New issue 2493: Unroll map(function, ${fixed width tuple})
https://bitbucket.org/pypy/pypy/issues/2493/unroll-map-function-fixed-width-tuple
rowillia:
I met Phil Jenvey last night at the SF Python meetup and he suggested I file an
issue on this:
We've been experimenting with PyPy, and we've got some math-heavy workloads
that we've optimized with Cython, for example computing the haversine distance
between two lat/lngs, below is a code snippet:
```python
from math import sin, cos, asin, sqrt
radians_per_degree = 0.0174532925
AVG_EARTH_RADIUS = 6371.0
MILES_PER_KM = 0.621371
def radians(theta):
return radians_per_degree*theta
def maybe_convert_to_miles(distance, miles):
if miles:
return MILES_PER_KM * distance
else:
return distance
def _haversine(lat1, lng1, lat2, lng2):
# convert all latitudes/longitudes from decimal degrees to radians
lat1, lng1, lat2, lng2 = map(radians, (lat1, lng1, lat2, lng2))
# calculate haversine
lat = lat2 - lat1
lng = lng2 - lng1
l1 = sin(lat * 0.5)
l2 = sin(lng * 0.5)
d = (l1 * l1) + cos(lat1) * cos(lat2) * (l2 * l2)
return 2 * AVG_EARTH_RADIUS * asin(sqrt(d))
def haversine(point1, point2, miles=0):
result = _haversine(point1[0], point1[1], point2[0], point2[1])
return maybe_convert_to_miles(result, miles)
LYON = (45.7597, 4.8422)
PARIS = (48.8567, 2.3508)
MUNICH = (48.14287, 11.5770)
```
Last night I noted to Phil that the Cython version running in CPython was 2x
faster than the pure python version running in PyPy. This was actually
incorrect, the Cython version manually unrolled ` lat1, lng1, lat2, lng2 =
map(radians, (lat1, lng1, lat2, lng2))` into
```python
lat1 = radians(lat1)
lng1 = radians(lng1)
lat2 = radians(lat2)
lng2 = radians(lng2)
```
Which resulted in a big win. Doing the same in PyPy resulted in the PyPy code
only being 5% slower!
It looks like there might be an opportunity for PyPy to unroll map calls in
cases like this where we've got a fixed width tuple.
Thanks for the amazing project!
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