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If __debug__ were referenced from the code object's co_consts instead of checking locals, globals and builtins (LOAD_NAME), then optimize=1 would work consistently for a given code object. 
Currently in 3.4.1:
 >>> dis.dis(compile("if __debug__: x = 1", "", "exec", optimize=0))
 1 0 LOAD_CONST 0 (1)
 3 STORE_NAME 0 (x)
 6 LOAD_CONST 1 (None)
 9 RETURN_VALUE
 >>> dis.dis(compile("if __debug__: x = 1", "", "exec", optimize=1))
 1 0 LOAD_CONST 0 (None)
 3 RETURN_VALUE
 >>> dis.dis(compile("x = __debug__", "", "exec", optimize=0))
 1 0 LOAD_NAME 0 (__debug__)
 3 STORE_NAME 1 (x)
 6 LOAD_CONST 0 (None)
 9 RETURN_VALUE
 >>> dis.dis(compile("x = __debug__", "", "exec", optimize=1))
 1 0 LOAD_NAME 0 (__debug__)
 3 STORE_NAME 1 (x)
 6 LOAD_CONST 0 (None)
 9 RETURN_VALUE
With the current design, an exec can override builtins.__debug__ in globals or locals:
 >>> exec("print(__debug__)", globals(), {"__debug__": "spam"})
 spam