[llvm-dev] Optionally using value numbering in Simplify*

[Daniel Berlin via llvm-dev]

On Fri, Mar 3, 2017 at 12:39 PM, Friedman, Eli <efriedma at codeaurora.org>
wrote:

On 3/3/2017 11:51 AM, Daniel Berlin via llvm-dev wrote:

So i have a testcase (see PR31792, and cond_br2.llin GVN) that current GVN
can simplify because it replaces instructions as it goes.  It's an example
of a larger issue that pops up quite a lot
I would appreciate thoughts on what to do about it
it amounts to something like this (but again, it happens a lot):

live = gep thing, 0
live2 = gep thing, 1
branch i1 provablytrue,, mergeblock, otherbb
otherbb:
dead = something else
br mergeblock
merge block
a = phi(live, dead)
b = live2
result = icmp sge a, b

both GVN and NewGVN prove provablytrue to be true, and phi to be equivalent
to live.

GVN transforms this piece at time, and so by the time simplifycmpinst sees
the icmp, it is

result = icmp sge <live2, live>

It proves result true.

NewGVN is an analysis, and so it doesn't transform the ir, and
simplifycmpinst (rightfully) does not try to walk everything, everywhere,
to prove something. It also couldn't know that dead is dead. So it doesn't
see that result is true.


Why aren't we calling SimplifyCmpInst(pred, live, live2, ...)?


We do.
The example is a bit contrived, the real example has a phi in the way of
computing the gep offset, and SimplifyCmpInst does walking and matching, so
this won't work anyway.

See computePointerICmp:

  Constant *LHSOffset = stripAndComputeConstantOffsets(DL, LHS);
  Constant *RHSOffset = stripAndComputeConstantOffsets(DL, RHS);

This in turn walks and collects the offsets.  One of those is a phi we know
to be equivalent to a constant ...


Or are you expecting SimplifyCmpInst(pred, live, dead, ...) to call back
into GVN to find values equivalent to "dead"?


The top level call we already get right.
But all of these simplifiers do not just do top level things. Some go
looking, so we need them to call back in in some cases.




The upshot is that it takes two passes of newgvn to get the same result as
gvn.

I'm trying to decide what to about this case. As i said, it happens a lot.

It would be pretty trivial to make a "VNImpl" interface that has a few
functions (that can be fulfilled by any value numbering that is an
analysis), have newgvn implement it, and use it in Simplify*.

(It would take work to make GVN work here because of how many places it
modifies the ir during value numbering. It also modifies as it goes, so the
only advantage would be from unreachable blocks it discovers)

But before i add another argument to functions taking a ton already[1], i
wanted to ask whether anyone had any opinion on whether it's worth doing.

VNImpl would probably look something like:
class VNImpl{
// return true if A and B are equivalent
bool areEquivalent(Value *A, Value *B);
// find a value that dominates A that is equivalent to it
Value *findDominatingEquivalent(Value *A);
// obviousn
bool isBlockReachable(BasicBock *BB);
}


I'm not sure how you expect InstructionSimplify to use
findDominatingEquivalent.


Most places it uses strict equality and doesn't care. In all of those cases
newgvn has a single unique (but not always dominating) leader it could give
and we could call it findEquivalent

But simplify* does expect the end result to dominate the original
instruction, and this is guaranteed by the docs :P.
We could give up on these, or we could actually just use it at the end.
Any instruction that it returns we could just find the equivalent that
dominates the original operand, or return null.
Besides that, there is one place (valueDominatesPhi) that actually checks
dominance where we would change.  Not doing so is just a missed opt.


Does it have to call findDominatingEquivalent every time it tries to
match() on a Value (or otherwise examine it)?

It depends on how many places go walking.
As I said, we get the top level calls right, and that's enough for a *lot*
of cases.
Just not a few that want to do some sort of collection or matching of
operands of operands.  We could make a vmatch that understands value
equivalency if we need to.

That seems extremely invasive in the sense that there would be a lot of
places to change, and no good way to make sure we actually caught all the
places which need to be changed.

 First, all are just missed optimization if you miss them, not correctness.


Second actually, for newgvn there is.  we can assert that for anything it
uses, lookupOperandLeader(V) == V.
We add these at the beginning of each function for the rhs and lhs
arguments (and others as appropriate), and along with vmatch, should catch
just about every case if not every one.
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