52 lines
2.2 KiB
Plaintext
52 lines
2.2 KiB
Plaintext
[/
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Copyright 2011 - 2020 John Maddock.
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Copyright 2013 - 2019 Paul A. Bristow.
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Copyright 2013 Christopher Kormanyos.
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt).
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]
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[section:overhead The Overhead in the Number Class Wrapper]
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Using a simple [@../../performance/arithmetic_backend.hpp backend class] that wraps any __fundamental arithmetic type
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we can measure the overhead involved in wrapping a type inside the `number` frontend, and the effect that turning
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on expression templates has. The following table compares the performance between `double` and a `double` wrapped
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inside class `number`:
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[table Bessel Functions (16 digit precision)
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[[Type][Time]]
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[[arithmetic_backend<double>][2.09301 (0.00133409s)]]
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[[arithmetic_backend<double> - no expression templates][1 (0.000637403s)]]
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[[double][1.07956 (0.000688113s)]]
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]
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As you can see whether or not there is an overhead, and how large it is depends on the actual situation,
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but the overhead is in any cases small. Expression templates generally add a greater overhead the
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more complex the expression becomes due to the logic of figuring out how to best unpack and evaluate
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the expression, but of course this is also the situation where you save more temporaries. For a
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"trivial" backend like this, saving temporaries has no benefit, but for larger types it becomes
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a bigger win.
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The following table compares arithmetic using either `long long` or `number<arithmetic_backend<long long> >`
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for the [@../../performance/voronoi_performance.cpp voronoi-diagram builder test]:
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[table
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[[Type][Relative time]]
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[[`int64_t`][[*1.0](0.0128646s)]]
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[[`number<arithmetic_backend<int64_t>, et_off>`][1.005 (0.0129255s)]]
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]
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This test involves mainly creating a lot of temporaries and performing a small amount of arithmetic on them,
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with very little difference in performance between the native and "wrapped" types.
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[table:platform Platform Details
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[[Platform][Linux 5.3.0-24-generic, version \#26-Ubuntu SMP Thu Nov 14 01:33:18 UTC 2019, x86_64]]
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[[Compiler][GNU C++ version 9.2.1 20191008]]
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[[Boost][1.72.0]]
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[[Run date][Dec 13 2019]]
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]
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[endsect]
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