On 08.04.2012 18:18, H. S. Teoh wrote:
[snip]
1. Every time a function is generated (or pretty much any symbol)
not only a size calculated but also a checksum* of it's data.
(If we go for link-time optimization we should find a place to stick
it to in the object file)
We'd have to make sure the checksum doesn't end up in the final
executable though, otherwise the bloat may negate any gains we've made.
Easy the symbol size is in object file (obviously) but it surely not
present in the executable. If I worth anything legacy formats have a
plenty of slack space reserved for future. Nwo is the future :)
[snip]
3. After any function was generated compiler checks an entry in the
duplicate table that matches size, followed by matching checksum and
only then (if required) doing a straight memcmp. If it happens that
there is a match compiler just throws generated code away and
_aliases_ it's symbol to that of a matched entry.
(so there has to be an alias table if there isn't one already)
I think you don't even need an alias table; IIRC the OS dynamic linker
can easily handle symbols that have the same value (i.e. that point to
the same place). All you have to do is to change the value of the
duplicated symbols so that they all point to the same address.
Nice to know.
[...]
Applicability:
It's not only const-immutable bloat, it can be alleviated with
inout. Yet there are plenty of places the exact same code is being
generated: e.g. sealed containers of int vs uint, std.find on
dchar[] vs uint[]/int[] an so on.
In general, the coarse grained parametrization is the root of all
evil and it is inevitable since we are just humans after all.
I'm not sure I understand the last sentence there, but duplicate code
elimination is definitely a big plus. It will also mean that we can use
templates more freely without having to worry about template bloat.
It's easy - define a template on type T. Code it up.
Now how many times you did consider that e.g. you can parametrize on the
size of the type instead of the type itself? I'm making a point that
it's almost always the case with sealed containers of PODs for instance.
(now multiply the argument for total number of parameters)
Notes:
1. If we do checksum calculation on the fly during codegen it gets
at virtually no cost as the data is in CPU data cache. Preliminary
version can avoid hacking this part of backend though.
2. By _alias_ I mean the ability of compiler to emit references to a
given symbol as if it was some other symbol (should be really
straight forward).
Like I said, I think this isn't even necessary, if the compiler can just
generate the same value for the duplicated symbols.
OK, I'm not much into how *exactly* linker works these days. I know the
basics though.
4. Ironically the same exact thing works with any kind of immutable
data structures. It looks like string pooling is superseded by this
proposal.
[...]
Not really... string pooling can take advantage of overlapping
(sub)strings, but I don't think you can do that with code. But I think
your idea has a lot of merit. I'm for making linkers smarter than they
currently are.
Sorry, it's just me running ahead of train somewhat. Basically once this
initial version is in place I have one cool refinement for it in mind.
For now we just need to keep the hash function transitive and
associative for the gods sake. 128/64bit checksum please ;)
--
Dmitry Olshansky
