[Patch][Aarch64] memcpy IFUNC for Cavium ThunderX2

[Szabolcs Nagy]

On 15/02/18 00:04, Steve Ellcey wrote:
> This patch adds a new memcpy ifunc for Cavium ThunderX2.  The difference
> between this and the Thunderx version is in the prefetching.  ThunderX2
> has different cache characteristics and so uses a different prefetching
> strategy.  Note that I prefetch past the end of the buffer being copied
> but my understanding is that that is legal and should never generate any
> errors.  I tried adding code to not prefetch past the end of the source
> but those changes slowed down memcpy so I did not include them.
> 
> I did not copy memcpy_thunderx.S to memcpy_thunderx2.S but just use
> memcpy_thunderx2.S to set some macros and then include memcpy_thunderx.S.
> This is to reduce duplicate code.
> 
> I have attached the memcpy benchmark output files from a ThunderX2 run,
> the main differences are in bench-memcpy-large.out.
> 
> Tested with no regressions, OK to checkin?
> 

the code looks ok, and it is ok to commit if you think this gives
benefit on thunderx2 (it should not affect other targets other
than code bloat).

i prefer not to add a new memcpy every time there is a new uarch,
so i think in the long term old ones should be removed or merged
(i'm not yet sure what's the right policy here, e.g. if a target
is not available to anyone in the community for benchmarking it
will be removed or if there is not enough performance benefit).

i don't see a huge performance difference in the benchmark logs
and there are a few weird cases e.g. in bench-memcpy.out

     {
      "length": 1888,
      "align1": 0,
      "align2": 59,
      "timings": [151.016, 1905.47, 150.547, 257.656, 147.969, 151.172]
     },

the memcpy_thunderx2 is very slow (and memcpy_falkor is the fastest).

> Steve Ellcey
> sellcey@cavium.com
> 
> 
> 2018-02-14  Steve Ellcey  <sellcey@cavium.com>
> 
> 	* sysdeps/aarch64/multiarch/Makefile (sysdep_routines):
> 	Add memcpy_thunderx2.
> 	* sysdeps/aarch64/multiarch/ifunc-impl-list.c (MAX_IFUNC):
> 	Increment to 4.
> 	(__libc_ifunc_impl_list): Add __memcpy_thunderx2.
> 	* sysdeps/aarch64/multiarch/memcpy.c (libc_ifunc): Add IS_THUNDERX2
> 	and IS_THUNDERX2PA checks.
> 	* sysdeps/aarch64/multiarch/memcpy_thunderx.S (USE_THUNDERX2):
> 	Use macro to set name appropriately.
> 	(memcpy): Use USE_THUNDERX2 macro to modify prefetches.
> 	* sysdeps/aarch64/multiarch/memcpy_thunderx2.S: New file.
> 	* sysdeps/unix/sysv/linux/aarch64/cpu-features.h (IS_THUNDERX2PA):
> 	New macro.
> 	(IS_THUNDERX2): New macro.
>