OpenEuphoria

Re: find_from and match_from

• Posted by Derek Parnell <ddparnell at bigpond.com> Mar 21, 2007
• 673 views

Matt Lewis wrote:
> 
> Derek Parnell wrote:
> > 
> > Robert Craig wrote:
> > > the current match() and find()
> > 
> > Robert, I vaguely remember someone saying that a slice doesn't create a new
> > sequence until such time as the slice is actually written to. Is that true?
> 
> Not true.  I disassembled Igor's example:
> }}}
<eucode>
> function find_from_to(integer N, integer L, object a, sequence b)
>      L=find(a,b[N .. L])
>      if L then
>        return L-1+N
>      else
>        return L
>      end if
> end function
> </eucode>
{{{

> 
> Notice line 7, where the slice is created an assigned to the temp.
> An additional slow down occurs here through the arithmetic to convert
> from the slice to the sequence as a whole.
> 
> SubProgram [find_from_to:00116]
>      1: 096 117                          # INTEGER_CHECK: [N:117]
>      3: 096 118                          # INTEGER_CHECK: [L:118]
>      5: 097 120                          # SEQUENCE_CHECK: [b:120]
>      7: 046 120 117 118 121              # RHS_SLICE: [b:120] [N:117]..[L:118]
> => [_temp_:121]
>     12: 077 119 121 118                  # FIND: [a:119], [_temp_:121] =>
>     [L:118]
>     16: 020 118 32                       # IF: [L:118] = 0 goto 0032
>     19: 010 118 122 121                  # MINUS: [L:118], [LIT 1:122] =>
>     [_temp_:121]
>     23: 011 121 117 121                  # PLUS: [_temp_:121], [N:117] =>
>     [_temp_:121]
>     27: 028 116 121                      # RETURNF: [_temp_:121]
>     30: 023 35                           # ELSE goto 0035
>     32: 028 116 118                      # RETURNF: [L:118]
>     35: 043                              # BADRETURNF:
> End SubProgram [find_from_to:00116]

This is not conclusive. Although the IL says to create a slice and assign it to
the temp, does the opcode RHS_SLICE actually allocate RAM for the subsequence and
copy data to the new RAM, or does it just create a reference to the existing RAM?
As the function never assigns to the slice, a reference is all it really needs.

-- 
Derek Parnell
Melbourne, Australia
Skype name: derek.j.parnell