> On Tue, 3 Jul 2001, Olin Lathrop wrote:
>
> > > >    MOVLW h'3A'
> > > >
> > > > That could be the low or high byte of an address, or just an arbitrary
> > > > constant.  There are lots of other gotchas too.  You need to know the
> > > > original intent of the code to do any meaningful optimizations.
> > >
> > > Olin's example above is not really clear, so let me elaborate slightly.
> >
> > Sorry, I'll explain why this is a show-stopping gotcha.  A HEX file contains
> > raw instructions after address symbols have been resolved and linked.  To do
> > any meaningful optimizing, you will have to move instructions around.  Since
> > some instructions will now be at different addresses, you have to adjust the
> > references to those addresses.  That can be done with GOTO and CALL
> > instructions, but what about the example above?  Here are a few examples of
> > the code that could have produced that MOVLW instruction:
> >
> >    MOVLW   h'3A'     ;get the official number of glorks per snorp
> >    MOVLW   low table ;get low byte of table start address
> >    MOVLW   high table ;get high byte of table start address
> >    MOVLW   low blog+3 ;get address of the count field in the BLOG structure
> >
> > You're screwed because there is no way to distinguish these cases, so you
> > have no idea how, if at all, to adjust the 3Ah value.
>
> Actually, Olin, I think you mean ADDLW, IORLW or XORLW instead of MOVLW.
> Consecutive RETLW's make sense for tables. It makes no sense to have two MOVLW's
> in a row unless there is a jump that some how chooses which is executed:
>
>     skpc
>      movlw   42
>     movlw    69
>
> Scott

The consecutive MOVLW's are just examples with the literal 3A meaning 4 different things.
At the hex file stage, there isn't any way for the optimizer to determine what the 3a refers to.
If it's a constant, don't change it.  If it's the start address of a data table and the optimizer moves
the table, then the 3a has be be changed to the tables new address.  It's easier to do at the
source stage or object code stage if we have it.

Doug

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