Optimise range() iteration despite type casts on the step value

Given the code below, all of the functions compile except for mysum2.

cdef double mysum0(double[:] a) nogil:
    cdef size_t i
    cdef double r = 0
    for i in range(<size_t>len(a)):
        r += a[i]
    return r


cdef double mysum1(double[:] a) nogil:
    cdef size_t i
    cdef double r = 0
    for i in range(<size_t>0, <size_t>len(a)):
        r += a[i]
    return r


cdef double mysum2(double[:] a) nogil:
    cdef size_t i
    cdef double r = 0
    for i in range(<size_t>0, <size_t>len(a), <size_t>1):
        r += a[i]
    return r

The errors generated by Cython for mysum2 are as follows.

Error compiling Cython file:
------------------------------------------------------------
...


cdef double mysum2(double[:] a) nogil:
    cdef size_t i
    cdef double r = 0
    for i in range(<size_t>0, <size_t>len(a), <size_t>1):
            ^
------------------------------------------------------------

test_range.pyx:20:13: Coercion from Python not allowed without the GIL

Error compiling Cython file:
------------------------------------------------------------
...


cdef double mysum2(double[:] a) nogil:
    cdef size_t i
    cdef double r = 0
    for i in range(<size_t>0, <size_t>len(a), <size_t>1):
            ^
------------------------------------------------------------

test_range.pyx:20:13: Iterating over Python object not allowed without gil

Error compiling Cython file:
------------------------------------------------------------
...


cdef double mysum2(double[:] a) nogil:
    cdef size_t i
    cdef double r = 0
    for i in range(<size_t>0, <size_t>len(a), <size_t>1):
                 ^
------------------------------------------------------------

test_range.pyx:20:18: Calling gil-requiring function not allowed without gil

Error compiling Cython file:
------------------------------------------------------------
...


cdef double mysum2(double[:] a) nogil:
    cdef size_t i
    cdef double r = 0
    for i in range(<size_t>0, <size_t>len(a), <size_t>1):
                 ^
------------------------------------------------------------

test_range.pyx:20:18: Constructing Python tuple not allowed without gil

Error compiling Cython file:
------------------------------------------------------------
...


cdef double mysum2(double[:] a) nogil:
    cdef size_t i
    cdef double r = 0
    for i in range(<size_t>0, <size_t>len(a), <size_t>1):
                  ^
------------------------------------------------------------

test_range.pyx:20:19: Converting to Python object not allowed without gil

Error compiling Cython file:
------------------------------------------------------------
...


cdef double mysum2(double[:] a) nogil:
    cdef size_t i
    cdef double r = 0
    for i in range(<size_t>0, <size_t>len(a), <size_t>1):
                             ^
------------------------------------------------------------

test_range.pyx:20:30: Converting to Python object not allowed without gil

Error compiling Cython file:
------------------------------------------------------------
...


cdef double mysum2(double[:] a) nogil:
    cdef size_t i
    cdef double r = 0
    for i in range(<size_t>0, <size_t>len(a), <size_t>1):
                                             ^
------------------------------------------------------------

test_range.pyx:20:46: Converting to Python object not allowed without gil

Details about the environment used below. This was run on my Mac, but expect the error to occur independently of the OS used.

name: cython
channels:
  - conda-forge
  - defaults
dependencies:
  - appdirs=1.4.3=py_1
  - appnope=0.1.0=py36_0
  - asn1crypto=0.24.0=py_1
  - attrs=18.1.0=py_1
  - automat=0.7.0=py36_0
  - backcall=0.1.0=py_0
  - blas=1.1=openblas
  - bleach=2.1.3=py_0
  - ca-certificates=2018.4.16=0
  - certifi=2018.4.16=py36_0
  - cffi=1.11.5=py36_0
  - constantly=15.1.0=py_0
  - cryptography=2.3=py36hdffb7b8_0
  - cryptography-vectors=2.3=py36_0
  - cython=0.28.4=py36hfc679d8_0
  - decorator=4.3.0=py_0
  - entrypoints=0.2.3=py36_1
  - html5lib=1.0.1=py_0
  - hyperlink=17.3.1=py_0
  - idna=2.7=py36_2
  - incremental=17.5.0=py_0
  - ipykernel=4.8.2=py36_0
  - ipython=6.4.0=py36_0
  - ipython_genutils=0.2.0=py_1
  - jedi=0.12.0=py36_0
  - jinja2=2.10=py_1
  - jsonschema=2.6.0=py36_1
  - jupyter_client=5.2.3=py_1
  - jupyter_core=4.4.0=py_0
  - libffi=3.2.1=3
  - libgfortran=3.0.0=0
  - libsodium=1.0.16=0
  - line_profiler=2.1.2=py36h470a237_1
  - markupsafe=1.0=py36_0
  - mistune=0.8.3=py36_1
  - nbconvert=5.3.1=py_1
  - nbformat=4.4.0=py_1
  - ncurses=6.1=0
  - notebook=5.6.0=py36_0
  - numpy=1.14.5=py36_blas_openblashd3ea46f_201
  - openblas=0.2.20=8
  - openssl=1.0.2o=0
  - pandoc=2.2.2=hde52d81_1
  - pandocfilters=1.4.2=py_1
  - parso=0.3.0=py_0
  - pexpect=4.6.0=py36_0
  - pickleshare=0.7.4=py36_0
  - prometheus_client=0.3.0=py_0
  - prompt_toolkit=1.0.15=py36_0
  - ptyprocess=0.6.0=py36_0
  - pyasn1=0.4.3=py_0
  - pyasn1-modules=0.2.1=py_0
  - pycparser=2.18=py_1
  - pygments=2.2.0=py_1
  - pyhamcrest=1.9.0=py_2
  - pyopenssl=18.0.0=py36_0
  - python=3.6.5=1
  - python-dateutil=2.7.3=py_0
  - pyzmq=17.1.0=py36hae99301_0
  - readline=7.0=haf1bffa_1
  - scipy=1.1.0=py36_blas_openblashd3ea46f_201
  - send2trash=1.5.0=py_0
  - service_identity=17.0.0=py_0
  - setuptools=40.0.0=py36_0
  - simplegeneric=0.8.1=py_1
  - six=1.11.0=py36_1
  - sqlite=3.20.1=0
  - terminado=0.8.1=py36_0
  - testpath=0.3.1=py36_0
  - tk=8.6.8=0
  - tornado=5.1=py36_0
  - traitlets=4.3.2=py36_0
  - wcwidth=0.1.7=py_1
  - webencodings=0.5.1=py36_0
  - xz=5.2.3=0
  - zeromq=4.2.5=hfc679d8_4
  - zlib=1.2.11=h470a237_3
  - zope.interface=4.5.0=py36h470a237_0
  - twisted=17.5.0=py36_0

[jimy-byerley]

That’s what I have done so far. 😐 There was nothing else at the time

[scoder]

Note that completing PR #368 would probably get us most of the benefit – this issue is then only a small optimisation on top. See #1106.