[math] Implement pow2 function

[nagayev]

BPO	36415
Nosy	@mdickinson, @serhiy-storchaka, @nagayev, @tirkarthi

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-03-25.08:44:51.306>
created_at = <Date 2019-03-24.11:33:10.455>
labels = ['3.8', 'type-bug', 'library']
title = '[math] Implement pow2 function'
updated_at = <Date 2019-03-25.08:44:51.305>
user = 'https://github.com/nagayev'

bugs.python.org fields:

activity = <Date 2019-03-25.08:44:51.305>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2019-03-25.08:44:51.306>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2019-03-24.11:33:10.455>
creator = 'nagayev'
dependencies = []
files = []
hgrepos = []
issue_num = 36415
keywords = []
message_count = 4.0
messages = ['338728', '338729', '338730', '338792']
nosy_count = 4.0
nosy_names = ['mark.dickinson', 'serhiy.storchaka', 'nagayev', 'xtreak']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue36415'
versions = ['Python 3.8']

[nagayev]

Hello.
I want to implement pow2 function special for powers of 2.
pow2 works 110 times faster if we pow 4 to 10**6
pow2 implementaion on Python:
def pow2(a,b):
p=log2(a)*b
if not p.isinteger():
raise TypeError('a isn't power of 2!')
return 1«p

[nagayev]

110 times is the result on MacBook 2017 13
In my phone this works 187 times faster!

[tirkarthi]

See also bpo-21420 in a previous attempt to optimize powers of 2 calculation in current code.

[serhiy-storchaka]

See also bpo-31980. The objections exposed there are applicable to this issue.