Am Samstag, 14. Mai 2005 21:39 schrieb Alexandre Oliva:
> On May 14, 2005, BjÃrn Haase <[EMAIL PROTECTED]> wrote:
> > I.e. expand
> > would insert two instructions after the double-set instruction that
> > contain the two individual sets and an additional "use" statement. I.e.
> > above sequence after expand then would look like
> >
> > (parallel[
> > (set reg:SI 100) (minus:SI (reg:SI 101) (reg:SI 102))
> > (set reg:CC CC_xxx) (compare (reg:SI 101) (reg:SI 102)))])
> > (parallel[ (set reg:SI 100) (minus:SI (reg:SI 101) (reg:SI 102))
> > (use (reg:SI 100) ])
> > (parallel[ (set reg:CC_xxx CC) (compare (reg:SI 101) (reg:SI 102)))
> > (use (reg:CC])
>
> You'd then have to some how arrange for the second and third insns to
> not be removed as redundant, and come up with some additional work
> around for the case when there's an overlap between output and input.
Yes. I agree with both of your remarks.
Concerning removal of the redundant expressions: It seems that the fact that
condition codes could be reused is identified already during CSE or GCSE, and
at this time the passes removing dead code seem not to have removed the
redundant patterns. At least, since this worked for the case of "divmod", I
am having hope that this would work as well for condition codes.
Concerning your second remark: Indeed one probably would need to rewrite the
third pattern as
(parallel[ (set reg:CC_xxx CC) (compare (reg:SI 100) (const_int 0)))
(use (reg:CC])
. For the divmod issue, I have observed a very similar difficulty (there
however there is no problem with overlapping in- and outputs due to a couple
of additional moves, but the problem there is that the register allocator
falsely assumes that the input register operands still need to be alive for
the announcement instruction and this prevents some optimizations.)
What might be a bit hairy is to make sure that the real compare instructions
(i.e. the possibly "unnecessary" compares I am aiming to remove) that are
emitted at expand result in RTL that match as well as possible the
"announcement" patterns that are generated by the arithmetic/logic
operations.
Anyway, I have hope that by using this method one would at least be able to
handle condition code reuse for code segments like
int a,b,c;
c = a - b;
if (c > 0)
{
// do something
;
}
without requiring to use peepholes.
Yours,
BjÃrn
