[llvm-dev] RFC: DenseMap grow() slowness

[Hal Finkel via llvm-dev]

----- Original Message -----

> From: "via llvm-dev" <llvm-dev at lists.llvm.org>
> To: "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Tuesday, March 15, 2016 5:07:29 PM
> Subject: [llvm-dev] RFC: DenseMap grow() slowness

> There’s a few passes in LLVM that make heavy use of a big DenseMap,
> one that potentially gets filled with up to 1 entry for each
> instruction in the function. EarlyCSE is the best example, but
> Reassociate and MachineCSE have this to some degree as well (there
> might be others?). To put it simply: at least in my profile,
> EarlyCSE spends ~1/5 of its time growing DenseMaps. This is kind of…
> bad.

> grow() is inherently slow because it needs to rehash and reinsert
> everything. This means growing a DenseMap costs much, much more than
> growing, for example, a vector. I talked about this with a few
> people and here are some possibilities we’ve come up with to improve
> this (some of which probably aren’t what we want):

> 1. Use a map that doesn’t require rehashing and reinsertion to grow.
> Chaining lets you do this, but std::unordered_map is probably so
> much slower than DenseMap we’d lose more than we gain.
> 2. Include the hash code in the map so that we don’t have to rehash.
> 32 bits more per entry (or whatever), and it might not help that
> much, since we still have to do the whole reinsertion routine.
> 3. Pre-calculate an estimate as to the map size we need. For example,
> in EarlyCSE, this is possibly gross overestimate of size needed:

> unsigned InstCount = 0 ;
> unsigned LoadCount = 0 ;
> unsigned CallCount = 0 ;
> for ( inst_iterator FI = inst_begin ( F ), FE = inst_end ( F ); FI !=
> FE; ++FI) {
> if (FI-> mayReadOrWriteMemory ())
> ++LoadCount;
> else if ( isa < CallInst >(*FI))
> ++CallCount;
> else
> ++InstCount;
> }
> AvailableValues . resize (InstCount);
> AvailableLoads . resize (LoadCount);
> AvailableCalls . resize (CallCount);

> But it does the job, and saves ~20% of time in EarlyCSE on my
> profiles. Yes, iterating over the entire function is way cheaper
> than grow(). Downsides are that while it’s still bounded by function
> size, it could end up allocating a good bit more depending on — in
> EarlyCSE’s case — the control flow/dominator structure.
This last option makes perfect sense to me. One thing we might be able to do regarding the extra memory overhead is, instead of actually resizing up front, to start with a relatively small map, but use the function size to set the growth factor so that we grow only a small number of times (say once) in the worst case. 

-Hal 

> Any thoughts on this, or other less ugly alternatives? I estimate
> that, at least in our pass pipeline, we’re losing at least ~1% of
> total time to avoidable DenseMap::grow() operations, which feels a
> little bit… unnecessary.

> —escha
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-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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