...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Using a simple backend
class that wraps any fundamental
(built-in) arithmetic type we can measure the overhead involved in
wrapping a type inside the number
frontend, and the effect that turning on expression templates has. The following
table compares the performance between double
and a double
wrapped inside
class number
:
Table 1.12. Bessel Functions (16 digit precision)
Type |
Time |
---|---|
arithmetic_backend<double> |
2.09301 (0.00133409s) |
arithmetic_backend<double> - no expression templates |
1 (0.000637403s) |
double |
1.07956 (0.000688113s) |
As you can see whether or not there is an overhead, and how large it is depends on the actual situation, but the overhead is in any cases small. Expression templates generally add a greater overhead the more complex the expression becomes due to the logic of figuring out how to best unpack and evaluate the expression, but of course this is also the situation where you save more temporaries. For a "trivial" backend like this, saving temporaries has no benefit, but for larger types it becomes a bigger win.
The following table compares arithmetic using either long
long
or number<arithmetic_backend<long long> >
for the voronoi-diagram
builder test:
Type |
Relative time |
---|---|
|
1.0(0.0128646s) |
|
1.005 (0.0129255s) |
This test involves mainly creating a lot of temporaries and performing a small amount of arithmetic on them, with very little difference in performance between the native and "wrapped" types.
Table 1.13. Platform Details
Platform |
Linux 5.3.0-24-generic, version #26-Ubuntu SMP Thu Nov 14 01:33:18 UTC 2019, x86_64 |
---|---|
Compiler |
GNU C++ version 9.2.1 20191008 |
Boost |
1.72.0 |
Run date |
Dec 13 2019 |