On 23.03.2012 9:57, Comrad wrote:
On Thursday, 22 March 2012 at 10:43:35 UTC, Trass3r wrote:
What is the status at the moment? What compiler and with which
compiler flags I should use to achieve maximum performance?
In general gdc or ldc. Not sure how good vectorization is though, esp.
auto-vectorization.
On the other hand the so called vector operations like a[] = b[] +
c[]; are lowered to hand-written SSE assembly even in dmd.
I had such a snippet to test:
1 import std.stdio;
2 void main()
3 {
4 double[2] a=[1.,0.];
5 double[2] a1=[1.,0.];
6 double[2] a2=[1.,0.];
7 double[2] a3=[0.,0.];
Here is a culprit, the array ops [] are tuned for arbitrary long(!)
arrays, they are not plain 1 simd SEE op. They are handcrafted loops(!)
on SSE ops, cool and fast for arrays in general, not fixed
pairs/trios/etc. I believe it might change in future, if compiler is
able to deduce that size is fixed, and use more optimal code for small
sizes.
8 foreach(i;0..1000000000)
9 a3[]+=a[]+a1[]*a2[];
10 writeln(a3);
11 }
And I compared with the following d code:
1 import std.stdio;
2 void main()
3 {
4 double[2] a=[1.,0.];
5 double[2] a1=[1.,0.];
6 double[2] a2=[1.,0.];
7 double[2] a3=[0.,0.];
8 foreach(i;0..1000000000)
9 {
10 a3[0]+=a[0]+a1[0]*a2[0];
11 a3[1]+=a[1]+a1[1]*a2[1];
12 }
13 writeln(a3);
14 }
And with the following c code:
1 #include <stdio.h>
2 int main()
3 {
4 double a[2]={1.,0.};
5 double a1[2]={1.,0.};
6 double a2[2]={1.,0.};
7 double a3[2];
8 unsigned i;
9 for(i=0;i<1000000000;++i)
10 {
11 a3[0]+=a[0]+a1[0]*a2[0];
12 a3[1]+=a[1]+a1[1]*a2[1];
13 }
14 printf("%f %f\n",a3[0],a3[1]);
15 return 0;
16 }
The last one I compiled with gcc two previous with dmd and ldc. C code
with -O2
was the fastest and as fast as d without slicing compiled with ldc. d
code with slicing was 3 times slower (ldc compiler). I tried to compile
with different optimization flags, that didn't help. Maybe I used the
wrong ones. Can someone comment on this?
--
Dmitry Olshansky
