Floating point performance issue

[Vincent Lefevre]

On 2011-12-20 12:16:34 +0100, Vincent Lefevre wrote:
> Now, the test program is faster where the values are *closer* to 0.
> This contradicts what you say about subnormals.

Actually, after thinking a bit more about it, there are also more
operations on the subnormals in the case 0.9, so that the subnormals
could also be a problem, and indeed:

ypig:~> time ./a.out 0.9999999
./a.out 0.9999999  0.10s user 0.00s system 92% cpu 0.108 total

ypig:~> time ./a.out 0.9
./a.out 0.9  2.93s user 0.00s system 98% cpu 2.987 total

ypig:~> time ./a.out 0.1
./a.out 0.1  0.10s user 0.00s system 86% cpu 0.116 total

The fact that only the second case is slower tends to indicate that
the problem is the subnormals (whether they are implemented in
software and in hardware).

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