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Re: Check if all 256 bits are clear or set ?

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Bonita Montero <Bonita.Montero@gmail.com> writes:

> Am 03.11.2021 um 21:25 schrieb Bonita Montero:
>> In assembly:
>>      movzx    rax, dl
>>      add      rax, [rcx]
>>      setz     al
>>      mov      rdx, [rcx]
>>      xor      rdx, [rcx + 8]
>>      xor      rdx, [rcx + 16]
>>      xor      rdx, [rcx + 24]
>>      setz     dl
>>      and      al, dl
>>      ret
>> rcx is the pointer to the memory block with 256 bits and dl
>> if either 0 or 1, depending if the bits should be all 0 or 1.
>
> Can anyone beat this ?

But I don't think the code is correct:

  #include <stdio.h>
  #include <stdbool.h>
  #include <stdint.h>
   
  bool test256(char *p, bool set)
  {
       uint64_t *p64 = (uint64_t *)p;
       return !(p64[0] + set) & !(p64[0] ^ p64[1] ^ p64[2] ^ p64[3]);
  }
   
  int main(void)
  {
       char b[32] = {
            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
             0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0,  0
       };
       printf("%d\n", test256(b, true));
  }

prints 1.

In addition, there is undefined behaviour if the pointer p is not
correctly aligned for uint64_t.

-- 
Ben.

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Check if all 256 bits are clear or set ? skybuck2000 <skybuck2000@hotmail.com> - 2021-10-28 17:47 -0700
  Re: Check if all 256 bits are clear or set ? skybuck2000 <skybuck2000@hotmail.com> - 2021-10-28 18:02 -0700
    Re: Check if all 256 bits are clear or set ? skybuck2000 <skybuck2000@hotmail.com> - 2021-10-28 18:12 -0700
      Re: Check if all 256 bits are clear or set ? skybuck2000 <skybuck2000@hotmail.com> - 2021-10-28 19:18 -0700
        Re: Check if all 256 bits are clear or set ? skybuck2000 <skybuck2000@hotmail.com> - 2021-10-29 05:16 -0700
  Re: Check if all 256 bits are clear or set ? Peter 'Shaggy' Haywood <phaywood@alphalink.com.au> - 2021-11-03 11:11 +1100
    Re: Check if all 256 bits are clear or set ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-11-03 11:50 +0000
      Re: Check if all 256 bits are clear or set ? Peter 'Shaggy' Haywood <phaywood@alphalink.com.au> - 2021-11-06 16:08 +1100
        Re: Check if all 256 bits are clear or set ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-11-06 11:02 +0000
          Re: Check if all 256 bits are clear or set ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-11-06 15:04 -0700
            Re: Check if all 256 bits are clear or set ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-11-06 22:06 +0000
            Re: Check if all 256 bits are clear or set ? Peter 'Shaggy' Haywood <phaywood@alphalink.com.au> - 2021-11-13 11:54 +1100
          Re: Check if all 256 bits are clear or set ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-11-06 15:22 -0700
            Re: Check if all 256 bits are clear or set ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-11-07 00:08 +0000
              Re: Check if all 256 bits are clear or set ? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-11-07 08:12 -0800
        Re: Check if all 256 bits are clear or set ? skybuck2000 <skybuck2000@hotmail.com> - 2021-11-23 08:23 -0800
      Re: Check if all 256 bits are clear or set ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-11-06 15:01 +0100
  Re: Check if all 256 bits are clear or set ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-11-03 21:04 +0100
    Re: Check if all 256 bits are clear or set ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-11-03 21:09 +0100
      Re: Check if all 256 bits are clear or set ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-11-03 21:11 +0100
        Re: Check if all 256 bits are clear or set ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-11-03 21:25 +0100
          Re: Check if all 256 bits are clear or set ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-11-04 07:05 +0100
            Re: Check if all 256 bits are clear or set ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-11-04 11:37 +0000
              Re: Check if all 256 bits are clear or set ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-11-04 17:58 +0100
                Re: Check if all 256 bits are clear or set ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-11-04 17:13 +0000
                Re: Check if all 256 bits are clear or set ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-11-04 18:15 +0100
                Re: Check if all 256 bits are clear or set ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-11-04 17:27 +0000
                Re: Check if all 256 bits are clear or set ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-11-04 18:29 +0100
                Re: Check if all 256 bits are clear or set ? scott@slp53.sl.home (Scott Lurndal) - 2021-11-04 17:28 +0000
                Re: Check if all 256 bits are clear or set ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-11-05 11:56 +0100
                Re: Check if all 256 bits are clear or set ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-11-05 18:20 +0100
                Re: Check if all 256 bits are clear or set ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-11-05 18:32 +0100
                Re: Check if all 256 bits are clear or set ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-11-04 18:13 +0100
                Re: Check if all 256 bits are clear or set ? Manfred <noname@add.invalid> - 2021-11-04 19:28 +0100
                Re: Check if all 256 bits are clear or set ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-11-04 20:29 +0100
                Re: Check if all 256 bits are clear or set ? Manfred <noname@add.invalid> - 2021-11-05 02:06 +0100
                Re: Check if all 256 bits are clear or set ? Bonita Montero <Bonita.Montero@gmail.com> - 2021-11-05 08:57 +0100
              Re: Check if all 256 bits are clear or set ? Bart <bc@freeuk.com> - 2021-11-04 18:01 +0000

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