Re: [PATCH] powerpc: Use aligned stores in memset

["Tulio Magno Quites Machado Filho" <tuliom@linux.vnet.ibm.com>]

Rajalakshmi Srinivasaraghavan <raji@linux.vnet.ibm.com> writes:

> The powerpc hardware does not allow unaligned accesses on non cacheable
> memory.  This patch avoids misaligned stores for sizes less than 8 in
> memset to avoid such cases.  Tested on powerpc64 and powerpc64le.

This commit message is misleading.  I think it's necessary to improve with:

 1. Remove the first line.
 2. Mention the performance impact and what causes it.
 3. Reference the section "3.1.4.2 Alignment Interrupts" of the "POWER8
    Processor User's Manual for the Single-Chip Module", which describes
    this behavior.
 4. Mention which kind of programs are affected by the old behavior.

> 2017-08-17  Rajalakshmi Srinivasaraghavan  <raji@linux.vnet.ibm.com>
>
> 	* sysdeps/powerpc/powerpc64/power8/memset.S: Store byte by byte
> 	for unaligned inputs if size is less than 8.
> ---
>  sysdeps/powerpc/powerpc64/power8/memset.S | 68 ++++++++++++++++++++++++++++++-
>  1 file changed, 66 insertions(+), 2 deletions(-)
>
> diff --git a/sysdeps/powerpc/powerpc64/power8/memset.S b/sysdeps/powerpc/powerpc64/power8/memset.S
> index 7ad3bb1b00..504bab0841 100644
> --- a/sysdeps/powerpc/powerpc64/power8/memset.S
> +++ b/sysdeps/powerpc/powerpc64/power8/memset.S
> @@ -377,7 +377,8 @@ L(write_LT_32):
>  	subf	r5,r0,r5
>
>  2:	bf	30,1f
> -	sth	r4,0(r10)
> +	stb	r4,0(r10)
> +	stb	r4,1(r10)

Needs a comment to prevent future mistakes in the future.

> @@ -437,11 +438,74 @@ L(tail5):
>  	/* Handles copies of 0~8 bytes.  */
>  	.align	4
>  L(write_LE_8):
> -	bne	cr6,L(tail4)
> +	/* Use stb instead of sth which is safe for
> +	   both aligned and unaligned inputs.  */

I don't think "safe" is the correct term.  What about this?

    Use stb instead of sth because it doesn't generate alignment interrupts
    on cache-inhibited storage.

> +	bne	cr6,L(LE7_tail4)
> +	/* If input is word aligned, use stw, Else use stb.  */

s/Else/else/


-- 
Tulio Magno