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11
app/views/pages/index.html.haml
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@ -76,8 +76,8 @@
=link_to "classes, types" , "/rubyx/parfait.html"
methods and basic types.
%li
=ext_link "Risc machine abstraction" , "https://github.com/ruby-x/rubyx/tree/master/lib/risc"
(includes extensible instruction)
=ext_link "Risc machine abstraction" , "/rubyx/layers.html#risc"
(with SSA and register allocation)
%li
A minimal ARM and ELF implementation to create
= succeed "." do
@ -86,10 +86,11 @@
%p
But there is still a lot of work, here are some of the next few topics
%ul
%li Dynamic Memory management
%li Benchmarks for calling and integer
%li Inlining and static memory analysis
%li Start stdlib with String and files
By then we may be in the foothills, but nowhere near even basecamp, let alone there.
%li Dynamic Memory management
There are also many small things anybody can
=ext_link "start with." , "https://github.com/ruby-x/rubyx/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+newbie%22"
.tripple
%h2.center Docs
%p

100
app/views/posts/2020/_03-22-register-allocation-in-a-ruby-compiler.haml
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@ -27,23 +27,26 @@
%h2 Motiviation for change
%p
There are two major problems with the simple solution outlined above. The first is
sub-optimal now, the second restrictive in the future.
that it is sub-optimal now, the second that it is restrictive in the future.
%p
The current problem, is actually many-fold. There is the obvious difficulty of keeping
track of registers as they are used and returned. While i thought that that the by hand
approach was not too bad, after finishing i checked the automatic way uses only half the
aamount of registers. This is nevertheless peanuts compared to the second problem,
which means that the code is forever locked into a subset of the SlotMachine, ie
no optimisations can be done across SlotMachine Instruction borders. That is quite
serious, and sort of biids in with the third problem. Namely there were some implicit
track of registers as they are used and returned. While i thought that the
%i by hand
approach was not too bad, after finishing this work, i checked the automatic way uses
only half the amount of registers. This is nevertheless peanuts compared to the second
problem, which means that the code is forever locked into a subset of the SlotMachine,
ie no optimisations can be done across SlotMachine Instruction borders. That is quite
serious, and sort of binds in with the third problem. Namely there were some implicit
register assumptions being made, but there were implicit, ie could not be checked
and would only show up if broken.
%p
But the real motiviation for this work came from the future, or the lack of
expandability with this approach. Basically i found from benchmarking that inlining
would have to happen quite soon. Even it would *only* be with more Macros at first.
would have to happen quite soon. Even it would
%em only
be with more Macros at first.
Inlining with this super simple allocation would not only be super hard, but also
much less efficient than could be. After all there are 10+ registers than one can
much less efficient than could be. After all there are 10+ registers that one can
keep things in, thus avoiding reloading, and i already noticed constant loading
was a thing.
@ -54,7 +57,7 @@
%p
So then we come to the way that it is coded now. The basic idea (followed by most
compilers) is to assign new names for every new usage of a register. I'll talk
about that first, then the second step is something called liveliness analysis,
about that first. Then the second step is something called liveliness analysis;
basically determining when register are not used anymore, and the third is the
allocation.
@ -63,54 +66,64 @@
A
=ext_link "Static Single Assignent form" , "https://en.wikipedia.org/wiki/Static_single_assignment_form"
of the Instructions is one where every register
name is assigned only once. Hence the Single Assignent. The static part means that
this single sssignment is only true for a static analysis, so at run time the code
may assign many times. But it would be the **same** code doing several assignemnts.
name is assigned only once. Hence the
%em Single
Assignent. The static part means that this single assignment is only true for a static
analysis, so at run time the code may assign many times.
But it would be the
%em same
code doing several assignemnts.
%p
SSA is often achieved by first naming registers according to the variables they hold,
and to derive subsequent names by increasing an subsript index. This did not sound
very fitting. For one, RubyX does not really have "floating" variables (that later
may get popped on some stack), rather every variable is an instance variable.
Assigning to a variable does not create a new register value, but needs to be stored
in memory. In a concurrent environment it is not save to bypass that.
in memory. In a concurrent environment it is not safe to bypass that.
%p
New variables may be created by "traversing" into a instane of the object (if type is
New variables may be created by "traversing" into a instance of the object (if type is
known off course). This lead to a dot syntax naming convention, where almost every
variable starts off from *message* or as a constant, eg "message.return_value". This
variable starts off from
%b message
or as a constant, eg "message.return_value". This
is as "single" as we need it to be (i think), and implementing this naming scheme
was about half the work.
was about half the work (more than half of that in tests, where register names
were and are checked).
%p
The great benefit from this renaming of registers is that even the risc code is still
quite readable, which is great for both debugging and tests. This is because
the registers now have meaningful name (instance variable names), and it is always
clear where it came from.
The great benefit from this renaming of registers is that even the risc code is now
quite readable, which is great for debugging and tests. This is because
the registers now have meaningful names (instance variable names), and it is always
clear what a register is used for.
%h3 Liveliness
%p
the next big step was to determine liveliness of registers. This is something i have not
The next big step was to determine liveliness of registers. This is something i have not
found good literature on, and in documents about Register Allocation it is often
taken as the starting point.
%p
Basic reasoning lead me to believe that a simple backward scan is at least a safe
estimate. If you imagine going trough the list of instructions and marking the
first occurence of ay register (name) use. By going backwards through the list
you thus get the last useage, and that is the point where we can recycle that
first occurence of any register use. By going backwards through the list
you thus get the last usage, and that is the point where we can recycle that
register.
%p
I spent some time trying to figure ot if backward branches changes the fact that
I spent some time trying to figure out if backward branches changes the fact that
you can release the register, but could not come to a conclusion (brain melt
every time i tried). Intuitively i think that you can, because on the first
run through such a loop you could not use results from a register that because of
the ssa would have had to be created later, but there you go. Even rereading that
run through such a loop you could not use results from a register that, because of
the ssa, would have had to be created later, but there you go. Even rereading that
hurts. My final argument was that a backward jump is a while loop, and a ruby
while loop would have to store it's data in ruby variables and not new registers
while loop would have to store its data in ruby variables and not new registers,
or so i hope).
%p
I did read about phi nodes in ssa's and i did not implement that. Phi nodes are a way to
ensure that different brnaches of an if produce the same registers, or the same
registers are meaningfully filled after the merge of an if. My hope is
that the ruby variable argument gets us out of that, and for some risc function
i added some transfers to ensure things work as they should.
I did read about phi nodes in ssa and i did not implement that. Phi nodes are a way to
ensure that different branches of an
%b if
produce the same registers, or the same
registers are meaningfully filled after the merge of an
%b if.
My hope is that the ruby variable argument from above gets us out of that, and for some
risc functions i added some transfers to ensure things work as they should.
%h3 Register Allocation
%p
@ -124,23 +137,28 @@
order. But because of the liveliness analysis, we can release registers after their
last use, and reuse them immediately off course. I noticed that this results in
surprisingly many registers being used only for a single instruction.
And the total number of registers used went **down by half**.
And the total number of registers used went
%b down by half.
%h2 The future
%p
As i said that this was mostly for the future, what is the future going to hold?
Well,
%b inlining
is high up, that's for sure.
%p
Well, **inlining** is number one, that's for sure.
%p
But also there is something called escape analysis. This basically means reclaming
objects that get created in a method, but never passed out. A sort of immediate GC,
But also there is something called escape analysis. This essentially means reclaming
objects that are created in a method, but never get passed out. A sort of immediate GC,
thus not only saving on GC time, but also on allocation. Mostly though it would
allow to run larger benchmarks, because there is no GC and integers get created
allow to run larger benchmarks, because there is no GC, and integers get created
a lot before a meaningfull number of milliseconds has elapsed.
%p
On the register front, some low hanging fruits are redundant transfer elimination and
double load elimination. Since methods have not grown to exhaust registers,
unloading register is undone and thus is looming. Which brings with it code cost
analysis.
analysis. So much more fun to be had!!
%p
So much more fun to be had!!
I am happy to annoounce that RubyX is part of
=ext_link "Rails Girls Summer of Code" , "https://railsgirlssummerofcode.org/"
and some interest is being show. Since i have enjoyed my last RGSoC summer,
i am looking forward to some mentoring, and outside participation.