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>]

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.

 
> 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
> >   
>