llvm dev - Sep 2016 - [RFC] Interprocedural MIR-level outlining pass

>
>
> Yes, this was exactly my point. We want to recognize
> structurally-equivalent sequences of instructions on inequivalent operands.
>
Yes, and my point is "none of the vn and vn-dag generating algorithms
care".

you can define equivalent to be "structural", you can define it to be
"these two variables are equivalent if they both start with
"a"", you can
define it however you want.
They will still give you the dags you want.

This is as simple as substituting a hash and equality function.

To whit: Actual compilers do it this way.

So i'm entirely unsure why there is such an argument that it hard or
impossible, or even strange.

It is in fact quite easy, the same way GCC has had the VN pass that
produces expression DAG output, and had it used to  code hoisting, to do
PRE, to do folding, to do whatever.  It has been used for all of these
thing.

Some of these use a more standard  VN definition of equivalence that is
useful for redundancy elimination.
Some of them use one that is meant for folding (and would be illegal to use
for straight redundancy elimination).

If you want to build a pass that basically does the same thing, it seems
silly, but feel free!
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(and in particular, the definition of equivalence used by code folding to
make the dags is STH like  "two VNDAG expressions are equivalent if their
operands come from VNDAG expressions with the same opcode")

Thus,

VN2 = VN0 + VN1
VN3 = VN1 + VN2

is considered equivalent to

VN2 = VN0 + VN5
VN3 = VN1 + VN2

Despite the fact that this is completely illegal for straight redundancy
elimination.


But again, as I said if y'all want to make a pass that basically generates
a new type of expression DAG, have fun :)

(I'm going to just leave this thread be now)



On Wed, Aug 31, 2016 at 4:17 PM, Daniel Berlin <dberlin at dberlin.org>
wrote:
>
>> Yes, this was exactly my point. We want to recognize
>> structurally-equivalent sequences of instructions on inequivalent
operands.
>>
>
> Yes, and my point is "none of the vn and vn-dag generating algorithms
> care".
>
> you can define equivalent to be "structural", you can define it
to be
> "these two variables are equivalent if they both start with
"a"", you can
> define it however you want.
> They will still give you the dags you want.
>
> This is as simple as substituting a hash and equality function.
>
> To whit: Actual compilers do it this way.
>
> So i'm entirely unsure why there is such an argument that it hard or
> impossible, or even strange.
>
> It is in fact quite easy, the same way GCC has had the VN pass that
> produces expression DAG output, and had it used to  code hoisting, to do
> PRE, to do folding, to do whatever.  It has been used for all of these
> thing.
>
> Some of these use a more standard  VN definition of equivalence that is
> useful for redundancy elimination.
> Some of them use one that is meant for folding (and would be illegal to
> use for straight redundancy elimination).
>
> If you want to build a pass that basically does the same thing, it seems
> silly, but feel free!
>
>
>
>
>
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----- Original Message -----
> From: "Daniel Berlin" <dberlin at dberlin.org>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Andrey Bokhanko" <andreybokhanko at gmail.com>,
"llvm-dev"
> <llvm-dev at lists.llvm.org>
> Sent: Wednesday, August 31, 2016 7:02:57 PM
> Subject: Re: [llvm-dev] [RFC] Interprocedural MIR-level outlining
> pass
> (and in particular, the definition of equivalence used by code
> folding to make the dags is STH like "two VNDAG expressions are
> equivalent if their operands come from VNDAG expressions with the
> same opcode")
> Thus,
> VN2 = VN0 + VN1
> VN3 = VN1 + VN2
> is considered equivalent to
> VN2 = VN0 + VN5
> VN3 = VN1 + VN2
> Despite the fact that this is completely illegal for straight
> redundancy elimination.
> But again, as I said if y'all want to make a pass that basically
> generates a new type of expression DAG, have fun :)
I don't think that anyone wants to do anything unnecessary, or re-invent any
wheels. I'm just trying to understand what you're saying.

Regarding you example above, I certainly see how you might do this simply by
defining an equivalence class over all "external" inputs. I don't
think this is sufficient, however, for what is needed here. The problem is that
we need to recognize maximal structurally-similar subgraphs, and I don't see
how what you're proposing does that (even if you have some scheme where you
don't hash every part of each instruction). Maybe if you had some
stream-based similarity-preserving hash function, but that's another matter.

Let's say we have this: 

q = a + b 
r = c + d 
s = q + r 
t = q - r 

and later we have: 

u = e + f 
v = g - h 
w = u + v 
x = u - v 

Now, what I want to recognize is that the latter two instructions in each group
are structurally similar. Even if I define an equivalence class over external
inputs, in this case, E = { a, b, c, d, e, f, g, h}, then I have:

q = E + E 
r = E + E 
s = q + r 
t = q - r 

and: 

u = E + E 
v = E - E 
w = u + v 
x = u - v 

but then r and v are still different, so how to we find that {s, t} can be
abstracted into a function, because we've found the isomorphism { s -> w,
t -> x }, and then thus reuse that function to evaluate {w, x}?

Thanks again, 
Hal 
> (I'm going to just leave this thread be now)
> On Wed, Aug 31, 2016 at 4:17 PM, Daniel Berlin < dberlin at dberlin.org
> > wrote:
> > > Yes, this was exactly my point. We want to recognize
> > > structurally-equivalent sequences of instructions on inequivalent
> > > operands.
> > 
> 
> > Yes, and my point is "none of the vn and vn-dag generating
> > algorithms
> > care".
> 
> > you can define equivalent to be "structural", you can define
it to
> > be
> > "these two variables are equivalent if they both start with
"a"",
> > you can define it however you want.
> 
> > They will still give you the dags you want.
> 
> > This is as simple as substituting a hash and equality function.
> 
> > To whit: Actual compilers do it this way.
> 
> > So i'm entirely unsure why there is such an argument that it hard
> > or
> > impossible, or even strange.
> 
> > It is in fact quite easy, the same way GCC has had the VN pass that
> > produces expression DAG output, and had it used to code hoisting,
> > to
> > do PRE, to do folding, to do whatever. It has been used for all of
> > these thing.
> 
> > Some of these use a more standard VN definition of equivalence that
> > is useful for redundancy elimination.
> 
> > Some of them use one that is meant for folding (and would be
> > illegal
> > to use for straight redundancy elimination).
> 
> > If you want to build a pass that basically does the same thing, it
> > seems silly, but feel free!
> 
-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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