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JimmyZ is blocked on alias_analysis branch because luajit using trace compiler and he does't know how to search the related ops in cfg to use alias analysis in a cheap way. 08:18
brrt ok, JimmyZ, I think I can help you there a bit 08:24
although my knowledge of this is purely theorethical 08:25
(is it theorethical or theoretical?)
spesh computes the SSA 'form' of the graph 08:26
which means that each register operand of each instruction has the original register index as well as the ssa-register index 08:27
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brrt this ssa register index is really the same as a identifier to when the value was last set 08:28
so if you have an instruction with a source register operand and whish to know where this was computed, you walk back to find an instruction with a destination register operand with the same ssa index 08:30
i... think this is always possible walking back 08:31
but jnthn might know better
(because of phi nodes)
it is potentially possible, if a basic block is out of order, to use a ssa register which was defined later, but at least it must be on the list of predecessors 08:34
in fact, that seems to be only possible if there is a single predecessor, otherwise there must be a phi node?
or the code would use an undefined register 08:35
e.g. suppose we have the following linear sequence: a -> b -> c
and the following code sequence (using unconditional jumps): a -> c -> b
and b uses register 42, which has been set in c but not in a 08:36
then you should be able to find register 42's (only) assignment by walking from b to it's predecessor c, but not in a 08:37
now what i'm not sure of, is if the linear predecessor is necessarily a predecessor in spesh, i think not 08:38
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JimmyZ It's really hard :) 09:09
brrt: I want to exchange orig and i in MVMSpeshOperand, but it cause segfault in jit 09:12
disable jit works fine 09:13
brrt don't excahnge orig and i
that doesn't make sense
JimmyZ why?
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brrt for one thing, because some i's start from (1 << 16) and upward 09:14
which is much larger than the register size 09:15
if any code uses the reg.i in the JIT, then that is very much a bug
(moarvm register array size that is)
JimmyZ I want to do it because I want to cover reg to lit_i64 for esay test is two Oprerand is equal or not
brrt but also because the orig refer to actual locations that are actually allocated, and i is really an identifier
cover = convert? 09:16
JimmyZ yeah
brrt hmmm
when do you do that?
JimmyZ alias analysis 09:17
brrt but... i'm not sure i get it 09:18
how do you wish to do that?
is there a commit that does this?
JimmyZ brrt: github.com/MoarVM/MoarVM/blob/alia.../mem.c#L77
brrt uhm... why'd you do that 09:22
there are plenty of cases in which the upper bits of the i64 are not set :-)
you might accidently see them as equal, or not as equal
JimmyZ I need test orig and i together? 09:23
brrt hmmm
why not check for equality of reg
and have the compiler figure the details out
JimmyZ test.c:21:11: error: invalid operands to binary == (have ‘struct <anonymous>’ and ‘struct <anonymous>’) 09:25
if(test.r == test2.r) {
y
like this?
brrt hmmm
ok, when i'm wrong i'm wrong
yes like that :-)
JimmyZ luajit does the way by convert it to a value 09:26
brrt luajit makes a whole lot of assumptions about luajit
mostly because it was made by one man 09:27
but yeah, does this crash in the JIT?
JimmyZ brrt, it crash if I changed it
brrt hmmm
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brrt if this code would crash the JIT it would be a JIT error :-) so that's one thing 09:28
JimmyZ and LARGE_INTEGER on windows does same thing like covert two small int to big int
brrt ok, let's suppose it is ok :-) 09:34
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brrt i would think that having the same i would imply the same orig 09:36
JimmyZ but same version not means same orig 09:37
is i means version? 09:38
brrt no, i is identifier 09:44
and it identifies an assignment :-)
so - unless something like set is special cased and keeps i equal over different registers - same i means same orig 09:45
but i really doubt that set is special cased
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JimmyZ for r(1) ,Is i = 1? 09:50
looking at codes in rename_locals, looks like it is 09:51
r2(1) 09:52
brrt yes 09:53
hmmm
ok, maybe jnthn can shed some light there
because that is very much true :-)
maybe i is really only an identifier for the same register
JimmyZ yeah, but I still need to test if it's the same reg, convert them into a value means I only need to test once 09:55
if I exchange orig/i then I can do it,but it breaks jit :(
brrt how do you wish to exchange orig and i 10:14
if i understand it like you say it, then it doesn't make sense, not just for the JIT 10:16
JimmyZ better memory algin 10:17
brrt ah..a
as in, in the struct
ok, that's pretty wrong 10:18
:-)
ok, can you file a github issue with a patch so that i can test it?
JimmyZ gist.github.com/zhuomingliang/6c25...fcf4d538ad
brrt: I have a test, if int32 is after int16, then .r is not the same 10:19
what patch? exchange? alias_analysis branch has the patch 10:20
so you could test it on that branch
brrt ok, let me read that :-) 10:21
JimmyZ github.com/MoarVM/MoarVM/commit/e7...bb2d52L198
brrt quite right 10:23
if you change them, they're not equal
huh 10:24
doesn't that add two orig fields
oh no
i'm wrong
nm
hmm 10:25
i still don't like the sneaky conversion to lit_i64 for comparison :-)
but still
if that breaks the JIT, that's a JIT bug
and I have to investigate
the JIT should be reading .orig at all times
JimmyZ brrt: I don't like conversion too, but since it saves a test op ... and others does the same way :P 10:30
decommute
brrt that doesn't convince me though 10:37
but i'll change it and see what happens
jnthn Do NOT compare MVMSpeshOperand.reg by converting it. Just write a function that compraes the orig and the i. Too much risk of screw-up on machines of different endianness etc otherwise. Not to mention confusing code. 10:54
This stuff is hard enough already. Being more clever than you need to be in the code is just unhelpful.
I read the allocation sinking stuff on the plane. It's not especially clear how to apply it in a MoarVM scenario. 10:55
Because it is muchly defined in terms of trace JIT things.
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brrt (on the plane? flying again) 10:57
jnthn A few days ago :) 10:58
brrt :-)
jnthn To family, for Christmas
brrt nice
i'll have to go by train tomorrow
and i'll be reading a long reader on system analysis and design rather than code :-)
jnthn Doing that after Christmas to get to other bit of family. :) 10:59
brrt i suspect the JIT uses lit_i16 where it should use reg.orig somewhere
jnthn Yeah, that is possible
orig is the actual register in the original bytecode, and i is the SSA version. We actually do this just like the paper, that is write variables with a version index. 11:00
Then conversion out of SSA form just involves dropping the versions again.
brrt so i is a version index? why do some of them get so large
or is that an artefact of the proces 11:01
jnthn Usually because long routine + compiler tries to re-use registers, and SSA is unpicking all of that.
brrt aha
hmm
so when i said that some start counting from 1<<16 up, i was wrong? 11:02
:-)
jnthn I thought it went from 0 for versions
And they're numbered via the dominance tree iirc
Anyways, seems family have food ready for me so I should go and eat it :) 11:03
Have a nice Christmas, y'all :)
brrt yes, you too 11:05
merry christmas
(also, how is everybody already celebrating christmas, /me is still at work)
FROGGS__ I'm off work since monday, which was a good decision because there was so much to prep 11:06
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JimmyZ jnthn: merry christmas 11:48
JimmyZ still is at work too 12:00
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JimmyZ jnthn: about Because it is muchly defined in terms of trace JIT things. Didn't we have a roadmap about Trace JIT? ;) 15:14
timotimo JimmyZ: what does the alias analysis thing do so far? or what can it already do/what can be implemented using it? 15:15
JimmyZ timotimo: the Allocation sinking need it. and I find moarvm's alias analysis is much more complex than luajit :( 15:17
timotimo i feared as much :( 15:18
JimmyZ well, not only Allocation sinking need it, itself is very nice thing 15:19
timotimo right 15:20
JimmyZ i.e: x = obj.x; .....; y = obj.x + i; and make the later obj.x a alias to x is very nice
or, e = @a[x+1]; ..... ; y = @a[x+1] + i; 15:21
timotimo ah
so it's a tiny bit like common subexpression elimination 15:22
JimmyZ oh yeah, a bit like 15:26
x+1 part is cse and @a[x] is aa, me thinks. 15:27
and sometimes I'm confused ...
timotimo: alias analysis mostly is for two object, cse is not 15:31
like obj1.x and obj2.x, is obj1 and obj2 may be ailas 15:33
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JimmyZ so the two example is actullay cse .... and ojb1 vs obj2 is aa 15:46
dalek arVM/alias_analysis: 8f5a7ab | (Jimmy Zhuo)++ | src/spesh/mem.c:
Add basic alias analysis for object
 15:54
JimmyZ timotimo: en.wikipedia.org/wiki/Alias_analysis
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