[LLVMdev] alias set collapse and LICM

[Philip Reames]

On 06/12/2015 02:10 PM, Andrew Trick wrote:
>> On Jun 12, 2015, at 2:06 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
>>
>> On Fri, Jun 12, 2015 at 2:03 PM, Andrew Trick <atrick at apple.com> wrote:
>>> On Jun 12, 2015, at 1:51 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
>>>
>>> So, you can't have disjoint sets, and have one of set that says "i
>>> access everything". Because it would contain everything :)
>>>
>>> Thus, I assume by your description you meant "we want to maintain
>>> multiple disjoint partitions of the same set of accesses" (which is
>>> not quite the same as just having one special partition).
>>>
>>> When one partition is a refinement of the other (as should be the case
>>> here), this is known as the nested set union problem, and in fact,
>>> some very recent research was done into it:
>>>
>>> http://www.cs.princeton.edu/~dhlarkin/papers/esa14-nested-set-union.pdf
>>>
>>>
>>> I’m suggesting that the tracker have a special set that is not included in
>>> the partition and assumed to alias with everything in the partition. So we
>>> would have a partition over the subset of memory accesses that are actually
>>> interesting. All the sets in that partition are disjoint. The set outside of
>>> the partition is not. Every time you query for aliasing, you need to check
>>> if one of the accesses is in the special set.
>> Ah, so this seems identical to what Sanjoy suggested initially :)
> I think he suggested maintaining two partitions, one for Refs and one for Mods. ModRef accesses would be in both partitions. That might also be helpful, but might not be necessary if all the readonly calls are simply removed from the partition.
I just want to point out that the readonly case is illustrative of a 
broader issue.  The alias set tracker mechanism is currently inherently 
transitive.  That is, a load can be considered clobbered even if doesn't 
alias a store; if it aliases another load which aliases the store, it's 
considered clobbered.  The readonly call is the case we've hit and the 
one that really matters to us, but it's worth keeping the broader 
problem in mind.

I came across this issue again this week and ended up implementing a 
much less principled fix in our local tree.  For sufficiently small 
loops, we now resort to pair wise modref checks if AST reports a 
possible clobber.  In several test cases I looked at, the precise 
aliasing was sufficient to enable LICM sufficient for us to recognize 
vectorizable loops which were otherwise getting missed. As you can 
imagine, that's a major performance win on those benchmarks.

For upstream, I now know of four possible implementation strategies:

Option 1 - Resort to pairwise modref queries for sufficiently small 
loops.  This has the advantage of being already implemented and easy to 
upstream, but would introduce a capped N^2 algorithm and thus a 
performance cliff.  It might be worth upstreaming this with the 
threshold defaulted to 0 (i.e. nop) just as a testing mechanism for 
identifying places AST's imprecision is hurting us.

Option 2 - As a variant of option 1, we can keep track of potentially 
clobbering instructions seen during a single walk of the loop.  With 
this, the above N^2 algorithm becomes #L*#S, and we can easily bound the 
maximum number of clobbering instructions we track.  I think this is 
actually a reasonable compromise and is worth considering.  This gives 
us fully precise aliasing for most loops within LICM at reasonable cost.

Option 3 - We can track readonly calls separately within AST.  This 
wouldn't change the inherent purpose of AST (i.e. it would still be 
about transitive aliasing), but would enable more precise results in the 
specific case we care about.  This only helps the readonly call case and 
does not help with general aliasing collapse issue.  It also requires 
every consumer of the AST interface to be updated to explicit check the 
"alias all" set.  (Note: This could help other "unknown instruction" 
cases as well as readonly calls.  It just doesn't solve the imprecise 
aliasing transitivity for known instructions.)

Option 4 - We can split AST into two sets of alias sets.  One set of set 
is based on aliasing ref behaviour, the other on mod behaviour. The 
rules for adding to the first set of sets are the existing AST rules.  A 
load would only be added to an mod based set if a store within the set 
aliased it directly.  We would still merge all of sets that a load 
belong to.  The difference is that a load would not be considered to 
belong to a set unless a store within that set aliases it, not just 
another load.  With LICM, we'd use the mod-set for hoisting of loads, 
and the ref-set for sinking of stores.  This is a fairly involved change.

My mild preference would be for option 2, followed by 4, then 1, then 
3.  What do others think?  I'm willing to implement any of 1, 2, and 
likely 3.  Option 4 is likely more work than I can commit to given that 
the symptom biting me is now fixed locally.

Philip