Re: [PATCH 1/2] powerpc: Zero pad using memset in strncpy/stpncpy

["Gabriel F. T. Gomes" <gftg at linux dot vnet dot ibm dot com>]

Thank you for reviewing, Adhemerval.

Pushed as 72c11b353ede72931cc474c9071d143d9a05c0d7.

On Tue, 26 Apr 2016 11:16:48 -0300
Adhemerval Zanella <adhemerval.zanella@linaro.org> wrote:

> On 26/04/2016 10:41, Gabriel F. T. Gomes wrote:
> > On Wed, 20 Apr 2016 18:25:37 -0300
> > Adhemerval Zanella <adhemerval.zanella@linaro.org> wrote:
> >   
> >> I am ok with this patch, the only nit is it now calls memset for all
> >> padding size, where for small padding (less than 16 or maybe 32) I think it would
> >> be still faster than calling memset (since dcbtst will be used only for
> >> large inputs). However I still not sure if embedding the memset write_LT_32
> >> would yield much gain as well.  
> > 
> > Hi, Adhemerval. Thanks for your review.
> > 
> > I also expected that for small amounts of padding, calling memset would
> > harm performance. However, I ran some tests, where memset was only
> > called for padding amounts greater than 32 bytes, and I didn't notice
> > any significant changes in performance. That's the reason why I sent
> > this version of the patch, which always calls memset.
> > 
> > I also noticed that bench-strncpy exposes a load-hit-store problem,
> > which might be suppressing the difference for small paddings. I have
> > plans to fix this in the future, and, if necessary, limit memset to
> > greater paddings.
> > 
> > However, right now, I don't see a benefit in adding the check.
> > 
> >    
> 
> Fair enough then.
> 
> >> On 20-04-2016 17:55, Gabriel F. T. Gomes wrote:  
> >>> Call __memset_power8 to pad, with zeros, the remaining bytes in the
> >>> dest string on __strncpy_power8 and __stpncpy_power8.  This improves
> >>> performance when n is larger than the input string, giving ~30% gain for
> >>> larger strings without impacting much shorter strings.
> >>>
> >>> 2016-04-18  Gabriel F. T. Gomes  <gftg@linux.vnet.ibm.com>
> >>>
> >>> 	* sysdeps/powerpc/powerpc64/power8/strncpy.S: Call memset to pad the
> >>> 	remaining bytes in the dest string, with zeros.
> >>> ---
> >>>  sysdeps/powerpc/powerpc64/power8/strncpy.S | 123 +++++++++++++----------------
> >>>  1 file changed, 56 insertions(+), 67 deletions(-)
> >>>
> >>> diff --git a/sysdeps/powerpc/powerpc64/power8/strncpy.S b/sysdeps/powerpc/powerpc64/power8/strncpy.S
> >>> index 17c3afb..0bb3bd4 100644
> >>> --- a/sysdeps/powerpc/powerpc64/power8/strncpy.S
> >>> +++ b/sysdeps/powerpc/powerpc64/power8/strncpy.S
> >>> @@ -24,6 +24,8 @@
> >>>  # define FUNC_NAME strncpy
> >>>  #endif
> >>>  
> >>> +#define FRAMESIZE (FRAME_MIN_SIZE+48)
> >>> +
> >>>  /* Implements the function
> >>>  
> >>>     char * [r3] strncpy (char *dest [r3], const char *src [r4], size_t n [r5])
> >>> @@ -54,8 +56,7 @@ EALIGN (FUNC_NAME, 4, 0)
> >>>  	addi	r10,r4,16
> >>>  	rlwinm	r9,r4,0,19,19
> >>>  
> >>> -	/* Since it is a leaf function, save some non-volatile registers on the
> >>> -	   protected/red zone.  */
> >>> +	/* Save some non-volatile registers on the stack.  */
> >>>  	std	r26,-48(r1)
> >>>  	std	r27,-40(r1)
> >>>  
> >>> @@ -69,6 +70,14 @@ EALIGN (FUNC_NAME, 4, 0)
> >>>  	std	r30,-16(r1)
> >>>  	std	r31,-8(r1)
> >>>  
> >>> +	/* Update CFI.  */
> >>> +	cfi_offset(r26, -48)
> >>> +	cfi_offset(r27, -40)
> >>> +	cfi_offset(r28, -32)
> >>> +	cfi_offset(r29, -24)
> >>> +	cfi_offset(r30, -16)
> >>> +	cfi_offset(r31, -8)
> >>> +
> >>>  	beq	cr7,L(unaligned_lt_16)
> >>>  	rldicl	r9,r4,0,61
> >>>  	subfic	r8,r9,8
> >>> @@ -180,74 +189,58 @@ L(short_path_loop_end):
> >>>  	ld	r31,-8(r1)
> >>>  	blr
> >>>  
> >>> -	/* This code pads the remainder dest with NULL bytes.  The algorithm
> >>> -	   calculate the remanining size and issues a doubleword unrolled
> >>> -	   loops followed by a byte a byte set.  */
> >>> +	/* This code pads the remainder of dest with NULL bytes.  The algorithm
> >>> +	   calculates the remaining size and calls memset.  */
> >>>  	.align	4
> >>>  L(zero_pad_start):
> >>>  	mr	r5,r10
> >>>  	mr	r9,r6
> >>>  L(zero_pad_start_1):
> >>> -	srdi.	r8,r5,r3
> >>> -	mr	r10,r9
> >>> -#ifdef USE_AS_STPNCPY
> >>> -	mr	r3,r9
> >>> +	/* At this point:
> >>> +	     - r5 holds the number of bytes that still have to be written to
> >>> +	       dest.
> >>> +	     - r9 points to the position, in dest, where the first null byte
> >>> +	       will be written.
> >>> +	   The above statements are true both when control reaches this label
> >>> +	   from a branch or when falling through the previous lines.  */
> >>> +#ifndef USE_AS_STPNCPY
> >>> +	mr	r30,r3       /* Save the return value of strncpy.  */
> >>> +#endif
> >>> +	/* Prepare the call to memset.  */
> >>> +	mr	r3,r9        /* Pointer to the area to be zero-filled.  */
> >>> +	li	r4,0         /* Byte to be written (zero).  */
> >>> +
> >>> +	/* We delayed the creation of the stack frame, as well as the saving of
> >>> +	   the link register, because only at this point, we are sure that
> >>> +	   doing so is actually needed.  */
> >>> +
> >>> +	/* Save the link register.  */
> >>> +	mflr	r0
> >>> +	std	r0,16(r1)
> >>> +	cfi_offset(lr, 16)
> >>> +
> >>> +	/* Create the stack frame.  */
> >>> +	stdu	r1,-FRAMESIZE(r1)
> >>> +	cfi_adjust_cfa_offset(FRAMESIZE)
> >>> +
> >>> +	bl	__memset_power8
> >>> +	nop
> >>> +
> >>> +	/* Restore the stack frame.  */
> >>> +	addi	r1,r1,FRAMESIZE
> >>> +	cfi_adjust_cfa_offset(-FRAMESIZE)
> >>> +	/* Restore the link register.  */
> >>> +	ld	r0,16(r1)
> >>> +	mtlr	r0
> >>> +
> >>> +#ifndef USE_AS_STPNCPY
> >>> +	mr	r3,r30       /* Restore the return value of strncpy, i.e.:
> >>> +				dest.  For stpncpy, the return value is the
> >>> +				same as return value of memset.  */
> >>>  #endif
> >>> -	beq-	cr0,L(zero_pad_loop_b_start)
> >>> -	cmpldi	cr7,r8,1
> >>> -	li	cr7,0
> >>> -	std	r7,0(r9)
> >>> -	beq	cr7,L(zero_pad_loop_b_prepare)
> >>> -	addic.	r8,r8,-2
> >>> -	addi	r10,r9,r16
> >>> -	std	r7,8(r9)
> >>> -	beq	cr0,L(zero_pad_loop_dw_2)
> >>> -	std	r7,16(r9)
> >>> -	li	r9,0
> >>> -	b	L(zero_pad_loop_dw_1)
> >>> -
> >>> -	.align	4
> >>> -L(zero_pad_loop_dw):
> >>> -	addi	r10,r10,16
> >>> -	std	r9,-8(r10)
> >>> -	beq	cr0,L(zero_pad_loop_dw_2)
> >>> -	std	r9,0(r10)
> >>> -L(zero_pad_loop_dw_1):
> >>> -	cmpldi	cr7,r8,1
> >>> -	std	r9,0(r10)
> >>> -	addic.	r8,r8,-2
> >>> -	bne	cr7,L(zero_pad_loop_dw)
> >>> -	addi	r10,r10,8
> >>> -L(zero_pad_loop_dw_2):
> >>> -	rldicl	r5,r5,0,61
> >>> -L(zero_pad_loop_b_start):
> >>> -	cmpdi	cr7,r5,0
> >>> -	addi	r5,r5,-1
> >>> -	addi	r9,r10,-1
> >>> -	add	r10,r10,5
> >>> -	subf	r10,r9,r10
> >>> -	li	r8,0
> >>> -	beq-	cr7,L(short_path_loop_end)
> >>> -
> >>> -	/* Write remaining 1-8 bytes.  */
> >>> -        .align  4
> >>> -	addi	r9,r9,1
> >>> -	mtocrf	0x1,r10
> >>> -	bf	29,4f
> >>> -        stw     r8,0(r9)
> >>> -        addi	r9,r9,4
> >>> -
> >>> -        .align  4
> >>> -4:      bf      30,2f
> >>> -        sth     r8,0(r9)
> >>> -        addi	r9,r9,2
> >>> -
> >>> -        .align  4
> >>> -2:      bf	31,1f
> >>> -        stb	r8,0(r9)
> >>>  
> >>> -	/* Restore non-volatile registers.  */
> >>> -1:	ld	r26,-48(r1)
> >>> +	/* Restore non-volatile registers and return.  */
> >>> +	ld	r26,-48(r1)
> >>>  	ld	r27,-40(r1)
> >>>  	ld	r28,-32(r1)
> >>>  	ld	r29,-24(r1)
> >>> @@ -443,10 +436,6 @@ L(short_path_prepare_2_3):
> >>>  	mr	r4,r28
> >>>  	mr	r9,r29
> >>>  	b	L(short_path_2)
> >>> -L(zero_pad_loop_b_prepare):
> >>> -	addi	r10,r9,8
> >>> -	rldicl	r5,r5,0,61
> >>> -	b	L(zero_pad_loop_b_start)
> >>>  L(zero_pad_start_prepare_1):
> >>>  	mr	r5,r6
> >>>  	mr	r9,r8
> >>>     
> >>  
> >   
>