Re: [PATCH] powerpc64le: add optimized strlen for P9

[Paul E Murphy via Libc-alpha <libc-alpha@sourceware.org>]

On 6/4/20 8:55 AM, Paul E Murphy wrote:
> 
> 
> On 6/3/20 3:44 PM, Paul A. Clarke wrote:
>>> +    vctzlsbb  r3,v6
>>> +    blr
>>> +
>>> +    /* Test 64B 16B at a time.  The 64B vector loop is optimized for
>>> +       longer strings.  Likewise, we check a multiple of 64B to avoid
>>> +       breaking the alignment calculation below.  */
>>> +L(aligned):
>>> +    add      r4,r3,r9
>>> +    rldicl.      r5,r4,60,62  /* Determine the number of 48B loops 
>>> needed for
>>> +                                  alignment to 64B.  And test for 
>>> zero.  */
>>
>> Would it be bad to move the "rldicl." down...
>>
>>> +
>>> +    lxv      v0+32,0(r4)
>>> +    vcmpequb. v6,v0,v18
>>> +    bne      cr6,L(tail1)
>>> +
>>> +    lxv      v0+32,16(r4)
>>> +    vcmpequb. v6,v0,v18
>>> +    bne       cr6,L(tail2)
>>> +
>>> +    lxv      v0+32,32(r4)
>>> +    vcmpequb. v6,v0,v18
>>> +    bne       cr6,L(tail3)
>>> +
>>> +    lxv      v0+32,48(r4)
>>> +    vcmpequb. v6,v0,v18
>>> +    bne       cr6,L(tail4)
>>
>> ...to here, to avoid needlessly penalizing the cases above?
> 
> My thinking is the integer op is basically free while we're busy 
> executing vector ops (similarly for li).  Likewise, this may put enough 
> space between the conditional branch below to ease prediction.  I don't 
> claim to have run this against a cycle accurate simulator, so it may not 
> really matter.
> 
> Similarly, binutils 2.26 should support all P9 instructions here and in 
> Anton's other patches.  I did check before pushing the rawmemchr/stpcpy.

And pushed with extra space removed and rewritten comment for li r0,0. 
Thank you for the feedback Paul.