The following test can never succeed when PY_SSIZE_T_MAX is 63-bits (as that number of allocations would exceed possible time and memory).

    #define MAX_DEQUE_LEN (PY_SSIZE_T_MAX - 3*BLOCKLEN)

    if (len >= MAX_DEQUE_LEN) {
        PyErr_SetString(PyExc_OverflowError,
                        "cannot add more blocks to the deque");
        return NULL;
    }

Removing the test saves a recurring block of code through-out the module.  The block adds register pressure, triggers an unnecessary memory access and has a predictable test-and-jump.

Conditional compilation can leave the test in for builds with size_t under 64-bits.

We can remove this test at all, it can never succeed on 32-bit platform. deque takes at least 4 bytes (PyObject*) per element. In 32-bit address space the maximal deque size is less than 2**32/4 = 2**30 that is much less than MAX_DEQUE_LEN = 2**31-1-3*64.

New changeset 1733b3bd46db by Raymond Hettinger in branch 'default':
Issue #25414: Remove unnecessary tests that can never succeed.
https://hg.python.org/cpython/rev/1733b3bd46db