Re: Referring to array element by descriptive name
Thank you for all your answers. I was concerned because I'm dealing with a small function that is called many times and where the bulk of the calculations in the simulation takes place. So even 5% performance difference would be significant for me. But it is good to know that compilers are smart enough to optimize this.
Re: Referring to array element by descriptive name
On 01/14/2017 07:11 AM, albert-j wrote:
> Is it possible to refer to an array element by a descriptive name, just
> for code clarity, without performance overhead? E.g.
>
> void aFunction(double[] arr) {
> double importantElement = arr[3];
> ... use importantElement ...
> }
>
> But the above, I suppose, introduces an extra copy operation?
>
I've used nested functions before. Compiled with
-O -inline -boundscheck=off
even dmd produces exact code for the following three access methods:
import std.stdio;
void aFunction(double[] arr) {
ref importantElement() {
return arr[3];
}
writeln("Indexed element : ", arr[3]);
writeln("importantElement: ", importantElement);
double originalIdea = arr[3];
writeln("Original idea : ", originalIdea);
}
void main() {
}
Here are the three calls; comments added by me. The only difference is
RCX vs. RAX for one of the calls:
.text._D6deneme9aFunctionFAdZv segment
assume CS:.text._D6deneme9aFunctionFAdZv
_D6deneme9aFunctionFAdZv:
pushRBP
mov RBP,RSP
sub RSP,010h
mov -010h[RBP],RDI
mov -8[RBP],RSI
; arr[3]
mov EDX,offset FLAT:_TMP0@32
mov EDI,012h
mov RSI,RDX
mov RAX,-8[RBP]
movsd XMM0,018h[RAX]
call _D3std5stdio18__T7writelnTAyaTdZ7writelnFNfAyadZv@PC32
; importantElement:
mov EDX,offset FLAT:_TMP0@32
mov EDI,012h
mov RSI,RDX
mov RCX,-8[RBP]
movsd XMM0,018h[RCX]
call _D3std5stdio18__T7writelnTAyaTdZ7writelnFNfAyadZv@PC32
; originalIdea:
mov EDX,offset FLAT:_TMP0@32
mov EDI,012h
mov RSI,RDX
mov RAX,-8[RBP]
movsd XMM0,018h[RAX]
call _D3std5stdio18__T7writelnTAyaTdZ7writelnFNfAyadZv@PC32
mov RSP,RBP
pop RBP
ret
0f1f
add byte ptr [RAX],0
add [RAX],AL
.text._D6deneme9aFunctionFAdZv ends
Ali
Re: Referring to array element by descriptive name
On Saturday, 14 January 2017 at 15:11:40 UTC, albert-j wrote:
Is it possible to refer to an array element by a descriptive
name, just for code clarity, without performance overhead? E.g.
void aFunction(double[] arr) {
double importantElement = arr[3];
... use importantElement ...
}
But the above, I suppose, introduces an extra copy operation?
Is the array always a fixed size? Or what?
I wonder since you might get away with a union, or a struct that
simply redirects the information appropriately. However it's a
lot of writing for very little benefit at all.
But honestly for as little loss you'll get of copying the one
element and then copying it back (maybe if you change it) I doubt
it will mean much if you just ignore trying to do a 0-cost
aliasing as you are trying to do. You'd have to be doing it
millions of times for such a copy to be noticeable.
Re: Referring to array element by descriptive name
On Saturday, 14 January 2017 at 15:11:40 UTC, albert-j wrote:
Is it possible to refer to an array element by a descriptive
name, just for code clarity, without performance overhead? E.g.
void aFunction(double[] arr) {
double importantElement = arr[3];
... use importantElement ...
}
But the above, I suppose, introduces an extra copy operation?
Unless the item type of that array is a complex like a big
struct, copying basic types won't have much effect at all. You
wouldn't notice it.
You could point to that element with a pointer:
double* importantElement = [3];
But then you are going to define that pointer variable anyway. On
top of that, for every access, instead of using the available
data, CPU would look at the pointed memory address to get the
value again and again (ignoring the cache).
