Subject: Re: [boost] sorting floats by casting to integer
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2009-07-05 12:17:09
Steven Ross wrote:
>>> I've been investigating why float_sort is so much faster than std::sort on
>>> x86 processors, and why sorting integers is so much faster than
>>> and came to the conclusion that floating-point comparisons are extremely
>>> slow on x86.
If the problem were simply that "x86 FP is very slow", then this
problem would be evident with std::sort as well as your algorithm.
I've therefore done a simple test with a program like the following:
static inline bool compare_floats_as_ints(float f1, float f2)
int i1, i2;
std::cout << "t = " << t_end-t_start << "\n";
Note that I haven't actually checked if I get the right answers from
this, so please let me know if you can see any mistakes! Also note
that I have not done any fixup for negative floats, so some small
additional time would be needed for that.
I've run variations of this code with the following results:
vector<int>, std::sort(begin,end) : 3.71 s
vector<float>, std::sort(begin,end) : 9.12 s
vector<float>, std::sort(begin,end,compare_floats_as_ints) : 4.19 s
So it looks like integer comparisons are two to three times faster than
float comparisons on this hardware. This is the sort of difference
that I would expect. Sorting floats as integers is a bit slower than
sorting integers, but significantly faster than sorting them as floats
(but note that the fixup for negatives etc. is not implemented).
This is with g++-4.3 -O3 -fprofile-generate/use on Linux, on a 1.2 GHz
VIA C7-M. Using profile-driven optimisation is important to get good results.
I feel that your observation that FP comparisons are 100X (or even 20X)
slower than integer comparisons on Intel hardware are implausible.
There must be some other reason for the slowness.
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