llvm dev - Jul 2017 - failing to optimize boolean ops on cmps

On 07/13/2017 06:40 PM, Daniel Berlin via llvm-dev
wrote:
> Ah yes, it can only return one instruction and you need two.
>
> NewGVN could still handle it if instsimplify was changed to return the 
> right binary operators because it has infrastructure that can handle 
> insertion.
> (it would need a little work).
>
>
> (At this point, InstCombine seems to have become worse to me than 
> GCC's combining infrastructure, so i'd really like to see if we can
> stop putting things there. Otherwise, we just go farther down the path 
> of 'let's  just make everything one big graph rewrite)
Perhaps something of a digression:

My general impression is that if InstCombine were actually doing a 
principled graph rewrite, then we wouldn't have these problems. The 
problem we have, however, is that we have phase-ordering problems (even 
within the fixed-point iteration scheme): Patterns being matched depend 
on a canonical form, instructions nearly matching those patterns might 
be created, and then suboptimally consumed (i.e. further transformed), 
before being subject to the canonicalizing transformation(s) that would 
make the better pattern actually match.

Except for TableGen'ing the whole thing, and making an actual graph 
automaton to match the patterns, it's not clear to me how to efficiently 
fix this. In the mean time, what we've been doing in practice, is that 
we have InstCombine patterns depend on canonical forms, until we hit 
some phase-ordering problem, and then we extend the pattern in question 
to match some non-canonical forms. We might do this here as well (if 
that's the problem here), but at some point I suspect we'll end up 
redoing everything.

  -Hal
>
>
> On Thu, Jul 13, 2017 at 4:37 PM, Sanjay Patel <spatel at rotateright.com
> <mailto:spatel at rotateright.com>> wrote:
>
>     This can't be an instsimplify though? The values we want in these
>     cases do not exist already:
>       %res = or i8 %b, %a
>       %res = or i1 %cmp, %c
>
>
>
>     On Thu, Jul 13, 2017 at 5:10 PM, Daniel Berlin
>     <dberlin at dberlin.org <mailto:dberlin at dberlin.org>>
wrote:
>
>
>
>         On Thu, Jul 13, 2017 at 2:12 PM, Sanjay Patel
>         <spatel at rotateright.com <mailto:spatel at
rotateright.com>> wrote:
>
>             We have several optimizations in InstCombine for bitwise
>             logic ops (and/or/xor) that fail to handle compare
>             patterns with the equivalent bitwise logic. Example:
>
>             define i8 @or_and_not(i8 %a, i8 %b) {
>               %nota = xor i8 %a, -1
>               %and = and i8 %nota, %b
>               %res = or i8 %and, %a
>               ret i8 %res
>             }
>
>             define i1 @or_and_cmp_not(i32 %a, i32 %b, i1 %c) {
>               %cmp = icmp sgt i32 %a, %b
>               %cmp_inv = icmp sle i32 %a, %b ; this is 'not' of
%cmp
>               %and = and i1 %c, %cmp_inv
>               %res = or i1 %cmp, %and
>               ret i1 %res
>             }
>
>             $ ./opt -instcombine hidden_not.ll -S
>
>             define i8 @or_and_not(i8 %a, i8 %b) {
>               %res = or i8 %b, %a
>               ret i8 %res
>             }
>
>             define i1 @or_and_cmp_not(i32 %a, i32 %b, i1 %c) {
>               %cmp = icmp sgt i32 %a, %b
>               %cmp_inv = icmp sle i32 %a, %b
>               %and = and i1 %cmp_inv, %c
>               %res = or i1 %cmp, %and
>               ret i1 %res
>             }
>
>            
---------------------------------------------------------------------
>
>             Would adding to the existing InstCombine logic folds to
>             handle this kind of pattern be a welcome enhancement? I
>             don't know if it's possible to make the matchers handle
>             this case without adding a new matcher. Eg:
>
>
>         I would rather see this added to instsimplify than instcombine.
>         If you do that, GVN/et all will get this already.
>
>         This probably does require a special matcher though:
>
>
>         We have:
>          if (m_c_And(m_Cmp(Pred, m_Value(), m_Value()),
>                       m_Cmp(Pred, m_Value(), m_Value()))
>
>         and you really want:
>          if (m_c_And(m_Cmp(Pred, m_Value(), m_Value()),
>         m_Cmp(m_Opposite(Pred), m_Value(), m_Value()))
>
>
>
>
>
>
>
> _______________________________________________
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> llvm-dev at lists.llvm.org
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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On Thu, Jul 13, 2017 at 6:18 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>
> On 07/13/2017 06:40 PM, Daniel Berlin via llvm-dev wrote:
>
> Ah yes, it can only return one instruction and you need two.
>
> NewGVN could still handle it if instsimplify was changed to return the
> right binary operators because it has infrastructure that can handle
> insertion.
> (it would need a little work).
>
>
> (At this point, InstCombine seems to have become worse to me than GCC's
> combining infrastructure, so i'd really like to see if we can stop
putting
> things there. Otherwise, we just go farther down the path of 'let's
just
> make everything one big graph rewrite)
>
>
> Perhaps something of a digression:
>
> My general impression is that if InstCombine were actually doing a
> principled graph rewrite, then we wouldn't have these problems.
>
Define "these problems"?

:)
I mean, you are right it would be better in general.
But the notion of adding more and more graph rewrites to a single part of
the compiler, to get second/third/etc order effects, is endless.  Pretty
much everything can be expressed as a set of graph rewrites.  There is no
principled stopping point.  IE You have to decide what you want each pass
to do, and if you have one pass whose job is "perform all possible graph
rewrites", that pass is in fact, your compiler :)

That's not such a bad thing, mind you, there are entire libraries/languages
centered around this (see, e.g., stratego,
https://en.wikipedia.org/wiki/Stratego/XT and
http://www.metaborg.org/en/latest/source/langdev/meta/lang/stratego/strategoxt/index.html),
but it should be a deliberate thing.

> e problem we have, however, is that we have phase-ordering problems (even
> within the fixed-point iteration scheme): Patterns being matched depend on
> a canonical form, instructions nearly matching those patterns might be
> created, and then suboptimally consumed (i.e. further transformed), before
> being subject to the canonicalizing transformation(s) that would make the
> better pattern actually match.
>
> Except for TableGen'ing the whole thing, and making an actual graph
> automaton to match the patterns, it's not clear to me how to
efficiently
> fix this
>
That is what people used to do, make parsing grammars, etc.
Stratego is also  quite efficient at term rewriting, from what i remember.

. In the mean time, what we've been doing in practice, is that we
have
> InstCombine patterns depend on canonical forms, until we hit some
> phase-ordering problem, and then we extend the pattern in question to match
> some non-canonical forms. We might do this here as well (if that's the
> problem here), but at some point I suspect we'll end up redoing
everything.
>
The question i would have is "how do you know when you are done?". As
I
said, pretty much everything can be expressed as a graph transform, and the
more it gets added here, the more people want to run instcombine 50 times
because cse/gvn/you name it "doesn't do what they want".
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On 07/14/2017 10:27 AM, Daniel Berlin wrote:
>
>
> On Thu, Jul 13, 2017 at 6:18 PM, Hal Finkel <hfinkel at anl.gov 
> <mailto:hfinkel at anl.gov>> wrote:
>
>
>     On 07/13/2017 06:40 PM, Daniel Berlin via llvm-dev wrote:
>>     Ah yes, it can only return one instruction and you need two.
>>
>>     NewGVN could still handle it if instsimplify was changed to
>>     return the right binary operators because it has infrastructure
>>     that can handle insertion.
>>     (it would need a little work).
>>
>>
>>     (At this point, InstCombine seems to have become worse to me than
>>     GCC's combining infrastructure, so i'd really like to see
if we
>>     can stop putting things there. Otherwise, we just go farther down
>>     the path of 'let's  just make everything one big graph
rewrite)
>
>     Perhaps something of a digression:
>
>     My general impression is that if InstCombine were actually doing a
>     principled graph rewrite, then we wouldn't have these problems.
>
>
> Define "these problems"?
>
> :)
> I mean, you are right it would be better in general.
> But the notion of adding more and more graph rewrites to a single part 
> of the compiler, to get second/third/etc order effects, is endless.  
> Pretty much everything can be expressed as a set of graph rewrites.  
> There is no principled stopping point.  IE You have to decide what you 
> want each pass to do, and if you have one pass whose job is "perform 
> all possible graph rewrites", that pass is in fact, your compiler :)
>
> That's not such a bad thing, mind you, there are entire 
> libraries/languages centered around this (see, e.g., stratego, 
> https://en.wikipedia.org/wiki/Stratego/XT and 
>
http://www.metaborg.org/en/latest/source/langdev/meta/lang/stratego/strategoxt/index.html),
> but it should be a deliberate thing.
>
>     e problem we have, however, is that we have phase-ordering
>     problems (even within the fixed-point iteration scheme): Patterns
>     being matched depend on a canonical form, instructions nearly
>     matching those patterns might be created, and then suboptimally
>     consumed (i.e. further transformed), before being subject to the
>     canonicalizing transformation(s) that would make the better
>     pattern actually match.
>
>     Except for TableGen'ing the whole thing, and making an actual
>     graph automaton to match the patterns, it's not clear to me how to
>     efficiently fix this
>
>
> That is what people used to do, make parsing grammars, etc.
> Stratego is also  quite efficient at term rewriting, from what i remember.
AFAIK, that's true.
>
>     . In the mean time, what we've been doing in practice, is that we
>     have InstCombine patterns depend on canonical forms, until we hit
>     some phase-ordering problem, and then we extend the pattern in
>     question to match some non-canonical forms. We might do this here
>     as well (if that's the problem here), but at some point I suspect
>     we'll end up redoing everything.
>
>
> The question i would have is "how do you know when you are
done?". As
> I said, pretty much everything can be expressed as a graph transform, 
> and the more it gets added here, the more people want to run 
> instcombine 50 times because cse/gvn/you name it "doesn't do what
they
> want".
>
I think you're done when you get everything that you have that fits into 
a model of local transformations and that should be run to a fixed 
point. Certainly an interesting question, but I suspect that the 
practical answer is "what's in InstCombine." To do more in this
sense
probably requires some more-general scheme (e.g. equality saturation).

  -Hal

-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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