eth/vm/logic/arithmetic: handle exp(X,0) == 1. (#1217)

* eth/vm/logic/arithmetic: handle exp(X,0) == 1.

Caught by `exp8.json` test (not yet tracked in tests fixture).

Ref:

https://github.com/ethereum/tests/blob/10ab37c095bb87d2e781bcf112b6104912fccb44/VMTests/vmArithmeticTest/exp8.json

Accidentally caught in PR #1181.

* tests/opcodes: exponentiation: 0^0==1, 0^1==0.

eth/vm/logic/arithmetic.py

@@ -147,7 +147,9 @@ def exp(computation, gas_per_byte):
     bit_size = exponent.bit_length()
     byte_size = ceil8(bit_size) // 8
 
-    if base == 0:
+    if exponent == 0:
+        result = 1
+    elif base == 0:
         result = 0
     else:
         result = pow(base, exponent, constants.UINT_256_CEILING)

tests/core/opcodes/test_opcodes.py

@@ -123,6 +123,32 @@ def test_mul(vm_class, val1, val2, expected):
     assert result == expected
 
 
+@pytest.mark.parametrize(
+    'vm_class, base, exponent, expected',
+    (
+        (ByzantiumVM, 0, 1, 0,),
+        (ByzantiumVM, 0, 0, 1,),
+        (SpuriousDragonVM, 0, 1, 0,),
+        (SpuriousDragonVM, 0, 0, 1,),
+        (TangerineWhistleVM, 0, 1, 0,),
+        (TangerineWhistleVM, 0, 0, 1,),
+        (HomesteadVM, 0, 1, 0,),
+        (HomesteadVM, 0, 0, 1,),
+        (FrontierVM, 0, 1, 0,),
+        (FrontierVM, 0, 0, 1,),
+    )
+)
+def test_exp(vm_class, base, exponent, expected):
+    computation = prepare_computation(vm_class)
+    computation.stack_push(exponent)
+    computation.stack_push(base)
+    computation.opcodes[opcode_values.EXP](computation)
+
+    result = computation.stack_pop(type_hint=constants.UINT256)
+
+    assert result == expected
+
+
 @pytest.mark.parametrize(
     # Testcases from https://github.com/ethereum/EIPs/blob/master/EIPS/eip-145.md#shl-shift-left
     'vm_class, val1, val2, expected',