[GR-13915] Strength reduction on BigInteger multiplication

PullRequest: graalpython/441

Author: cosminbasca

graalpython/com.oracle.graal.python.test/src/tests/test_int.py

@@ -67,6 +67,11 @@ def test_boolean2int():
     assert int(False) == 0
 
 
+def test_bigint_mul():
+    assert 99999937497465632974931 * 1223432423545234234123123 == 122343165886896325043539375228725106116626429513
+    assert 99999937497465632974931 * (2**100) == 126764980791447734004805377032945185921379990352429056
+
+
 def test_int_from_custom():
     class CustomInt4():
         def __int__(self):

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/ints/IntBuiltins.java

@@ -637,27 +637,41 @@ PInt doLLOvf(long x, long y) {
             if (((ax | ay) >>> 31 != 0)) {
                 int leadingZeros = Long.numberOfLeadingZeros(ax) + Long.numberOfLeadingZeros(ay);
                 if (leadingZeros < 66) {
-                    return factory().createInt(op(BigInteger.valueOf(x), BigInteger.valueOf(y)));
+                    return factory().createInt(mul(BigInteger.valueOf(x), BigInteger.valueOf(y)));
                 }
             }
             return factory().createInt(r);
         }
 
         @Specialization
         PInt doPIntLong(PInt left, long right) {
-            return factory().createInt(op(left.getValue(), BigInteger.valueOf(right)));
+            return factory().createInt(mul(left.getValue(), BigInteger.valueOf(right)));
         }
 
         @Specialization
         PInt doPIntPInt(PInt left, PInt right) {
-            return factory().createInt(op(left.getValue(), right.getValue()));
+            return factory().createInt(mul(left.getValue(), right.getValue()));
+        }
+
+        @TruffleBoundary
+        BigInteger mul(BigInteger a, BigInteger b) {
+            if (b.and(b.subtract(BigInteger.ONE)).equals(BigInteger.ZERO)) {
+                return bigIntegerShift(a, b.getLowestSetBit());
+            } else {
+                return bigIntegerMul(a, b);
+            }
         }
 
         @TruffleBoundary
-        BigInteger op(BigInteger a, BigInteger b) {
+        BigInteger bigIntegerMul(BigInteger a, BigInteger b) {
             return a.multiply(b);
         }
 
+        @TruffleBoundary
+        BigInteger bigIntegerShift(BigInteger a, int n) {
+            return a.shiftLeft(n);
+        }
+
         @SuppressWarnings("unused")
         @Fallback
         PNotImplemented doGeneric(Object left, Object right) {