map question

[Siarhei Siamashka]

On Sunday, 23 January 2022 at 09:08:46 UTC, Stanislav Blinov 
wrote:
> Using `iota` here incurs additional computation and argument 
> copies that are actually never used, i.e. wasted work. So I'd 
> say go with `generate`, as that seems the intent.

Isn't this normally a compiler's job to eliminate all unused 
computations and copies?

```D
     auto foobar1(size_t n)
     {
         return n.iota.map!(_ => 123).array;
     }

     auto foobar2(size_t n)
     {
         return generate!(() => 123).take(n).array;
     }
```

LDC with "-O -release" command line options generates pretty much 
identical code for foobar1 and foobar2 (I'm only showing the main 
loop, but the rest is also the same):
```
   20:   48 39 c8                cmp    %rcx,%rax
   23:   74 18                   je     3d 
<_D2zz7foobar1FNaNbNfmZAi+0x3d>
   25:   c7 04 8a 7b 00 00 00    movl   $0x7b,(%rdx,%rcx,4)
   2c:   48 83 c1 01             add    $0x1,%rcx
   30:   48 39 cb                cmp    %rcx,%rbx
   33:   75 eb                   jne    20 
<_D2zz7foobar1FNaNbNfmZAi+0x20>
```

```
   20:   48 39 c8                cmp    %rcx,%rax
   23:   74 18                   je     3d 
<_D2zz7foobar2FNaNbNfmZAi+0x3d>
   25:   c7 04 8a 7b 00 00 00    movl   $0x7b,(%rdx,%rcx,4)
   2c:   48 83 c1 01             add    $0x1,%rcx
   30:   48 39 cb                cmp    %rcx,%rbx
   33:   75 eb                   jne    20 
<_D2zz7foobar2FNaNbNfmZAi+0x20>
```

Do you have a better example to demonstrate `generate`'s 
superiority?