[PATCH] S390: Optimize __memset_z196.

[Stefan Liebler]

committed

On 6/25/20 10:18 AM, Stefan Liebler via Libc-alpha wrote:
> Just as information, if nobody opposes, I'll commit this patch tomorrow.
> 
> On 6/19/20 3:51 PM, Stefan Liebler wrote:
>> It turned out that an 256b-mvc instruction which depends on the
>> result of a previous 256b-mvc instruction is counterproductive.
>> Therefore this patch adjusts the 256b-loop by storing the
>> first byte with stc and setting the remaining 255b with mvc.
>> Now the 255b-mvc instruction depends on the stc instruction.
>> ---
>>  sysdeps/s390/memset-z900.S | 19 +++++++++----------
>>  1 file changed, 9 insertions(+), 10 deletions(-)
>>
>> diff --git a/sysdeps/s390/memset-z900.S b/sysdeps/s390/memset-z900.S
>> index ca3eac0522..1e0c334156 100644
>> --- a/sysdeps/s390/memset-z900.S
>> +++ b/sysdeps/s390/memset-z900.S
>> @@ -157,28 +157,27 @@ ENTRY(MEMSET_Z196)
>>  # if !defined __s390x__
>>  	llgfr	%r4,%r4
>>  # endif /* !defined __s390x__  */
>> -	ltgr    %r4,%r4
>> -	je      .L_Z196_4
>> +	clgfi	%r4,1
>> +	jl	.L_Z196_4	    # n == 0
>>  	stc     %r3,0(%r2)
>> +	je      .L_Z196_4	    # n == 1
>> +	aghi	%r4,-2
>>  	lgr     %r1,%r2
>> -	cghi    %r4,1
>> -	je      .L_Z196_4
>> -	aghi    %r4,-2
>> -	srlg    %r5,%r4,8
>> -	ltgr    %r5,%r5
>> -	jne     .L_Z196_1
>> +	risbg	%r5,%r4,8,128+63,56 # r5 = n / 256
>> +	jne     .L_Z196_1	    # Jump away if r5 != 0
>>  .L_Z196_3:
>>  	exrl    %r4,.L_Z196_17
>>  .L_Z196_4:
>>  	br      %r14
>>  .L_Z196_1:
>>  	cgfi	%r5,1048576
>> -	jh	__memset_mvcle	   # Switch to mvcle for >256MB
>> +	jh	__memset_mvcle	    # Switch to mvcle for >256MB
>>  .L_Z196_2:
>>  	pfd     2,1024(%r1)
>> -	mvc     1(256,%r1),0(%r1)
>> +	mvc     1(255,%r1),0(%r1)
>>  	aghi    %r5,-1
>>  	la      %r1,256(%r1)
>> +	stc     %r3,0(%r1)
>>  	jne     .L_Z196_2
>>  	j       .L_Z196_3
>>  .L_Z196_17:
>>
>