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Re: does a double cast to unsigned makes sense in any circumstance?

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Richard Damon <Richard@Damon-Family.org> writes:

> On 1/15/22 8:15 PM, Meredith Montgomery wrote:
>> Richard Damon <Richard@Damon-Family.org> writes:
>> 
>>> On 1/3/22 12:15 PM, Meredith Montgomery wrote:
>>>> I've seen this somewhere or I wrote this some time in the past for some
>>>> reason and I can't see how this make sense any longer.  In a procedure
>>>> to read a numeric string and turn it into a number, we need to convert
>>>> each char-digit to some kind of integer:
>>>>     (uint64_t) (unsigned char) (s[pos] - '0');
>>>> But the double cast here seems odd.  I could have written this by
>>>> copying it from somewhere else.  Today, what I would write is just
>>>>     (uint64_t) (s[pos] - '0');
>>>> Am I making a mistake now?  I can't anything wrong with this.  Isn't
>>>> a
>>>> char just an int?  I'm turning an int into an unsigned integer (of a
>>>> larger size).
>>>> Thank you!
>>>
>>> As others have said, it does change the behavior, but only if s[pos]
>>> might be less than '0', which means it holds something other than a
>>> digit, as the characters 0..9 are a required to be a consecutive
>>> increase sequence (which the code is likely counting on).
>>>
>>> If you know that s[pos] will always be > 0, then the double cast may
>>> produce faster code, as unless the compiler can also determine this,
>>> it may need to first sign extend to int, then zero extend to uint64_t,
>>> verse just zero extending to uint64_t.
>> What do you mean by ``zero extend''?  Are you qualifying the verb
>> ``to
>> extend''?  You lost me there.  Thank you.
>
> When the processor loads the 8 bit character into the bottom of the
> register, it generally leaves the rest of the register alone.
>
> If Char is signed, and int is 32 bits, then the implementation can
> convert it to an int by extending the sign of the bottom byte into the 
> rest of the register, and then to make it a unsigned 54 bit value, it
> needs to put zeros into the upper 32 bits of the register.
>
> By casting to unsigned character, then the implentation only needs to
> use an instruction that fills the upper 56 bits of the register to 0.
>
> Setting the upper bits of a register with a value is called 'extending'
>
> If it is copying the sign bit of the lower part of the register, it is
> called sign extending.
>
> If it is just fixing them to zero, it is called zero extending,
> because we are thinking of the always 0 'sign bits' that extend beyond
> the bits of the unsigned value.

Awesome information.  Thank you so much.

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does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-03 14:15 -0300
  Re: does a double cast to unsigned makes sense in any circumstance? Bart <bc@freeuk.com> - 2022-01-03 17:32 +0000
    Re: does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-15 22:09 -0300
  Re: does a double cast to unsigned makes sense in any circumstance? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-03 10:06 -0800
    Re: does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-15 22:12 -0300
      Re: does a double cast to unsigned makes sense in any circumstance? James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-15 23:12 -0500
        Re: does a double cast to unsigned makes sense in any circumstance? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-20 19:37 -0800
          Re: does a double cast to unsigned makes sense in any circumstance? "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2022-01-21 09:35 -0800
            Re: does a double cast to unsigned makes sense in any circumstance? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-21 17:19 -0800
  Re: does a double cast to unsigned makes sense in any circumstance? Bonita Montero <Bonita.Montero@gmail.com> - 2022-01-03 19:31 +0100
    Re: does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-15 22:14 -0300
      Re: does a double cast to unsigned makes sense in any circumstance? James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-15 23:24 -0500
        Re: does a double cast to unsigned makes sense in any circumstance? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-01-16 13:30 -0800
          Re: does a double cast to unsigned makes sense in any circumstance? James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-16 23:25 -0500
          Re: does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-17 09:43 -0300
            Re: does a double cast to unsigned makes sense in any circumstance? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-01-17 13:59 -0800
              Re: does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-28 22:13 -0300
                Re: does a double cast to unsigned makes sense in any circumstance? James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-28 21:29 -0500
                Re: does a double cast to unsigned makes sense in any circumstance? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-02-03 22:28 -0800
                Re: does a double cast to unsigned makes sense in any circumstance? "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2022-02-04 10:43 -0800
                Re: does a double cast to unsigned makes sense in any circumstance? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-01-28 19:37 -0800
                Re: does a double cast to unsigned makes sense in any circumstance? Manfred <noname@add.invalid> - 2022-01-30 03:45 +0100
                Re: does a double cast to unsigned makes sense in any circumstance? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-01-30 00:01 -0800
                Re: does a double cast to unsigned makes sense in any circumstance? Richard Damon <Richard@Damon-Family.org> - 2022-01-30 12:59 -0500
        Re: does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-17 09:41 -0300
  Re: does a double cast to unsigned makes sense in any circumstance? Richard Damon <Richard@Damon-Family.org> - 2022-01-03 14:37 -0500
    Re: does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-15 22:15 -0300
      Re: does a double cast to unsigned makes sense in any circumstance? Richard Damon <Richard@Damon-Family.org> - 2022-01-15 20:55 -0500
        Re: does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-17 09:44 -0300
    Re: does a double cast to unsigned makes sense in any circumstance? Bonita Montero <Bonita.Montero@gmail.com> - 2022-01-16 05:13 +0100
  Re: does a double cast to unsigned makes sense in any circumstance? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-15 18:53 -0800
    Re: does a double cast to unsigned makes sense in any circumstance? Meredith Montgomery <mmontgomery@levado.to> - 2022-01-17 09:46 -0300
    Re: does a double cast to unsigned makes sense in any circumstance? Bonita Montero <Bonita.Montero@gmail.com> - 2022-01-29 16:57 +0100
      Re: does a double cast to unsigned makes sense in any circumstance? Öö Tiib <ootiib@hot.ee> - 2022-01-29 09:54 -0800
        Re: does a double cast to unsigned makes sense in any circumstance? Bonita Montero <Bonita.Montero@gmail.com> - 2022-01-30 13:17 +0100
          Re: does a double cast to unsigned makes sense in any circumstance? Öö Tiib <ootiib@hot.ee> - 2022-01-30 23:53 -0800

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