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

[Adhemerval Zanella <adhemerval dot zanella at linaro dot org>]

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
>>>   
>>
>