That's useful to know, thanks. Ideally I'd like to put it behind a kind
of CSE compiler flag because this will add a degree of overhead to it
and make compilation slower (currently I'm only doing it for -O3).
Currently I'm only tracking "mov (ref),%reg" and "lea (ref),%reg"
instructions and will build from there.
You know I'm never one to turn down a challenge - honestly I'm surprised
I got this much success already!
I'll take a look at your commits. At present, to ensure mistakes are
not made, the sliding window is emptied/reset whenever the following is hit:
- A live label.
- An instruction that modifies the stack pointer (this one may prove
unnecessary due to register tracking).
- An instruction that writes to memory, since even if it writes to the
stack or global memory, there's no way to fully predict whether another
register is pointing to this location and is hence volatile.
Gareth aka. Kit
On 17/02/2022 21:38, Jonas Maebe via fpc-devel wrote:
On 17/02/2022 20:25, J. Gareth Moreton via fpc-devel wrote:
P.S. The term "sliding window" comes from the LZ77 compression
algorithm and is used to track repeated sequences in ZIP files, among
others. This prototype optimisation essentially uses the same
construct, but built for instructions instead of individual bytes.
The usual term for this kind of optimisations is common subexpression
elimination (CSE). We used to have one at the assembler level for i386
(my first main contribution to the compiler), but removed it in the
end because it was extremely prone to breaking — even more so than the
peephole optimiser. This happened in
a04cae2c4ba32bc5c7a8461b5851dee8e70c272b (it was supposed to have
happened in 1b439307490861afd739ba5a48dc175f09727a89, but I made a
mistake).
It doesn't mean it can't be implemented in a robust way, but I can
guarantee from experience that it is extremely hard.
Jonas
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