add test and fix inverse modulus

Author: timfel

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

@@ -117,3 +117,12 @@ def mypow_rev(a, b, c):
         assert mypow_rev(*args[:-1]) == args[-1], "%r -> %r == %r" % (args, mypow(*args[:-1]), args[-1])
 
     assert 2**1.0 == 2.0
+
+    try:
+        pow(12,-2,100)
+    except ValueError as e:
+        assert "base is not invertible for the given modulus" in str(e)
+    else:
+        assert False
+
+    assert pow(1234567, -2, 100) == 9

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

@@ -299,7 +299,7 @@ def test_pow():
     # (0xffffffffffffffff >> 63) is used to produce a non-narrowed int
     assert 2**(0xffffffffffffffff >> 63) == 2
 
-    if sys.implementation.name == "graalpython":
+    if sys.version_info.minor >= 8:
         # for some reason this hangs CPython on the CI even if it's just parsed
         from pow_tests import test_pow
         test_pow()

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

@@ -802,26 +802,24 @@ private Object objectOp(Object left, Object right, Object mod) {
             BigInteger bigMod = integerToBigInteger(mod);
             if (bigMod.signum() == 0) {
                 throw raise(ValueError, ErrorMessages.POW_THIRD_ARG_CANNOT_BE_ZERO);
-            } else if (bigRight.signum() >= 0 && bigMod.signum() > 0) {
-                return bigLeft.modPow(bigRight, bigMod);
-            } else if (bigMod.signum() < 0) {
-                BigInteger bigModPos = bigMod.abs();
+            } else {
+                BigInteger bigModPos;
+                if (bigMod.signum() < 0) {
+                    bigModPos = bigMod.abs();
+                } else {
+                    bigModPos = bigMod;
+                }
                 try {
                     BigInteger pow = bigLeft.modPow(bigRight, bigModPos);
-                    if (!BigInteger.ZERO.equals(pow)) {
+                    if (bigModPos != bigMod && !BigInteger.ZERO.equals(pow)) {
                         return pow.subtract(bigModPos);
                     } else {
                         return pow;
                     }
                 } catch (ArithmeticException e) {
-                    // bigModPos was used, so this exception must mean the exponent was negative
-                    return Math.pow(bigLeft.doubleValue(), bigRight.doubleValue()) % bigMod.doubleValue();
-                }
-            } else {
-                try {
-                    return bigLeft.modPow(bigRight, bigMod);
-                } catch (ArithmeticException e) {
-                    return Math.pow(bigLeft.doubleValue(), bigRight.doubleValue()) % bigMod.doubleValue();
+                    // a positive mod was used, so this exception must mean the exponent was
+                    // negative and the base is not relatively prime to the exponent
+                    throw raise(ValueError, ErrorMessages.POW_BASE_NOT_INVERTIBLE);
                 }
             }
         }

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/nodes/ErrorMessages.java

@@ -489,5 +489,6 @@ public abstract class ErrorMessages {
     public static final String ZIPIMPORT_CANT_READ_DATA = "zipimport: can't read data";
     public static final String ZIPIMPORT_WRONG_CACHED_FILE_POS = "zipimport: wrong cached file position";
     public static final String ACCESS_TO_INTERNAL_LANG_NOT_PERMITTED = "access to internal language %s is not permitted";
+    public static final String POW_BASE_NOT_INVERTIBLE = "base is not invertible for the given modulus";
     public static final String POW_THIRD_ARG_CANNOT_BE_ZERO = "pow() 3rd argument cannot be 0";
 }