Referring to array element by descriptive name

[Ali Çehreli via Digitalmars-d-learn]

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:
		push	RBP
		mov	RBP,RSP
		sub	RSP,010h
		mov	-010h[RBP],RDI
		mov	-8[RBP],RSI

; arr[3]
		mov	EDX,offset FLAT:_TMP0 at 32
		mov	EDI,012h
		mov	RSI,RDX
		mov	RAX,-8[RBP]
		movsd	XMM0,018h[RAX]
		call	  _D3std5stdio18__T7writelnTAyaTdZ7writelnFNfAyadZv at PC32

; importantElement:
		mov	EDX,offset FLAT:_TMP0 at 32
		mov	EDI,012h
		mov	RSI,RDX
		mov	RCX,-8[RBP]
		movsd	XMM0,018h[RCX]
		call	  _D3std5stdio18__T7writelnTAyaTdZ7writelnFNfAyadZv at PC32

; originalIdea:
		mov	EDX,offset FLAT:_TMP0 at 32
		mov	EDI,012h
		mov	RSI,RDX
		mov	RAX,-8[RBP]
		movsd	XMM0,018h[RAX]
		call	  _D3std5stdio18__T7writelnTAyaTdZ7writelnFNfAyadZv at PC32


		mov	RSP,RBP
		pop	RBP
		ret
		0f1f
		add	byte ptr [RAX],0
		add	[RAX],AL
.text._D6deneme9aFunctionFAdZv	ends

Ali