fix[ux]: fix compiler hang for large exponentiations (#3893)

this commit fixes a UX bug where the compiler would hang while trying to
constant-fold large exponentiations. it does a loose check to see if the
output would be vastly out of bounds and fails fast if so.

Author: charles-cooper

tests/functional/codegen/types/numbers/test_exponents.py

@@ -3,6 +3,18 @@
 from hypothesis import strategies as st
 
 from vyper.codegen.arithmetic import calculate_largest_base, calculate_largest_power
+from vyper.compiler import compile_code
+from vyper.exceptions import InvalidLiteral
+
+
+def test_compiler_hang():
+    code = """
+@external
+def f0():
+    lv0: uint256 = max_value(int128) ** max_value(int128)
+    """
+    with pytest.raises(InvalidLiteral):
+        compile_code(code)
 
 
 @pytest.mark.fuzzing

vyper/ast/nodes.py

@@ -3,6 +3,7 @@
 import copy
 import decimal
 import functools
+import math
 import operator
 import pickle
 import sys
@@ -1080,6 +1081,15 @@ def _op(self, left, right):
             raise TypeMismatch("Cannot perform exponentiation on decimal values.", self._parent)
         if right < 0:
             raise InvalidOperation("Cannot calculate a negative power", self._parent)
+        # prevent a compiler hang. we are ok with false positives at this
+        # stage since we are just trying to filter out inputs which can cause
+        # the compiler to hang. the others will get caught during constant
+        # folding or codegen.
+        # l**r > 2**256
+        # r * ln(l) > ln(2 ** 256)
+        # r > ln(2 ** 256) / ln(l)
+        if right > math.log(decimal.Decimal(2**257)) / math.log(decimal.Decimal(left)):
+            raise InvalidLiteral("Out of bounds", self)
         return int(left**right)