[Bug default/25276] create_import_tree performance improvements

["vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=25276

--- Comment #3 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Tom de Vries from comment #0)
> Looking at the perf data, we spend 51% of the time in dwz, 

> Furthermore, we spent 28% here:
> ...
>   7,02 │1481:   movb   $0x0,0x2c(%rax)
>        │            for (icu = ipu->next; icu; icu = icu->next)
>  28,04 │        mov    0x18(%rax),%rax
>   0,00 │        test   %rax,%rax
>   0,87 │      ↑ jne    1481
> ...
> which is this loop here:
> ...
>       for (icu = ipu->next; icu; icu = icu->next)
>         icu->seen = false;
> ...
> 

With current master, we now spent 41% of execution time in dwz, and of that 41%
we spent 59% in this loop:
...
  1,39 │2895:┌─→test   %rax,%rax
  0,01 │     │↑ je     1fcd
       │     │        icu->seen = false;
 11,10 │     │  movb   $0x0,0x34(%rax)
       │     │      for (icu = ipu->next; icu; icu = icu->next)
 47,87 │     │  mov    0x20(%rax),%rax
  0,36 │     └──jmp    2895
...

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