Speed of math function atan: comparison D and C++

[H. S. Teoh]

On Mon, Mar 05, 2018 at 06:39:21PM +0000, J-S Caux via Digitalmars-d-learn wrote:
[...]
> I've tested these two very basic representative codes:
> https://www.dropbox.com/s/b5o4i8h43qh1saf/test.cc?dl=0
> https://www.dropbox.com/s/zsaikhdoyun3olk/test.d?dl=0
> 
> Results:
> 
> C++:
> g++ (Apple LLVM version 7.3.0):  9.5 secs
> g++ (GCC 7.1.0):  10.7 secs
> 
> D:
> dmd :  35.5 secs
> dmd -release -inline -O : 29.5 secs
> ldc2 :  34.4 secs
> ldc2 -release -O : 31.5 secs
> 
> But now: using the core.stdc.math atan as per Uknown's suggestion:
> D:
> dmd:  9 secs
> dmd -release -inline -O :  6.8 secs
> ldc2 : 10 secs
> ldc2 -release -O :  6.5 secs   <- best
> 
> So indeed the difference is between the `std.math atan` versus the
> `core.stdc.math atan`. Thanks Uknown! Just knowing this trick could
> make the difference between me and other scientists switching over to
> D...
> 
> But now comes the question: can the D fundamental maths functions be
> propped up to be as fast as the C ones?

Walter has been adamant that we should always compute std.math.*
functions with the `real` type, which on x86 maps to the non-IEEE 80-bit
floats.  However, 80-bit floats have been deprecated for a while now,
and pretty much nobody cares to improve their performance on newer CPUs,
focusing instead on SSE/MMX performance with 64-bit doubles.  People
have been clamoring for using 64-bit doubles by default rather than
80-bit floats, but so far Walter has refused to budge.

But perhaps this time, we might have a strong case for pushing this into
D.  IMO, it has been long overdue.  I filed an issue for this:

	https://issues.dlang.org/show_bug.cgi?id=18559

If you have any additional relevant information, please post it there so
that we can build a strong case to convince Walter about this issue.


T

-- 
Heuristics are bug-ridden by definition. If they didn't have bugs, they'd be algorithms.