Re: [PATCH 07/27] S390: Optimize strlen and wcslen.

[OndÅej BÃlka <neleai at seznam dot cz>]

On Fri, Jun 26, 2015 at 01:51:32PM +0200, Stefan Liebler wrote:
> This patch provides optimized versions of strlen and wcslen with the z13 vector

Didn't read details about z13 so I will ask. These questions apply for
all functions.

> +	lghi	%r5,0		/* current_len = 0.  */
> +
> +	/* Align s to 16 byte.  */

This way of masking tends to be slow due inputs where you read only few
bytes on first check due alignment.

In my experience a fastest is first check for page cross and if not then
do unaligned load of 16 bytes. It looks possible with vll unless its
limited to cache line.

> +	risbg	%r3,%r2,60,128+63,0 /* Test if s is aligned and
> +				       %r3 = bits 60-63 'and' 15.  */
> +	je	.Lloop1		/* If s is aligned, loop aligned.  */

This is performance problem as its relatively unpredictable branch 
(29.8% calls are aligned to 16 bytes), and you save few cycles but lose
more.

> +	lghi	%r4,15
> +	slr	%r4,%r3		/* Compute highest index to load (15-x).  */
> +	vll	%v16,%r4,0(%r2) /* Load up to 16 byte boundary. (vll needs
> +				   highest index, remaining bytes are 0.)  */
> +	ahi	%r4,1		/* Work with loaded byte count.  */
> +	vfenezb	%v16,%v16,%v16	/* Find element not equal with zero search.  */
> +	vlgvb	%r5,%v16,7	/* Load zero index or 16 if not found.  */
> +	clr	%r5,%r4		/* If found zero within loaded bytes?  */
> +	locgrl	%r2,%r5		/* Then copy return value.  */
> +	blr	%r14		/* And return.  */
> +	lgr	%r5,%r4		/* No zero within loaded bytes,
> +				   process further bytes aligned.  */
> +	/* Find zero in 16 byte aligned loop.  */
> +.Lloop1:
> +	vl	%v16,0(%r5,%r2)	/* Load s.  */
> +	aghi	%r5,16
> +	vfenezbs %v16,%v16,%v16 /* Find element not equal with zero search.  */
> +	je	.Lfound		/* Jump away if zero was found.  */
> +	vl	%v16,0(%r5,%r2)
> +	aghi	%r5,16
> +	vfenezbs %v16,%v16,%v16
> +	je	.Lfound
> +	vl	%v16,0(%r5,%r2)
What addressing is allowed? If you could add offsets then following
looks faster
 
     vl       %v16,0(%r2)
     vfenezbs %v16,%v16,%v16
     je      .Lfound0
     vl       %v16,16(%r2)
     vfenezbs %v16,%v16,%v16
     je      .Lfound1



> +	aghi	%r5,16
> +	vfenezbs %v16,%v16,%v16
> +	je	.Lfound
> +	vl	%v16,0(%r5,%r2)
> +	aghi	%r5,16
> +	vfenezbs %v16,%v16,%v16
> +	jne	.Lloop1		/* No zero found -> loop.  */
> +
> +.Lfound:
> +	vlgvb	%r2,%v16,7	/* Load byte index of zero.  */
> +	slgfi	%r5,16		/* current_len -=16 */
> +	algr	%r2,%r5
> +	br	%r14
> +END(__strlen_vx)