[Vm-dev] ASM questions for tight loops

[Clément Bera]

On Sat, Jan 6, 2018 at 7:16 PM, Eliot Miranda <eliot.miranda at gmail.com>
wrote:

> Hi Clément,
>
> On Sat, Jan 6, 2018 at 12:00 AM, Clément Bera <bera.clement at gmail.com>
> wrote:
>
>> Hi Eliot, (sorry long mail but I think you will enjoy this discussion;
>> read it)
>>
>> Looking deeper into tight loops performance to get some boost out of
>> Sista, I have questions and remarks.
>>
>> First thing is when I compare 3 versions or array copies (code at the
>> bottom of mail).
>> Version [1] : copyArray : written in plain Smalltalk
>> Version [2] : fastCopyArray : sista generated code (only unchecked
>> operations)
>> Version [3] : primCopyArray : simply call the machine code primitive
>>
>> For small arrays everything is fine, sista code is good. But for large
>> arrays I get the following results (here a copy of 1000 elements):
>> 1) 85k/second
>> 2) 390k/second
>> 3) 2,000k/second
>>
>> So the sista version is ~5x compared to pure smalltalk code, but ~5x
>> slower to the primitive.
>>
>> My guess is that the reason for this is that the hand written primitive
>> is written with a 7 instructions tight loop (2 additions, 1 read, 1 write,
>> 2 jumps, 1 comp), while the sista generated code requires 6 temp read, 1
>> temp write and a couple cheap instructions in addition. We discussed
>> earlier than register allocation might bring a 20% performance boost, but
>> it seems tight loops are quite important and in this case the performance
>> difference is around 5x (500%).
>>
>
> This is for me very good news.  I thought there was potential in good
> register allocation and this confirms it.
>
>
>>
>> So big question is that should I had more macro-instructions (let's call
>> them stubs) in the sista extended bytecode set such as bytesEquals which
>> bulkCompares the bytes of 2 objects, since performance difference is bigger
>> than expected, or should I wait for better register allocation algorithm ?
>>
>
> I think *very much* we should take the register allocation route.  If we
> go the other route we're following the well-worn path of systems like
> Python where basic execution performance is poor and is solved by
> special-casing specific operations and implementing them with C
> primitives.  Eventually one gets to a system where all the performance
> critical parts are in C, which has a couple of really bad effects.  First,
> nothing one does to increase basic performance has any effect since all the
> time is in C, so one looses the impetus to improve basic language
> performance.  Second, much of the system is in large mono;ethic and
> unchangeable chunks of C, so the system looses its dynamic language derived
> flexibility.  We definitely want to move in the other direction.  So let's
> instead put in the effort to make register allocation work well.  We know
> what has to be done.  It isn't easy but the long-term benefits are great.
> We should be able to make many primitives entirely optional in the JIT.
>  [Brain fart: We may even be able to generate primitives for the
> Interpreter versions using our framework]
>
> I believe hand-written stub will always be faster than sista generated
>> code (since a couple things can't be expressed at the bytecode level), but
>> based on multiple benchmarks I think if the better register allocator
>> generates code with the same number of memory read-write (including temp
>> access on stack) the difference should be less than 20% performance
>> difference as we expected.
>>
>
> Good, then i think we are in agreement.  We can live without stubs.
>
>
>> It's not clear how many stubs would be needed. I have a prototype of
>> array copy, other things such as indexOf or fillObject are needed. Maybe I
>> can add only 3-4 for now to show massive speed-ups like we did with
>> bytesEquals.
>>
>
> I don't think it's the right way to go.  Having these for performaNc
> comparison is good, but having them as general features of inline
> primitives is, I think, a long-term strategic error, as outlined above.
>

Yeah my main concern is with maintaining huge Assembly code stubs. I did it
for byteEquals already, and this is hard to maintain (80 lines in total for
the stub ?). Having 5 of those is anooying.


>
>
>> Let's note that we could try to share the stubs with normal primitives
>> (primStringReplace with the arraycopy stub for example) to avoid too much
>> maintenance cost. The problem is that normal primitives have complexity
>> with all the checks at the beginning, then a generic case (copy without
>> knowing anything about the operands) while sista stubs have no checks, but
>> complexity is in generating better code when some operands are constants.
>> So it's not really the same thing.
>>
>
> Yes, this does seem interesting.  But medium-term interpreter performance
> isn't critical.  Let's get the JIT performance good first.
>

I mean primitive in the JIT like genPrimStringReplace versus inline
primitives.

I would love to have interpreter primitives and the main interpreter loop
generated from the JIT back-end, so switching from the interpreter
primitives and the interpreter loop to jitted code would be cheap (same
calling conventions, etc.). But let's do that later.

>
>
>> Second question is about RTL code for tight loops. I looked into V8 and
>> it seems that for tight loop they try to use backward conditional instead
>> of forward conditional + backward unconditional. What do you think will be
>> the difference if I do it for array copy (the copy is guaranteed not to be
>> empty at this point, example below Original-New) ? I wonder if conditional
>> back jumps are not slower than conditional forward jumps or something like
>> that (I am always confused with branch performance, and the performance is
>> different depending on caches so small benchs may not be relevant). What's
>> your take on this ?
>>
>
> That's a good question.  I don't know.  The current layout is dictated by
> the fact that backwards jumps are interrupt points along with
> byttecode-to-machine-code-pc-mappng.  The code look like this, this being
> from SequenceableCollection>>indexOf:startingAt:
>
> startIndex to: self size do: [ :index |
> (self at: index) == anElement ifTrue: [ ^index ] ].
> ^0
>
> The end of the loop's bytecodes are
>
> 61 <B0> send: +
> 62 <6A> popIntoTemp: 2
> 63 <A3 ED> jumpTo: 46
> 65 <75> pushConstant: 0
> 66 <7C> returnTop
>
> is compiled into
>
> 000010f2: movl $0x00100028=#+, %ecx : B9 28 00 10 00
> 000010f7: call .+0xfffff37c (0x00000478=ceSend1Args) : E8 7C F3 FF FF
> IsSendCall + bc 61/62:
> 000010fc: movl %edx, -28(%ebp) : 89 55 E4
> 000010ff: movl %ds:0x80000='stackLimit', %eax : A1 00 00 08 00
> 00001104: cmpl %eax, %esp : 39 C4
> 00001106: jnb .+0xffffff5e (0x0000106a=indexOf:startingAt:@6A) : 0F 83 5E
> FF FF FF
> 0000110c: call .+0xfffffa87 (0x00000b98=ceCheckForInterruptsTrampoline) :
> E8 87 FA FF FF
> IsRelativeCall:
> HasBytecodePC bc 62/63:
> 00001111: jmp .+0xffffff54 (0x0000106a=indexOf:startingAt:@6A) : E9 54 FF
> FF FF
> 00001116: movl $0x00000001, %edx : BA 01 00 00 00
> 0000111b: movl %ebp, %esp : 89 EC
> 0000111d: popl %ebp : 5D
> 0000111e: ret $0x000c : C2 0C 00
>
> So the suspension point is the pc following the call of
> ceCheckForInterruptsTrampoline, which is 00001111.  If execution
> continues after interruption the jump is taken and the loop continues,
> which is as required.  [I remember an early state in Cog;'s development at
> Qwaq when I had't implemented this and loops were terminating every time
> there was an interrupt, such as when one wiggled the mouse,  The system
> almost worked; it wad quite bizarre :-) ].
>
> But notice that the conditional jump at 00001106 *is* backwards as you
> desire.  It seems we're already doing the right thing, at least with the
> backwards jump.  What do we do with backwards jumps in SistaV1 inline
> primitives?
>

I have added backjumpNoInterrupt.


>
>
> And then the question is that should I be able to generate that kind of
>> loop from the sista back-end (I don't really like it, I prefer to limit the
>> number of back-jumps else it makes some things harder) ? Else I can do it
>> in the stubs for most critical performance things and ignore it in other
>> cases.
>>
>> *Original*
>> instr := cogit CmpR: startReg R: stopReg.
>> jumpFinished := cogit JumpBelow: 0.
>> cogit MoveXwr: repStartReg R: replReg R: TempReg.
>> cogit MoveR: TempReg Xwr: startReg R: arrayReg.
>> cogit AddCq: 1 R: startReg.
>> cogit AddCq: 1 R: repStartReg.
>> cogit Jump: instr.
>> jumpFinished jmpTarget: (jumpEmpty jmpTarget: cogit genPrimReturn).
>> Total 7 instructions in the loop
>> *New*
>> instr := cogit MoveXwr: repStartReg R: replReg R: TempReg.
>> cogit MoveR: TempReg Xwr: startReg R: arrayReg.
>> cogit AddCq: 1 R: startReg.
>> cogit AddCq: 1 R: repStartReg.
>>         cogit CmpR: startReg R: stopReg.
>> cogit JumpGreaterOrEqual: instr.
>>         jumpEmpty jmpTarget: cogit genPrimReturn.
>> Total 6 instructions in the loop
>> But a conditional back-jump.
>>
>
> Yes, the latter looks better and matches what we're doing with the
> backwards jump.
>
>
>>
>> NB : Eliot I tried incrementing the pointer instead of the index and use
>> a 0 displacement read/writes and it made no performance difference on
>> Intel. Saves a register in the loop but we don't need them. It makes a
>> difference in the inlined stub however since other registers can be used
>> for other things in the method where it's inlined.
>>
>
> In general we should always try to reduce register pressure.  So if we can
> write the primitive using incremented pointers instead of base registers
> and incremented indexes we should.  In some complex loop, once we have good
> register allocation, that extra register will be put to good use and we can
> expect that we would see increased performance.
>
>
>> *[1]* Smalltalk code
>> 1 to: (y - x + 1) do: [ :i |
>> array2 at: y2 + i put: (array at: x + i) ].
>> *[2] *(sista bytecodes of the tight loop)
>> 32 <45> pushTemp: 5
>> 33 <46> pushTemp: 6
>> 34 <F8 F3 87> smiLessOrEqual:
>> 37 <EF 1B> jumpFalse: 66
>> 39 <43> pushTemp: 3
>> 40 <45> pushTemp: 5
>> 41 <40> pushTemp: 0
>> 42 <45> pushTemp: 5
>> 43 <F8 10 88> pointerAt:
>> 46 <F8 B8 8B> pointerAt:put:
>> 49 <D8> pop
>> 50 <45> pushTemp: 5
>> 51 <E1 00 E8 01> pushConstant: 1
>> 55 <F8 D0 87> smiAdd:
>> 58 <D5> popIntoTemp: 5
>> 59 <E1 FF E8 DE> pushConstant: -34
>> 63 <F8 70 97> backjumpNoInterrupt
>>
>
> I'm confused by "59 <E1 FF E8 DE> pushConstant: -34"; what's that?  Are
> these actually extension bytes for the backwards jump?  Can't be because
> the E8 wouldn't be there.  So what is this?
>

-34 is the distance of the backward jump pushed on stack.
#backjumpNoInterrupt is an inlined primitive expecting the distance to be
on stack with a long form pushInteger bytecode (the guaranteed long form
simplifies Cogit's scan for fixups and latestContinuation). That way no
inline primitive require extensions and the callPrimitive bytecode does not
use any extensions. I changed the inlined primitives from 6000 and onwards
to be jumps with different number of parameters (jumpReadOnly, jumpYoung,
backwardJumpNotInterrupt, etc.). Works really well. I am finishing patches
to Scorch and evaluation of performance to commit to VMMaker.


>
> *[3]* primitive call
>> array2 replaceFrom: x to: y with: array startingAt: y2
>> Current tight loop RTL:
>> instr := cogit CmpR: startReg R: stopReg.
>> jumpFinished := cogit JumpBelow: 0.
>> cogit MoveXwr: repStartReg R: replReg R: TempReg.
>> cogit MoveR: TempReg Xwr: startReg R: arrayReg.
>> cogit AddCq: 1 R: startReg.
>> cogit AddCq: 1 R: repStartReg.
>> cogit Jump: instr.
>> jumpFinished jmpTarget: (jumpEmpty jmpTarget: cogit genPrimReturn).
>>
>
> Note that if you were to maintain a pointer and the delta between the
> source and the destination pointers, instead of a source pointer and a
> destination pointer, you'd only have to increment one register (for the one
> pointer) in the loop which would save another instruction.
>
>

Yeah I wanted to do that at some point. It can be improved.

I did the simplest that could possibly work and the performance is better
than the C primitive so I did not go any further. Remember that later we'll
need to compare this performance with the sista generated code. For byte
copies it needs to be improved (slower than C primitive).


>
>> --
>> Clément Béra
>> https://clementbera.wordpress.com/
>> Bâtiment B 40, avenue Halley 59650
>> <https://maps.google.com/?q=40,+avenue+Halley+59650%C2%A0Villeneuve+d'Ascq&entry=gmail&source=g>Villeneuve
>> d
>> <https://maps.google.com/?q=40,+avenue+Halley+59650%C2%A0Villeneuve+d'Ascq&entry=gmail&source=g>
>> 'Ascq
>> <https://maps.google.com/?q=40,+avenue+Halley+59650%C2%A0Villeneuve+d'Ascq&entry=gmail&source=g>
>>
>
> This is exciting!!
>
> _,,,^..^,,,_
> best, Eliot
>



-- 
Clément Béra
Pharo consortium engineer
https://clementbera.wordpress.com/
Bâtiment B 40, avenue Halley 59650 Villeneuve d'Ascq
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