🚀 go-pugleaf

In article <1993Jan21.114022.5930@cs.nott.ac.uk> smb@cs.nott.ac.uk (Simon Burrows) writes:
>A while ago Acorn sent out some guidelines on code sequences which should
>no longer be used if compatibility with future processors is to be maximised.
>They were issued before the ARM250 was brought out, so probably apply to that?
>The problem is that such things are only deemed to be of interest to
>registered developers!

Well, I'm not a registered developer, but I am very interested in this,
as I've built an, um "experimental" lazy functional language compiler
that generates ARM assembler as its target code.  I don't use any
kind of NOP instructions, but I would like to know if I'm doing anything
else naughty.

In a similar vain, one of the optimisations I do is to cause the
code to be generated such that the starting addresses of the
code sequences that can be jumped to are aligned to have an address
of &XXXXXX4.  This maximises the number of consequtive (sp?) sequential
memory cycles when using MEMC1a.  I have three main questions.

1) The effect of this alignment is quite significant on an ARM2
machine.  Would the effect be as significant on an ARM3 (or ARM250,
although I suspect the answer is yes, in this case)?

2) In the next generation of memory controllers, are different rules
likely to apply?  If so (I suspect this is highly likely), can anyone
give me an idea what they might be?

3) Is it possible that certain orderings of instructions are "better"
than others (ie, faster), even though they achieve the same effect?  For
example

        ADR    ad1,blk1                    ADR    ad1,blk1
        LDMIA  ad1,{r0-r7}   compared      ADR    ad2,blk2
        ADR    ad1,blk2        with        LDMIA  ad1,{r0-r7}
        STMIA  ad1,{r0-r7}                 STMIA  ad2,{r0-r7}

These sort of sequences occur a great deal in the code I'm generating.
If there is a difference, what are the rules?

Keep well,

Stephen Thomas

P.S. while I'm currently at the University of Kent, I will be very
shortly (in about a week!) be moving to the University of Nottingham.
My account here should still be active for a while, though.
--
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