Re: Fix performance of generic atomics

On Wed, Sep 6, 2017 at 12:01 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> This seems like a pretty sound argument to me.  I think Tom's probably
>> right that the changes in generic-gcc.h are the important ones, but
>> I'm not sure that's an argument against patching generics.h.  Given
>> that pg_atomic_compare_exchange_u32_impl is defined to update *old
>> there seems to be no reason to call pg_atomic_read_u32_impl every time
>> through the loop.
>
> Probably not.  I'm not quite 100% convinced of that, because of my
> observation that gcc is capable of generating different and better
> code for some of these primitives if it can prove that the return
> value is not needed.  It's not clear that that could apply in any
> of these uses of pg_atomic_compare_exchange_u32_impl, though.
> In any case, by my own argument, it shouldn't matter, because if
> any of these are really performance-critical then we should be
> looking for better ways.

It's not a question of whether the return value is used, but of
whether the updated value of *old is used.

Unless I'm confused.

-- 
Robert Haas
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