Why doesn't the optimizer eliminate High in a loop?

Question

People said that Delphi produces quite-nice optimized code on integer operations. I try the following example in Delphi 2007, and see its assembly code produced by the compiler. ``` program p1000; {$APPTYPE CONSOLE} procedure test; var arr: array of integer; i: integer; begin SetLength(arr, 100); for i := 0 to High(arr) do begin if (i = High(arr)) then begin arr[i] := -9; end; end; end; begin test; readln; end. ``` When building configuration is set to DEBUG, I can set a breakpoint and use shortkey Ctrl+Alt+D to see its assembly code, like this: ``` Project3.dpr.11: for i := 0 to High(arr) do 004045A1 8B45FC mov eax,[ebp-$04] 004045A4 E8F7FAFFFF call @DynArrayHigh 004045A9 8BF0 mov esi,eax 004045AB 85F6 test esi,esi 004045AD 7C1D jl $004045cc 004045AF 46 inc esi 004045B0 33DB xor ebx,ebx Project3.dpr.13: if (i = High(arr)) then 004045B2 8B45FC mov eax,[ebp-$04] 004045B5 E8E6FAFFFF **call @DynArrayHigh** 004045BA 3BD8 cmp ebx,eax 004045BC 750A jnz $004045c8 Project3.dpr.15: arr[i] := -…

Accepted Answer

This is not a answer but rather a (self destructing) comment :)

In my view, The compiler must not attempt to optimize this.

Why should the compiler attempt to optimize a (non-deterministic) `High` function as opposed to other? (such as `Length`)

The dynamic array length might be changed inside the loop either by `SetLenth`, or by other means. the array might be re-initilzed at run-time and your code might depend on that:

```
for i := 0 to High(arr) do
begin
  if (i = High(arr)) then
    arr[i] := -9
  else
    if foo() then
      arr := nil; // or SetLength(arr, 0);

if High(arr) = -1 then Exit; // arr is nil  
end;
```

How do you suggest this should be optimized? Should the compiler even attempt to optimized this? I don't see anything special about `High` functionm even if the compiler translates it to `@DynArrayHigh`.

If you want your code to be optimized , optimize it yourself .e.g:

```
var
  arrHigh: Integer;

arrHigh := High(arr);
  for i := 0 to arrHigh do
    if i = arrHigh then...
```

Why doesn't the optimizer eliminate High in a loop?

Solution

Problem