Hi everyone,
So one of my patch suggestions sought to give a small efficiency boost
to the post-peephole optimiser stage, since it is only designed to
convert instructions into abbreviated forms but which would likely
damage the peephole optimizer if done sooner.
https://bugs.freepascal.org/view.php?id=36437 Florian rejected it
because it made the stage act differently to the others and might be
considered hard to follow.
One thing that I've noted though is that the peephole optimizer only
investigates instructions, not other entries like labels or register
allocations (except to update UsedRegs), and it makes me wonder if it's
possible to convert the peephole optimiser stages so it doesn't enter
the platform-specific methods if the current list entry is not an
instruction. At the moment, it enters a platform-specific method, which
then checks to see if the current entry is an instruction, and
immediately exits out if it's not. Instead, I propose that the
platform-independent method does this check. It might not sound like
much, but it means that it cuts down on a number of procedure calls,
which might number several dozen when analysing a complex function. It
also means one can remove the specific checks for the entry type in the
platform-specific methods.
One thing Florian noted in the above issue is how the parameters were
still of type tai (class for a list entry) instead of taicpu (which is
specific for instructions). Thinking ahead, I don't think this can be
changed to taicpu without failing some optional sanity checks, because
if the current instruction gets deleted, most optimisation methods set
the parameter to the next entry in the list, which may not be an
instruction. One could get around this by demanding the methods call
GetNextInstruction and calling UpdateUsedRegs, but it's asking for
subtle bugs to be introduced if this gets forgotten sonewhere.
Gareth aka. Kit
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