Issue #12589 has been updated by vmakarov (Vladimir Makarov).


k0kubun (Takashi Kokubun) wrote:

> > An alternative approach can be useful but it might be a waste of your time at the end. But any performance work requires a lot alternative implementations (e.g. the current global RA in GCC was actually one of my seven different RA implementations), some temporary solutions might become permanent. who knows.
> 
> As I'm hacking Ruby as not work but just hobby to enjoy improving my Ruby core understanding, it wouldn't be a waste of time even if I end up with developing seven different JIT implementations :)
> 

Sorry, Takashi.  I was inaccurate.  I am agree.  Any serious problem solving (even if it does not result in MRI code change) makes anyone a better, more experienced MRI developer.

> > So the solution would be implementing analysis on RTL to use double values in JITted code of a method to avoid double->flonum and flonum->double conversions. RTL is a good fit to this.
> 
> Question for my better understanding: Do you mean GCC and Clang can't optimize double<->flonum conversion well even if all necessary code is inlined?

Yes. It is too complicated for them.  Tagging doubles manipulates with exponent and mantissa by constraining exponent range and using a part of exponent field to store few less significant bits of mantissa.  Even worse, processing 0.0 makes it even more complicated.  Optimizing compilers are not smart enough to see that untaging doubles is a reverse operation to tagging and vice versa. 

> If so, having special effort to optimize it in Ruby core makes sense. I'm not sure why we can't do that with stack-based instructions or just in JIT-ed C code generation process. Can't we introduce instruction specialization (to avoid double<->flonum conversion, not sure its details) without having all instructions as register-based?

You can do the optimization with stack insns.  You need to analyze all method(s) code and see where from operand values come.  It is easier to do with RTL.

But actually the worst part with using stack insns for optimizations is that you can not easily transform a program on them (e.g. move an invariant expression from the loop -- you need to introduce new local vars for this) because they process values only in a stack mode and optimized code can process values in any order.

In any case, if we are going to do some optimizations by ourself (and I see such necessity in the future) not only by GCC/LLVM, we need a convenient IR for this.  I tried to explain it in my presentation at RubyKaigi.

One simple case where we can avoid untagging is RTL insn with immediate operand (we can use double not VALUE for the immediate operand).  It is actually on my TODO list.
 
> 
> > Basic type inference could be another example for RTL necessity. I could find other examples.
> 
> Type inference at RTL instructions is interesting topic which I couldn't understand well from discussion with you at RubyKaigi. I'm looking forward to seeing the example!

https://github.com/dino-lang/dino/blob/master/DINO/d_inference.c is an example how a basic type inference can be implemented on RTL-like language.  It is a different approach to algorithm W in HindleyMilner type system.  The algorithm consists of the following steps

1.  Building a control flow graph (CFG) consisting of basic blocks and control flow edges connecting them.
2.  Calculating available results of RTL instructions  this is a forward data-flow problem on the CFG.
3.  Using the availability information, building def-use chains connecting possible operands and results of RTL instructions and variables.
4.  Calculating  the  types  of  RTL  instruction  operands  and results    this  is  a  forward  data-flow  problem  on  the  def-use graph.

The definition of availability and def-use chains can be found practically in any book about optimizing compilers.


----------------------------------------
Feature #12589: VM performance improvement proposal
https://bugs.ruby-lang.org/issues/12589#change-67363

* Author: vmakarov (Vladimir Makarov)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
  Hello.  I'd like to start a big MRI project but I don't want to
disrupt somebody else plans.  Therefore I'd like to have MRI
developer's opinion on the proposed project or information if somebody
is already working on an analogous project.

  Basically I want to improve overall MRI VM performance:

  * First of all, I'd like to change VM insns and move from
    **stack-based** insns to **register transfer** ones.  The idea behind
    it is to decrease VM dispatch overhead as approximately 2 times
    less RTL insns are necessary than stack based insns for the same
    program (for Ruby it is probably even less as a typical Ruby program
    contains a lot of method calls and the arguments are passed through
    the stack).

    But *decreasing memory traffic* is even more important advantage
    of RTL insns as an RTL insn can address temporaries (stack) and
    local variables in any combination.  So there is no necessity to
    put an insn result on the stack and then move it to a local
    variable or put variable value on the stack and then use it as an
    insn operand.  Insns doing more also provide a bigger scope for C
    compiler optimizations.

    The biggest changes will be in files compile.c and insns.def (they
    will be basically rewritten).  **So the project is not a new VM
    machine.  MRI VM is much more than these 2 files.**

    The disadvantage of RTL insns is a bigger insn memory footprint
    (which can be upto 30% more) although as I wrote there are fewer
    number of RTL insns.

    Another disadvantage of RTL insns *specifically* for Ruby is that
    insns for call sequences will be basically the same stack based
    ones but only bigger as they address the stack explicitly.

  * Secondly, I'd like to **combine some frequent insn sequences** into
    bigger insns.  Again it decreases insn dispatch overhead and
    memory traffic even more.  Also it permits to remove some type
    checking.

    The first thing on my mind is a sequence of a compare insn and a
    branch and using immediate operands besides temporary (stack) and
    local variables.  Also it is not a trivial task for Ruby as the
    compare can be implemented as a method.

  I already did some experiments.  RTL insns & combining insns permits
to speed the following micro-benchmark in more 2 times:

```
i = 0
while i<30_000_000 # benchmark loop 1
  i += 1
end
```

The generated RTL insns for the benchmark are

```
== disasm: #<ISeq:<main>@while.rb>======================================
== catch table
| catch type: break  st: 0007 ed: 0020 sp: 0000 cont: 0020
| catch type: next   st: 0007 ed: 0020 sp: 0000 cont: 0005
| catch type: redo   st: 0007 ed: 0020 sp: 0000 cont: 0007
|------------------------------------------------------------------------
local table (size: 2, temp: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 2] i
0000 set_local_val    2, 0                                            (   1)
0003 jump             13                                              (   2)
0005 jump             13
0007 plusi            <callcache>, 2, 2, 1, -1                        (   3)
0013 btlti            7, <callcache>, -1, 2, 30000000, -1             (   2)
0020 local_ret        2, 0                                            (   3)
```

In this experiment I ignored trace insns (that is another story) and a
complication that a integer compare insn can be re-implemented as a
Ruby method.  Insn bflti is combination of LT immediate compare and
branch true.

A modification of fib benchmark is sped up in 1.35 times:

```
def fib_m n
  if n < 1
    1
  else
    fib_m(n-1) * fib_m(n-2)
  end
end

fib_m(40)
```

The RTL code of fib_m looks like

```
== disasm: #<ISeq:fib_m / fm.rb>==========================================
local table (size: 2, temp: 3, argc: 1 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 2] n<Arg>
0000 bflti            10, <callcache>, -1, 2, 1, -1                   (   2)
0007 val_ret          1, 16
0010 minusi           <callcache>, -2, 2, 1, -2                       (   5)
0016 simple_call_self <callinfo!mid:fib_m, argc:1, FCALL|ARGS_SIMPLE>, <callcache>, -1
0020 minusi           <callcache>, -3, 2, 2, -3
0026 simple_call_self <callinfo!mid:fib_m, argc:1, FCALL|ARGS_SIMPLE>, <callcache>, -2
0030 mult             <callcache>, -1, -1, -2, -1
0036 temp_ret         -1, 16
```

In reality, the improvement of most programs probably will be about
10%.  That is because of very dynamic nature of Ruby (a lot of calls,
checks for redefinition of basic type operations, checking overflows
to switch to GMP numbers).  For example, integer addition can not be
less than about x86-64 17 insns out of the current 50 insns on the
fast path.  So even if you make the rest (33) insns 2 times faster,
the improvement will be only 30%.

A very important part of MRI performance improvement is to make calls
fast because there are a lot of them in Ruby but as I read in some
Koichi Sasada's presentations he pays a lot of attention to it.  So I
don't want to touch it.

  * Thirdly.  I want to implement the insns as small inline functions
    for future AOT compiler, of course, if the projects described
    above are successful.  It will permit easy AOT generation of C code
    which will be basically calls of the functions.

    I'd like to implement AOT compiler which will generate a Ruby
    method code, call a C compiler to generate a binary shared code
    and load it into MRI for subsequent calls.  The key is to minimize
    the compilation time.  There are many approaches to do it but I
    don't want to discuss it right now.

    C generation is easy and most portable implementation of AOT but
    in future it is possible to use GCC JIT plugin or LLVM IR to
    decrease overhead of C scanner/parser.

    C compiler will see a bigger scope (all method insns) to do
    optimizations.  I think using AOT can give another 10%
    improvement.  It is not that big again because of dynamic nature
    of Ruby and any C compiler is not smart enough to figure out
    aliasing for typical generated C program.

    The life with the performance point of view would be easy if Ruby
    did not permit to redefine basic operations for basic types,
    e.g. plus for integer.  In this case we could evaluate types of
    operands and results using some data flow analysis and generate
    faster specialized insns.  Still a gradual typing if it is
    introduced in future versions of Ruby would help to generate such
    faster insns.

  Again I wrote this proposal for discussion as I don't want to be in
a position to compete with somebody else ongoing big project.  It
might be counterproductive for MRI development.  Especially I don't
want it because the project is big and long and probably will have a
lot of tehcnical obstacles and have a possibilty to be a failure.




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