On Monday, 5 March 2018 at 06:01:27 UTC, J-S Caux wrote:
On Monday, 5 March 2018 at 05:40:09 UTC, rikki cattermole wrote:
On 05/03/2018 6:35 PM, J-S Caux wrote:
I'm considering shifting a large existing C++ codebase into D
(it's a scientific code making much use of functions like
atan, log etc).
I've compared the raw speed of atan between C++ (Apple LLVM
version 7.3.0 (clang-703.0.29)) and D (dmd v2.079.0, also
ldc2 1.7.0) by doing long loops of such functions.
I can't get the D to run faster than about half the speed of
C++.
double x = 0.0;
for (int a = 0; a < 1000000000; ++a) x += atan(1.0/(1.0 +
sqrt(1.0 + a)));
for C++ and
double x = 0.0;
for (int a = 0; a < 1_000_000_000; ++a) x += atan(1.0/(1.0 +
sqrt(1.0 + a)));
for D. C++ exec takes 40 seconds, D exec takes 68 seconds.
The performance problem with this code is that LDC does not yet
do cross-module inlining by default. GDC does. If you pass
`-enable-cross-module-inlining` to LDC, things should be faster.
In particular, std.sqrt is not inlined although it is profitable
to do so (it becomes one machine instruction). Things become
worse when using core.stdc.math.sqrt, because no implementation
source available: no inlining possible.
Another problem is that std.math.atan(double) just calls
std.math.atan(real). Calculations are more expensive on platforms
where real==80bits (i.e. x86), and that's not solvable with a
compile flag. What it takes is someone to write the double and
float versions of atan (and other math functions), but it
requires someone with the right knowledge to do it.
Your tests (and reporting about them) are much appreciated.
Please do file bug reports for these things. Perhaps you can take
a stab at implementing double-versions of the functions you need?
cheers,
Johan
