[PATCH] D25963: [LoopUnroll] Implement profile-based loop peeling

[Michael Kuperstein via llvm-commits]

mkuper added a comment.

Hi Gerolf,

In https://reviews.llvm.org/D25963#579918, @Gerolf wrote:

> Hi,
>
> Could you provide more background on this idea?


I can't take credit for the idea - this is something GCC already does.

> What is your motivational use case? When the trip count is low why optimize?

The motivational use case is a loop with a low trip count that is nested inside a loop with a high trip count.
Peeling the inner loop allows further passes in the optimization pipeline simplify the code for the iterations that actually run, making the outer loop's body better.
Consider something like this:

  for (int i = 0; i < bignum; ++i) {
    int ret = 0;
    for (int j = 0; j < param; ++j)
      ret += arr[i][j];
    out[i] = ret;
  }

Imagine param is usually 1.
We can then peel this into:

  for (int i = 0; i < bignum; ++i) {
    int ret = 0;
    if (param == 0)
      continue;
    ret += arr[i][0]
    for (int j = 1; j < param; ++j)
      ret += arr[i][j];
    out[i] = ret;
  }

Which then becomes something morally equivalent to:

  for (int i = 0; i < bignum; ++i) {
    if (param == 0)
       continue;
    if (param == 1) {
      out[i] = arr[i][0];
      continue;
    }
    ret = arr[i][0];
    for (int j = 1; j < param; ++j)
      ret += arr[i][j];
    out[i] = ret;
  }

So, we've improved the common case (param == 1) - we no longer have to initialize ret, we don't have to deal with the inner loop's IV, there's no add, just a direct assignment.

> If the profile is wrong and it actually is a hot loop for a regular/different input set peeling could hurt.

Sure, but this is true for any profile-guided optimization. PGO is only good with representative inputs. If an optimization was good regardless of input, we'd be doing it for non-PGO builds.

> There are also side effects on code size, register pressure etc. that could hurt performance.

Right. But that's not really different from any other kind of loop unrolling. Hence the thresholds, etc.

> Thanks
> Gerolf

Thanks,
Michael



================
Comment at: lib/Transforms/Utils/LoopUnrollPeel.cpp:87
+    VMap[*BB] = NewBB;
+    if (Header == *BB)
+      InsertTop->getTerminator()->setSuccessor(0, NewBB);
----------------
davidxl wrote:
> Can this be moved outside the loop?
> 
> assert(VMap[Header]);
> InsertTop->getTerminator()->setSuccessor(0, VMap[Header]);
Right, both this and the Latch handling should be moved outside the loop, thanks.


================
Comment at: lib/Transforms/Utils/LoopUnrollPeel.cpp:90
+
+    if (Latch == *BB) {
+      VMap.erase((*BB)->getTerminator());
----------------
davidxl wrote:
> This can be handled outside the loop too.
Right, thanks.


================
Comment at: lib/Transforms/Utils/LoopUnrollPeel.cpp:91
+    if (Latch == *BB) {
+      VMap.erase((*BB)->getTerminator());
+      BranchInst *LatchBR = cast<BranchInst>(NewBB->getTerminator());
----------------
davidxl wrote:
> What this this erase do?
Nothing, nice catch!
(It's stale - it's needed when you replace LatchBR instead of modifying it in-place.)


================
Comment at: lib/Transforms/Utils/LoopUnrollPeel.cpp:95
+      // If this is the last iteration, branch past the loop, otherwise
+      // branch to the next iteration (which may itself either be peeled off,
+      // or the loop's preheader)
----------------
davidxl wrote:
> Is this a stale comment?
No, but I guess it's not clear.

Let's say we're peeling off K iterations.

For iteration J in 1..K-1, we want the branch that terminates the copy of the latch to be:
if (cond) goto header(J+1) else goto exit

For iteration K, we want to set this branch to be:
if (cond) goto new-ph else goto exit.

Here, new-ph is the preheader of the new loop (that is, the loop that handles iterations >= K+1). Technically, this preheader should be empty, and only contains a branch to the loop header - the only reason it exists is to keep the loop in canonical form.
Does this make sense? If it does, I'll try to improve the comment.


================
Comment at: lib/Transforms/Utils/LoopUnrollPeel.cpp:101
+      // We no longer know anything about the branch probability.
+      LatchBR->setMetadata(LLVMContext::MD_prof, nullptr);
+    }
----------------
davidxl wrote:
> Why? I think we should update the branch probability here -- it depends on the what iteration of the peeled clone. If peel count < average/estimated trip count, then each peeled iteration should be more biased towards fall through. If peel_count == est trip_count, then the last peel iteration should be biased toward exit.
You're right, it's not that we don't know anything - but we don't know enough. I'm not sure how to attach a reasonable number to this, without knowing the distribution.
Do you have any suggestions? The trivial option would be to assume an extremely narrow distribution (the loop always exits after exactly K iterations), but that would mean having an extreme bias for all of the branches, and I'm not sure that's wise.


================
Comment at: lib/Transforms/Utils/LoopUnrollPeel.cpp:220
+  }
+
+  // Now adjust the phi nodes in the loop header to get their initial values
----------------
davidxl wrote:
> The profile meta data of the original loop's back branch should be adjusted too.
Right, I missed that, thanks.
But, as above - I'm not sure by how much to adjust it.


https://reviews.llvm.org/D25963