top-down vs. bottom-up list scheduling

Hello List!

I am looking at top-down vs. bottom-up list scheduling for simple(r) in-order
cores. First, for some context, below is a fairly representative pseudo-code
example of the sort of DSP-like codes I am looking at:

  uint64_t foo(int *pA, int *pB, unsigned N, unsigned C) {
    uint64_t sum = 0;
    while (N-- > 0) {
      A1 = *pA++;
      A2 = *pA++;
      B1 = *pB++;
      B2 = *pB++;
      sum += ((uint64_t) A1 * B1) >> C;
      sum += ((uint64_t) A2 * B2) >> C;
    return sum;

These kernels are very tight loops. In this sort of legacy codes it's not
uncommon that loops are manually tuned and unrolled with the available
registers in mind, so that all values just about fit in registers. In the
example above, we read from input streams A and B, and do 2 multiply-accumulate
operations. The loop is unrolled 2 times in this example, but 4, 8 or 16 would
more realistic, resulting in very high register pressure.

But legacy apps written in this way are not the only culprit; we see that with
(aggressive) loop unrolling we can end up in exactly the same situation: the
loopunroller does exactly what it needs to do, but it results in very high
register pressure.

Here's the (obvious) problem: all live values in these loops should just about
fit in registers, but suboptimal/wrong decisions are made resulting in a lot of
spills/reloads; the machine scheduler is making the life of the register allocator
very difficult. I am looking at regressions of about 30 - 40% for more
than a handful of kernels, and am thus very interested in what I could do about

The first observation is that it looks like we default to bottom-up scheduling.
Starting bottom-up, all these "sum" variables are scheduled first, and after
that the loads (this is simplifying things a bit). And thus it looks like we
create a lot of very long live-ranges, causing the problems mentioned above.
When instead we start top-down, the loads are picked up first, then the MAC, and
this repeated for all MACs. The result is a sequence LD, MAC, LD, MAC, etc.,
which is let's say a sequence more in program-order, also with shorter
live-ranges. This does exactly what I want for these sort of kernels, it generates
exactly the code we want.

While playing with this bottom-up/top-down order, it didn't take long to see
that the top-down approach is excellent for these kind of codes, but not for
some other benchmarks, and so solving this issue not just a matter of
defaulting to a new scheduling policy.

I am thinking about prototyping an approach where we start with the bottom-up
approach (under a new misched option), but when a register-pressure/live-range
trashhold value is reached, we bail and fall back to the top-down approach.
This is my first rough idea, but I haven't looked at this problem for very
long. My first few data points is suggesting this might be benificial, but I
could be missing a lot here. And also I am sure that I am looking at an
old/classic problem here and I'm sure others have looked at this problem
before. Thus I am wondering if people have experiences/opinions on this.

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Do you know why the existing register pressure heuristics don't work for your testcase with the bottom-up scheduler?


Some comments:
- It's all heuristics, so you may be lucky/unlycky in any configuration. I assume you know how to read -debug-only=machine-scheduler output and the GenericScheduler::tryCandidate() code to figure out how most decisions are made.
- In my experience bottom-up works better when optimizing for register pressure while top-down tends to work better when optimizing for latencies/stall reduction.
- The scheduler has a couple rules dealing with pressure. Hitting any of these limits will make it pick pressure reducing nodes more aggressively (look for tryPressure() in the tryCandidate() code)
    - Hitting the target limits (number of available registers)
    - Hitting the maximum of the region before scheduling
    - Generally attempt to minimmize pressure (this has lower priority though)

In the past I looked at some crypto code that also was carefully tuned by a human and that turned out to be very tricky for the heuristics to get right because most values had multiple users.
At the time I had a patch that would revert all scheduling decisions if it turned out that the scheduler couldn't find a solution with better or equal register pressure. It wasn't received too enthusiastically and I ended up tweaking some heuristics instead. But we may start that discussion again if you cannot find a know to tune the heuristic for your case.

- Matthias

Hi Matthias and Eli,

Thanks for your replies. Yes, I am first going to study this more, to really understand all the different bits and pieces involved here. I indeed need to see if we not accidentally miss something here, or if we are just unlucky with some heuristics. Your pointers are really useful for that! I will write again when I have more concrete ideas/plans.



Bottom-up scheduling would work for you in this case if the heuristics knew to minimize pressure at every point in the loop. Ideally, that could be detected from the shape of the DAG before list scheduling begins.

The closest thing we have to doing that in the current source is the “subtree” scheduling. See `computeDFSResult`.

…or maybe you find a simpler way to control the heurstics!

I’m not entirely sure why the top-down heuristics are working better for you in this case. What you’re proposing is similar in spirit to LLVM's bidirectional scheduling. The problem is that a list scheduler can dig itself into a hole in either direction so bidirectional doesn’t really save you unless you throw away the previous schedule and start over. LLVM’s normal no-backtracking approach is really designed to avoid compile-time issues.