RE: Popcount optimization using AVX512

> -----Original Message-----
> From: David Rowley <dgrowleyml@gmail.com>
> Sent: Wednesday, March 20, 2024 5:28 PM
> To: Amonson, Paul D <paul.d.amonson@intel.com>
> Cc: Nathan Bossart <nathandbossart@gmail.com>; Andres Freund
>
> I'm not sure about this "extern negates inline" comment.  It seems to me the
> compiler is perfectly free to inline a static function into an external function
> and it's free to inline the static function elsewhere within the same .c file.
> 
> The final sentence of the following comment that the 0001 patch removes
> explains this:
> 
> /*
>  * When the POPCNT instruction is not available, there's no point in using
>  * function pointers to vary the implementation between the fast and slow
>  * method.  We instead just make these actual external functions when
>  * TRY_POPCNT_FAST is not defined.  The compiler should be able to inline
>  * the slow versions here.
>  */
> 
> Also, have a look at [1].  You'll see f_slow() wasn't even compiled and the code
> was just inlined into f().  I just added the
> __attribute__((noinline)) so that usage() wouldn't just perform constant
> folding and just return 6.
> 
> I think, unless you have evidence that some common compiler isn't inlining the
> static into the extern then we shouldn't add the macros.
> It adds quite a bit of churn to the patch and will break out of core code as you
> no longer have functions named pg_popcount32(),
> pg_popcount64() and pg_popcount().

This may be a simple misunderstanding extern != static. If I use the "extern" keyword then a symbol *will* be generated
andinline will be ignored. This is NOT true of "static inline", where the compiler will try to inline the method. :)
 

In this patch set:
* I removed the macro implementation.
* Made everything that could possibly be inlined marked with the "static inline" keyword.
* Conditionally made the *_slow() functions "static inline" when TRY_POPCONT_FAST is not set.
* Found and fixed some whitespace errors in the AVX code implementation.

Thanks,
Paul