Re: [PATCH 09/17] string: Improve generic strcmp

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 03/09/22 00:31, Noah Goldstein wrote:
> On Fri, Sep 2, 2022 at 1:41 PM Adhemerval Zanella via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
>>
>> New generic implementation tries to use word operations along with
>> the new string-fz{b,i} functions even for inputs with different
>> alignments (with still uses aligned access plus merge operation
>> to get a correct word by word comparison).
>>
>> Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc64-linux-gnu,
>> and powerpc-linux-gnu by removing the arch-specific assembly
>> implementation and disabling multi-arch (it covers both LE and BE
>> for 64 and 32 bits).
>>
>> Co-authored-by: Richard Henderson  <rth@twiddle.net>
>> ---
>>  string/strcmp.c | 117 +++++++++++++++++++++++++++++++++++++++++-------
>>  1 file changed, 101 insertions(+), 16 deletions(-)
>>
>> diff --git a/string/strcmp.c b/string/strcmp.c
>> index d4962be4ec..c8acc5c0b5 100644
>> --- a/string/strcmp.c
>> +++ b/string/strcmp.c
>> @@ -15,33 +15,118 @@
>>     License along with the GNU C Library; if not, see
>>     <https://www.gnu.org/licenses/>.  */
>>
>> +#include <stdint.h>
>> +#include <string-extbyte.h>
>> +#include <string-fzb.h>
>> +#include <string-fzi.h>
>>  #include <string.h>
>> +#include <memcopy.h>
>>
>> -#undef strcmp
>> -
>> -#ifndef STRCMP
>> -# define STRCMP strcmp
>> +#ifdef STRCMP
>> +# define strcmp STRCMP
>>  #endif
>>
>> +static inline int
>> +final_cmp (const op_t w1, const op_t w2)
>> +{
>> +  unsigned char c1, c2;
>> +  for (size_t i = 0; i < sizeof (op_t); i++)
>> +    {
>> +      c1 = extractbyte (w1, i);
>> +      c2 = extractbyte (w2, i);
> 
> Is using extractbyte here better than just reloading indicices from memory?
> 
> As well, maybe (for 64 bit atleast)
> maybe worth cutting in half with a 32-bit xor on the lower half then
> maybe skipping forward
> 4-bytes.

Not sure in fact, I tried to replace with 'i = index_first_zero_ne(w1, w2);'
as Richard has suggested but the issue is we might have non initialized 
bytes that prevents us to use it.  I will check if I can simplify this
a bit.