[parisc-linux] Re: pa_memcpy: 2 small question

[Randolph Chung <randolph@tausq.org>]

> in the shift computing:
>         /* Calculate how to shift a word read at the memory operation
>            aligned srcp to make it aligned for copy.  */
>         sh_1 = 8 * (src % sizeof(unsigned int));
>         sh_2 = 8 * sizeof(unsigned int) - sh_1;
> 
> what means '8' (== 2 * word size; i.e. 2 * 32 bit because MERGE use shrpw 
> and so 2 (a pair of word)? )

no. 8 is # bits/byte. sh_1 is the number of bits to shift a 32-bit
integer.

what we are trying to achieve is that given two adjacent 32-bit numbers,
we want to extract a 32-bit number "in the middle" of the two (aligned)
32-bit values.

if you look carefully, actually the implementation of MERGE does not use
both sh_1 and sh_2. In the original implementation, MERGE was
implemented using two SHIFT operations plus an OR operation. This was
optimized to use shrpw because this PA insn can do all three operations
in a single step, so it saves a lot of cycles.

> next in
>          switch (len % 4)
>         {
> 
> is '4' because as mentioned in copy_dstalign() description this 'Handles 
> _4_ words per loop'

yes.

randolph
-- 
Randolph Chung
Debian GNU/Linux Developer, hppa/ia64 ports
http://www.tausq.org/
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