Re: [PATCH v8 6/6] elf: Optimize _dl_new_hash in dl-new-hash.h

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 16/05/2022 16:41, Alexander Monakov via Libc-alpha wrote:
> On Mon, 16 May 2022, Noah Goldstein wrote:
> 
>>> The empty asms are used to prevent compiler reassociating 'h*32 + (h + c)'
>>> to '(h*32 + h) + c' which looks fine in isolation, but significantly changes
>>> the dependency graph in context of the whole loop.
>>
>> Some architecture could have a really fast integer MADD instruction that
>> the barrier could either prevent from being emitted or add an extra ADD
>> instruction at the end of.
> 
> With the barrier I'd expect a shift-by-5 and two additions, no madd. Modern
> aarch64 cores have 3-cycle madd I believe, so it's 3 cycles if the compiler
> decides to emit madd vs. 2 cycles if it's only additions.
> 
> Alexander

How hard would to make compiler to make this very optimization? I raised 
this on weekly call because more and more it seems that tuning computation
dependencies for loop tuning seems to be more a compiler job than libc's
(although this not a blocker, but we have multiple smalls micro-optimizations
in the past that turned in dead code due compiler catching up).