[Openmcl-devel] A little more LAP stuff...

[Jon S. Anthony]

Thanks.  That clears that up.

Another quick one (I think..)

Is the basic difference between ($ n) and ($ 'n), (where n is some
positive integer) that the latter is a kind of shorthand indicating that
we are talking about fixnums and that this will be turned into the
equivalent of ($ (* 4 n)), i.e., fixnum n (where ?? == 32/64)?  While
the former is literally native value n as represented in full 32 bits?


/Jon


On Tue, 2010-02-16 at 17:33 -0500, R. Matthew Emerson wrote:
> On Feb 16, 2010, at 2:18 PM, Jon S. Anthony wrote:
> 
> > Looking over some LAP functions and disassembling some user level lisp
> > functions, my question(s) is what (or whether) there is any difference
> > between the two approaches used in these for the X86 register toggle
> > tagging between immediate/node indications.
> > 
> > In user level lisp, this typically seems to use btrl to reset bit
> > indicating immediate and btsl to set bit indicating node.  For example:
> > 
> > 
> >  [13]    (btrl ($ 1) (@ (% fs) 8)) ; indicate immo is immediate
> >  [23]    (movl (@ -2 (% arg_z)) (% imm0)) ; load it up...
> > ...
> >  [46]    (btsl ($ 1) (@ (% fs) 8)) ; indicate immo is node
> > 
> > 
> > This same thing in LAP functions where (mark-as-imm ...) and
> > (mark-as-node ...) are used is:
> > 
> >          ;; (mark-as-imm immo)
> >  [30]    (andb ($ 254) (@ (% fs) 8)) ; indicate immo is immediate
> > ...
> >          ;; (mark-as-node imm0)
> >  [84]    (xorl (% imm0) (% imm0)) ; always there, but unclear about it
> >  [86]    (orb ($ 1) (@ (% fs) 8)) ; indicate immo is node
> > 
> > Is there any real difference between these for the intended task?
> 
> They do the same thing (set/clear a bit in the node-regs-mask word stored in the TCR).
> 
> I'm not sure why they're different.  I like the btr/bts for clarity, though I think it may be the case that the btr/bts instructions are a byte longer than equivalent logical operations would be.  I think the lap macro uses logical operations because I had a sort of vague intention to make it support reclassifying the category of multiple registers at once, i.e., I thought that one might say
> 
> (mark-as-imm temp0 temp1)
> ...
> (mark-as-node temp0 temp1)
> 
> and have that expand to
> 
> (andb ($ 249) (@ (% fs) 8))
> ...
> (xorl (% temp0) (% temp0))
> (xorl (% temp1) (% temp1))
> (orb ($ 6) (@ (% fs) 8))
> 
> but I never got around to doing that.
> 
> 
> 
>