[PATCH v2] powerpc: optimize strcpy and stpcpy for POWER9/10

[Adhemerval Zanella Netto]

On 26/06/24 11:34, MAHESH BODAPATI wrote:
> This patch modifies the current POWER9 implementation of strcpy and
> stpcpy to optimize it for POWER9/10.
> 
> Since no new POWER10 instructions are used, the original POWER9
> strcpy is modified instead of creating a new implementation for POWER10.
> 
> The changes also affect stpcpy, which uses the same implementation
> with some additional code before returning.
> 
> Improvements compared to POWER9 version:
> 
> Use simple comparisons for the first ~512 bytes
>   The main loop is good for long strings, but comparing 16B each time is
>   better for shorter strings. After aligning the address to 16 bytes, we
>   unroll the loop four times, checking 128 bytes each time. There may be
>   some overlap with the main loop for unaligned strings, but it is better
>   for shorter strings.
> 
> Loop with 64 bytes for longer bytes
>   using 4 consecutive lxv/stxv instructions.
> 
> Showed an average improvement of 13% and the degradations for smaller
> strings are not consistent.

Other implementations either focus on small sizes or add fast-paths code
assuming small sizes are more common than larger one.  You might reevaluate
if this implementation if really worth if you are seeing smaller size
regressions.