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

via llvm-dev
Tue, 15 Mar 2016 22:07:29 -0000

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.

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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