On Sunday, 3 February 2013 at 09:23:01 UTC, timotheecour wrote:
Static arrays suffer from:
1) bad implementation that allocates on the heap when we do:
"int[3]=[1,2,3];"
2) lack of syntactic sugar to declare them on on the fly, eg
when we want to pass a static array to a function without
declaring an intermediate variable. See my proposal for "auto
x=[1,2,3]s" here:
http://permalink.gmane.org/gmane.comp.lang.d.general/90035,
which would allow one to pass a static array to a function eg:
fun([1,2,3]s) without having to do int[3] temp; fun(temp), when
it's not passed by ref.
If 90035 won't get implemented, what about a library solution?
Below, we can construct a static array on the fly as:
"auto x=S(1,2,3);"
as opposed to:
"int[3] x=[1,2,3]"
advantages:
a) can directly pass to a function without creating temp
variable
b) less verbose
c) no heap allocations
d) 40 times faster in the example below (even 2.3x faster than
C, for some reason which eludes me)
----
import std.stdio,std.conv;
import std.traits:CommonType;
auto S(T...)(T a) if(!is(CommonType!T == void )){ //check to
prevent illegal stuff like S([],2)
alias CommonType!T T0;
T0[T.length]ret;
foreach(i,ai;a)
ret[i]=ai;
return ret;
}
void main(){
size_t n=1000000,z=0,i=0,j=0;
for(i=0;i<n;i++){
// auto
a=S(cast(size_t)i,i+1,i+2,i+3,i+4,i+5,i+6,i+7,i+8,i+9); //time:
0.351 with LDC
size_t[10] a=[i,i+1,i+2,i+3,i+4,i+5,i+6,i+7,i+8,i+9]; //time:
14.049 with LDC, 16s with dmd (-inline -O -release)
for(j=0;j<9;j++){z+=a[j];}
}
TOC;
writeln(z); //to prevent optimizing away result (?)
}
----
interestingly, this seems faster than the C version below. Why
is that so?
----
//test.c:
#include <stdio.h>
int main(){
size_t n=100000000,z=0,i=0,j=0;
for(i=0;i<n;i++){
size_t a[10]={i,i+1,i+2,i+3,i+4,i+5,i+6,i+7,i+8,i+9};
for(j=0;j<9;j++){z+=a[j];}
}
printf("%lu\n",z);
return 0;
}
----
gcc -O2 test.c -o test && time ./test
real 0m0.803s
Very interesting! Anything that beats c performance is a very big
plus for D.
Btw, you can replace the loop in S with
ret[] = a[];
Which should be even faster.
Also, to check that the assignment is being optimised away, try
using different data in each pass.
