a);
+    mk_big(result);
+    MPZ_BEGIN_CRITICAL();
+    mpz_tdiv_r_2exp(*result.m_ptr, a1(), k);
+    MPZ_END_CRITICAL();
+#endif
+    return result;
+}
+
 template<bool SYNCH>
 void mpz_manager<SYNCH>::neg(mpz & a) {
     STRACE(mpz, tout << "[mpz] 0 - " << to_string(a) << " == ";); 
@@ -1140,13 +1213,9 @@ unsigned mpz_manager<SYNCH>::size_info(mpz const & a) {
 
 template<bool SYNCH>
 struct mpz_manager<SYNCH>::sz_lt {
-    mpz_manager<SYNCH> & m;
-    mpz const *          m_as;
-    
-    sz_lt(mpz_manager<SYNCH> & _m, mpz const * as):m(_m), m_as(as) {}
-
+    mpz const * m_as;
     bool operator()(unsigned p1, unsigned p2) {
-        return m.size_info(m_as[p1]) < m.size_info(m_as[p2]);
+        return size_info(m_as[p1]) < size_info(m_as[p2]);
     }
 };
 
@@ -1180,8 +1249,7 @@ void mpz_manager<SYNCH>::gcd(unsigned sz, mpz const * as, mpz & g) {
         sbuffer<unsigned, 1024> p;
         for (i = 0; i < sz; ++i)
             p.push_back(i);
-        sz_lt lt(*this, as);
-        std::sort(p.begin(), p.end(), lt);
+        std::sort(p.begin(), p.end(), sz_lt{as});
         TRACE(mpz_gcd, for (unsigned i = 0; i < sz; ++i) tout << p[i] << ":" << size_info(as[p[i]]) << " "; tout << "\n";);
         gcd(as[p[0]], as[p[1]], g);
         for (i = 2; i < sz; ++i) {
@@ -1695,15 +1763,11 @@ void mpz_manager<SYNCH>::display(std::ostream & out, mpz const & a) const {
     else {
 #ifndef _MP_GMP
         if (a.m_val < 0)
-            out << "-";
-        if (sizeof(digit_t) == 4) {
-            sbuffer<char, 1024> buffer(11*size(a), 0);
-            out << m_mpn_manager.to_string(digits(a), size(a), buffer.begin(), buffer.size());
-        }
-        else {
-            sbuffer<char, 1024> buffer(21*size(a), 0);
-            out << m_mpn_manager.to_string(digits(a), size(a), buffer.begin(), buffer.size());
-        }
+            out << '-';
+
+        auto sz = sizeof(digit_t) == 4 ? 11 : 21;
+        sbuffer<char, 1024> buffer(sz * size(a), 0);
+        out << m_mpn_manager.to_string(digits(a), size(a), buffer.begin(), buffer.size());
 #else
         // GMP version
         size_t sz = mpz_sizeinbase(*a.m_ptr, 10) + 2;
@@ -1835,7 +1899,7 @@ template<bool SYNCH>
 std::string mpz_manager<SYNCH>::to_string(mpz const & a) const {
     std::ostringstream buffer;
     display(buffer, a);
-    return buffer.str();
+    return std::move(buffer).str();
 }
 
 template<bool SYNCH>