Correctly detect+handle overflow in mp_get_int64

Handles the fact that for 64-bit 2's complement numbers the maximum
positive value is 2**63 - 1, but the maximum negative value is 2**63.

Uses the mp_get_long_long now available with the newer version of
libtommath. Also, instead of counting the bits every time, only do it
during the too-large-value error path.

src/6model/reprs/P6bigint.c

@@ -4,30 +4,27 @@
    #define MIN(x,y) ((x)<(y)?(x):(y))
 #endif
 
-/* A forced 64-bit version of mp_get_long, since on some platforms long is
- * not all that long. */
-static MVMuint64 mp_get_int64(MVMThreadContext *tc, mp_int * a) {
-    int i, bits;
+/* Handles both int and uint cases. */
+static MVMuint64 mp_get_int64(MVMThreadContext *tc, mp_int * a, int sign) {
     MVMuint64 res;
+    MVMuint64 signed_max = 9223372036854775807ULL;
 
     if (a->used == 0) {
-         return 0;
+        return 0;
     }
 
-    bits = mp_count_bits(a);
-    if (bits > 64) {
-        MVM_exception_throw_adhoc(tc, "Cannot unbox %d bit wide bigint into native integer", bits);
+    /* For 64-bit 2's complement numbers the positive max is 2**63-1
+     * but the negative max is 2**63. */
+    if (sign && MP_NEG == SIGN(a)) {
+        ++signed_max;
     }
 
-    /* get number of digits of the lsb we have to read */
-    i = MIN(a->used,(int)((sizeof(MVMuint64)*CHAR_BIT+DIGIT_BIT-1)/DIGIT_BIT))-1;
-
-    /* get most significant digit of result */
-    res = DIGIT(a,i);
-
-    while (--i >= 0) {
-        res = (res << DIGIT_BIT) | DIGIT(a,i);
+    res = mp_get_long_long(a);
+    if (res == 0 || (sign && res > signed_max)) {
+        /* The mp_int was either so big it overflowed a MVMuint64 or was bigger than a signed result could be. */
+        MVM_exception_throw_adhoc(tc, "Cannot unbox %d bit wide bigint into native integer", mp_count_bits(a));
     }
+
     return res;
 }
 
@@ -92,10 +89,10 @@ static MVMint64 get_int(MVMThreadContext *tc, MVMSTable *st, MVMObject *root, vo
     if (MVM_BIGINT_IS_BIG(body)) {
         mp_int *i = body->u.bigint;
         if (MP_NEG == SIGN(i)) {
-            return -mp_get_int64(tc, i);
+            return -mp_get_int64(tc, i, 1);
         }
         else {
-            return mp_get_int64(tc, i);
+            return mp_get_int64(tc, i, 1);
         }
     }
     else {
@@ -123,7 +120,7 @@ static MVMuint64 get_uint(MVMThreadContext *tc, MVMSTable *st, MVMObject *root,
         if (MP_NEG == SIGN(i))
             MVM_exception_throw_adhoc(tc, "Cannot unbox negative bigint into native unsigned integer");
         else
-            return mp_get_int64(tc, i);
+            return mp_get_int64(tc, i, 0);
     }
     else {
         return body->u.smallint.value;