As described in the Jira issue, this replaces the implementation of
isPowerOf2() and power(). It also revamps floorLog2 so that there only
needs to be one implementation and no assumptions about how big certain
types are.
The way power() used to work was to raise a number n to an exponent e
by multiplying n times itself e times. As a warning in this function
explains, this can be quite slow for large e. A much more efficient
way to raise a number to an exponent is to square n over and over, and
to multiply in the current square if that bit of e is set.
n ^ 15 = (n^1) * (n^2) * (n^4) * (n^8)
n^8 = (n^4)^2
n^4 = (n^2)^2
n^2 = n^2
n^1 = n
So that takes 6 multiplications, n^2, (n^2)^2, (n^4)^2, and then each
multipy to compute the final result, instead of 14.
The difference is more pronounced for larger exponents, although you'd
quickly start to overflow a uint64_t.
Jira Issue: https://gem5.atlassian.net/browse/GEM5-140
Change-Id: I0ae05aeba1b5882d2a616613b1679e6206b4cbfe
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26164
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Testing intmath.hh and intmath.cc. Here is the
list of the functions that are tested.
intmath.isPowerOf2, intmath.power, intmath.floorLog2,
intmath.ceilLog2, intmath.divCeil, intmath.roundUp,
intmath.roundDown. Other functions are not tested,
because they are not currently used and are dead code.
Change-Id: I150ac1b5cead93c6698a8c9e9cec80bd87ef181a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22081
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Mahyar Samani <msamani@ucdavis.edu>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
