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To: pdcaw...@bofh.org.uk (Piers Cawley)
Subject: Re: One more thing...
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References: <a06100514bcb59e5f2c78@[10.0.1.3]> <200404300935.i3U9Zqg07670@thu8.leo.home> <a06100502bcb7f7775cfc@[10.0.1.3]> <200405051002.i45A2Kb13601@thu8.leo.home> <m2y8o4wfew.fsf@obelisk.bofh.org.uk> <200405071337.i47Dbpj30143@thu8.leo.home> <m2lljz2b2x.fsf@obelisk.bofh.org.uk> <40A135E4.1080308@toetsch.at> <m23c66dx22.fsf@obelisk.bofh.org.uk>
Reply-To: l...@toetsch.at
Cc: perl6-intern...@perl.org
Date: Wed, 12 May 2004 08:36:40 +0200
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From: l...@toetsch.at (Leopold Toetsch)

Piers Cawley <pdcaw...@bofh.org.uk> wrote:
> Leopold Toetsch <l...@toetsch.at> writes:

[ calculating registers to save ]

>> ... once per sub per location where the sub is called from. But there
>> isn't any knowledge that a sub might be called. So the cost is actually
>> more per PMC instruction that might eventually run a PASM MMD. This is,
>> when its done right, or ...

> No. Once per compilation unit.

An example:

 .sub foo

   # a lot of string handling code
   # and some PMCs
   $P0 = concat $P0, $S0            # <<< 1) calculate: save P, S here
   # now a lot of float code
   # no strings used any more
   # and no branch back to 1)
   $N1 = 47.11                      # $N1's live starts here
   $P0 = $P1 + $N1                  # <<< 2) calculate: save P, N regs
   $P2 = $P0 + $N1                  # <<< 3) calculate: save P regs
   # no N reg used here
 .end

At 1) the caller is not interested in preserving N-registers, these
aren't used there. Saving everything, the caller needs saving, ends up
with C<saveall> in non trivial subroutines.

Using your proposal would need a lot of storage for the saved
register ranges.

If the calculation is done based on the called subroutine, it's not
unlikely that only a few registers have to be preserved, e.g. no
N-registers for the overloaded C<concat> and no string registers for the
overloaded C<add>.

This doesn't violate the principle of caller saves: all that needs
preserving from the caller's POV is preserved.

leo