map question

[Stanislav Blinov]

On Sunday, 23 January 2022 at 09:38:57 UTC, Siarhei Siamashka 
wrote:
> 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?

It is the programmer's job long before it is the compiler's. It 
can do wonders on your "minimal" code but it's not its job to 
read your mind.

> ```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?

Try actual work instead of returning a literal :) Like e.g. 
calling dice(50, 50).

One thing to note though - `generate` will always eagerly call 
its function at least once. Which of course should also be 
considered in choosing the desired implementation. I.e. if your 
`n` comes from user and is allowed to be 0, then `generate` 
becomes an inferior choice.