[PartialInl] Enhance partial inliner to handle more complex conditions

[davidxl]

Summary

This handles case :

if (a || b)

return;

// Large_body of slow path code
do_something here.

[eraman]

I've a couple of trivial comments so far. Could you please generate a diff with a larger context?

lib/Transforms/IPO/PartialInlining.cpp

40

The comments doesn't convey what the function is doing.

47

nit: change -> chain

50

Why is this named Entries_ and not Entries?

[davidxl]

Addressed review comments .

[davidxl]

lib/Transforms/IPO/PartialInlining.cpp

40

Fixed.

50

Fixed.

[wmi]

lib/Transforms/IPO/PartialInlining.cpp

40

nit: ghain --> chain

110–119

Here we add a limitation that the function only has one return BB to be a partial inline candidate. Is it required? Can we walk forward from entry instead of backward from the return block so as to avoid the requirement of finding single return BB?

158–166

Here we allows a node on the chain with more than two successors. Will such case bring us problem later during extracting?

test/Transforms/CodeExtractor/MultiConditions.ll

6

we can use opt -instnamer to rename the temporaries.

[davidxl]

lib/Transforms/IPO/PartialInlining.cpp

110–119

the restriction no it is not required. Changed as suggested.

(Note if the extracted region has other return instruction, extra code will be generated (to handle the return status in caller), so the call to the outlined function is slightly higher).

158–166

Changed the code to limit 2 successors. More than one successors can cause problems or lead to very inefficient outline call (to handle 'multiple entries').

[davidxl]

Addressed review comments. Added more test including multi return case.

[wmi]

I am thinking of another two issues:

1. Is it possible that some of the BBs on the chain may be very big and we don't want to partial inline them?
2. The existing pattern handles if (a || b || c ...) case, but it may not be easy to extend for cases like (a && b && c) and ((a && b) || c). Basically, we want to find a collection of bbs with small sizes starting from entry. The bb collection only have two exits. one of them is ReturnBlock and the other is NonReturnBlock. The edges inside of the collection can be of any pattern.

Sorry I didn't come up with them at the first beginning and dive into details.

[davidxl]

Is it possible that some of the BBs on the chain may be very big and we don't want to partial inline them?

Yes, i was thinking adding a limit on the chain length as an option. Probably do this a a follow up -- which also needs to look at profile data. (Note that Partialinlininer is currently not enabled in any optimization pipeline yet).

The existing pattern handles if (a || b || c ...) case, but it may not be easy to extend for cases like (a && b && c) and ((a && b) || c). Basically, we want to find a collection of bbs with small sizes starting from entry. The bb collection only have two exits. one of them is ReturnBlock and the other is NonReturnBlock. The edges inside of the collection can be of any pattern.

a&b&c case is already partially handled -- the partial inliner can partially peel the first iteration which may be good enough. However I do like your suggestion of computing the entry predicate region idea.

[eraman]

Yes, i was thinking adding a limit on the chain length as an option. Probably do this a a follow up -- which also needs to look at profile data. (Note that Partialinlininer is currently not enabled in any optimization pipeline yet).

I think instruction count will be better limit than chain length.

lib/Transforms/IPO/PartialInlining.cpp

193

This will look cleaner if this is made to while (CommonSucc) and peel the else part outside of the while loop (since you always break from there). Perhaps you can even peel the first iteration out of the loop: first find the candidate non-return block, then walk the chain in a loop as long as one successor equals non-return block and the other succesor has a single predecessor. Some more comments will also increase readability.

[davidxl]

Address review feedback. The new version now can handle more general conditions (see test cases).

[davide]

lib/Transforms/IPO/PartialInlining.cpp

43

typo, partially inlined. BTW, do you have numbers for different values of MaxNumInlineBlocks ?

66–68

SmallVector<> maybe

120–131

These could probably live in BasicBlockUtils.

134–139

Also this one. Please remove else after return

162–166

Can you add comments about why we break in this case? (i.e. we don't outline blocks without exactly two successors, etc...)

203–205

braces unneeded.

319

explicit type maybe?

326

explicit type maybe?

335–337

return BB == NewReturnBlock || NewEntries.count(BB) , maybe?

374–376

unrelated change, also braces are unneeded.

401

unrelated?

[davidxl]

lib/Transforms/IPO/PartialInlining.cpp

43

Currently it is set to a high number for stress testing purpose. When partial inliner is enabled in the pipeline, the default value needs to be tuned depending on opt level.

120–131

will leave it here for now.

134–139

removed else.

[davidxl]

addressed review comments.

[wmi]

Some minor comments.

lib/Transforms/IPO/PartialInlining.cpp

68

I find NonReturnBlockPreds has been set but not used.

136

we can change the else if to if.

222–223

should we use an assertion here instead of an early exit, so we can catch mistake instead of giving up silently.

340–343

ToBeInlined(NewNonReturnBlock) is also false, so can we just do:

for (BasicBlock &BB : *DuplicateFunction)
  if (!ToBeInlined(&BB))
    ToExtract.push_back(&BB);

[davidxl]

lib/Transforms/IPO/PartialInlining.cpp

68

Removed

222–223

No. This is designed in way such that the candidate found may be illegal (to simplify the code), so a check here is necessary.

340–343

NewNonReturnBlock needs to be the head of the blocks to be cloned. It is already pushed in, so the filter is needed.

[davidxl]

Address Wei's review feedback. Also fixed a bug found in clang self build test.

[wmi]

LGTM.

[eraman]

lib/Transforms/IPO/PartialInlining.cpp

42

Expressing this as instructions will make this somewhat similar to inlining cost.

147

Cleaner to do return std::make_tuple(Succ1, Succ2) here and the following if

183

No need to initialize it here since GetCommonSucc would return a (nullptr, nullptr) tuple if there is no triangle shape.

[davidxl]

lib/Transforms/IPO/PartialInlining.cpp

42

This will be done in a follow up patch before the final patch that turn it on.

[davidxl]

Address review comments.

[eraman]

lib/Transforms/IPO/PartialInlining.cpp

160

Some comment here will be useful. You want to keep space for 1 return block and the CurrEntry and that's why you've the above. I initially thought the condition was wrong.

237

The HasNonEntryPred lambda checks for the presence of preds in Entries (the DenseSet above), but you're not updating the Entries in this loop when you insert Cand into OutliningInfo->Entries. So this loop will return after 1 iteration right?

[davidxl]

lib/Transforms/IPO/PartialInlining.cpp

160

Fixed. Added a member function.

237

good catch.Fixed.

[davidxl]

Addressed review feedback by eraman.

[eraman]

LGTM.

Please add a comment to explain the condition for exiting the loop for exceeding MaxNumInlineBlocks.

lib/Transforms/IPO/PartialInlining.cpp

160

You should still add a comment in the member function about the +1 (the return block) and here (we still have a pending CurrEntry block).

[gyiu]

llvm/trunk/lib/Transforms/IPO/PartialInlining.cpp

299 ↗

(On Diff #96932)

@davidxl Please correct me if I'm wrong, but in this loop, if the first instruction in the BB is not a PHINode we give up? If that's the case, then wouldn't 'Phi' be the first and only Phi we'll find?

[davidxl]

llvm/trunk/lib/Transforms/IPO/PartialInlining.cpp

299 ↗

(On Diff #96932)

The method returns the first phi of the block if it exists. It should add this:

if (isa<DbgInfoIntrinsic>(I))

continue;

lib/Transforms/IPO/PartialInlining.cpp

   bool run(Module &M);
   Function *unswitchFunction(Function *F);
 
+  // Returns the ReturnBlock and NonReturnBlock
+  std::tuple<BasicBlock *, BasicBlock *> findEntryBlocks(Function *F);
+
 private:
+  // A chain of basic blocks starting from the function entry point.
+  // Except for the function entry block, each block in the chain
+  // has the previous block in the vector as its single predecessor and
+  // all the blocks in the change have a common non return block
+  // as their successor. The last block in the chain also has the return
+  // block as its successor.
+  std::vector<BasicBlock *> Entries_;
   InlineFunctionInfo IFI;
 };
 struct PartialInlinerLegacyPass : public ModulePass {

 };
 }
 
-Function *PartialInlinerImpl::unswitchFunction(Function *F) {
-  // First, verify that this function is an unswitching candidate...
+std::tuple<BasicBlock *, BasicBlock *>
+PartialInlinerImpl::findEntryBlocks(Function *F) {
   BasicBlock *EntryBlock = &F->front();
   BranchInst *BR = dyn_cast<BranchInst>(EntryBlock->getTerminator());
   if (!BR || BR->isUnconditional())
-    return nullptr;
+    return std::make_tuple<BasicBlock *, BasicBlock *>(nullptr, nullptr);
 
   BasicBlock *ReturnBlock = nullptr;
   BasicBlock *NonReturnBlock = nullptr;
   unsigned ReturnCount = 0;
+
   for (BasicBlock *BB : successors(EntryBlock)) {
     if (isa<ReturnInst>(BB->getTerminator())) {
       ReturnBlock = BB;
       ReturnCount++;
-    } else
+    } else {
       NonReturnBlock = BB;
+    }
+  }
+
+  if (ReturnCount == 1) {
+    Entries_.push_back(EntryBlock);
+    return std::make_tuple(ReturnBlock, NonReturnBlock);
+  } else if (ReturnCount != 0) {
+    return std::make_tuple<BasicBlock *, BasicBlock *>(nullptr, nullptr);
+  }
+
+  // Handle more complicated case where the ReturnBlock is guarded by
+  // a chain of predicates:
+  // First find the return block:
+  NonReturnBlock = nullptr;
+
+  for (auto &BB : *F) {
+    TerminatorInst *TI = BB.getTerminator();
+    if (isa<ReturnInst>(TI)) {
+      ReturnBlock = &BB;
+      ReturnCount++;
+    }
   }
 
   if (ReturnCount != 1)
+    return std::make_tuple<BasicBlock *, BasicBlock *>(nullptr, nullptr);
+
+  // Returns true if Succ is BB's successor
+  auto IsSuccessor = [](BasicBlock *Succ, BasicBlock *BB) {
+    return is_contained(successors(BB), Succ);
+  };
+
+  if (succ_end(EntryBlock) - succ_begin(EntryBlock) == 2) {
+    BasicBlock *Succ1 = *succ_begin(EntryBlock);
+    BasicBlock *Succ2 = *(succ_begin(EntryBlock) + 1);
+    if (IsSuccessor(Succ1, Succ2))
+      NonReturnBlock = Succ1;
+    else if (IsSuccessor(Succ2, Succ1))
+      NonReturnBlock = Succ2;
+  }
+
+  if (!NonReturnBlock)
+    return std::make_tuple<BasicBlock *, BasicBlock *>(nullptr, nullptr);
+
+  // Now walk backward from the return block to find the last block in the
+  // entry chain:
+  BasicBlock *LastEntryInChain = nullptr;
+  for (BasicBlock *BB : predecessors(ReturnBlock)) {
+    // Find a predecessor that has NonReturnBlock as its
+    // successor:
+    if (IsSuccessor(NonReturnBlock, BB)) {
+      // found more than one, bail ..
+      if (LastEntryInChain) {
+        LastEntryInChain = nullptr;
+        break;
+      } else {
+        LastEntryInChain = BB;
+      }
+    }
+  }
+
+  if (LastEntryInChain == nullptr)
+    return std::make_tuple<BasicBlock *, BasicBlock *>(nullptr, nullptr);
+
+  // Now walk up the chain till the function entry is reached:
+  BasicBlock *CurrEntryInChain = LastEntryInChain;
+  do {
+    Entries_.push_back(CurrEntryInChain);
+    if (CurrEntryInChain == EntryBlock)
+      break;
+
+    CurrEntryInChain = CurrEntryInChain->getSinglePredecessor();
+  } while (CurrEntryInChain && IsSuccessor(NonReturnBlock, CurrEntryInChain));
+
+  if (CurrEntryInChain != EntryBlock)
+    return std::make_tuple<BasicBlock *, BasicBlock *>(nullptr, nullptr);
+
+  std::reverse(Entries_.begin(), Entries_.end());
+  return std::make_tuple(ReturnBlock, NonReturnBlock);
+}
+
+Function *PartialInlinerImpl::unswitchFunction(Function *F) {
+
+  BasicBlock *ReturnBlock = nullptr;
+  BasicBlock *NonReturnBlock = nullptr;
+
+  std::tie(ReturnBlock, NonReturnBlock) = findEntryBlocks(F);
+
+  if (!ReturnBlock)
     return nullptr;
 
+#ifndef NDEBUG
+  BasicBlock *EntryBlock = Entries_[0];
+  assert(EntryBlock && EntryBlock == &F->front());
+  assert(NonReturnBlock);
+#endif
+
   // Clone the function, so that we can hack away on it.
   ValueToValueMapTy VMap;
   Function *DuplicateFunction = CloneFunction(F, VMap);
   DuplicateFunction->setLinkage(GlobalValue::InternalLinkage);
-  BasicBlock *NewEntryBlock = cast<BasicBlock>(VMap[EntryBlock]);
+  // Last block in the entry chain:
+  BasicBlock *NewLastEntryBlock = cast<BasicBlock>(VMap[&*Entries_.back()]);
   BasicBlock *NewReturnBlock = cast<BasicBlock>(VMap[ReturnBlock]);
   BasicBlock *NewNonReturnBlock = cast<BasicBlock>(VMap[NonReturnBlock]);
+  DenseSet<BasicBlock *> NewEntryChains;
+  for (BasicBlock *BB : Entries_) {
+    NewEntryChains.insert(cast<BasicBlock>(VMap[BB]));
+  }
 
   // Go ahead and update all uses to the duplicate, so that we can just
   // use the inliner functionality when we're done hacking.

     Ins = NewReturnBlock->getFirstNonPHI();
 
     RetPhi->addIncoming(&*I, PreReturn);
-    RetPhi->addIncoming(OldPhi->getIncomingValueForBlock(NewEntryBlock),
-                        NewEntryBlock);
-    OldPhi->removeIncomingValue(NewEntryBlock);
+    RetPhi->addIncoming(OldPhi->getIncomingValueForBlock(NewLastEntryBlock),
+                        NewLastEntryBlock);
+    OldPhi->removeIncomingValue(NewLastEntryBlock);
 
     ++I;
   }
-  NewEntryBlock->getTerminator()->replaceUsesOfWith(PreReturn, NewReturnBlock);
+  NewLastEntryBlock->getTerminator()->replaceUsesOfWith(PreReturn, NewReturnBlock);
+
+  // Returns true if the block is to be partial inlined into the caller
+  // (i.e. not to be extracted to the out of line function)
+  auto ToBeInlined = [=](BasicBlock *BB) {
+    if (BB == NewReturnBlock || NewEntryChains.count(BB))
+      return true;
+    return false;
 
+  };
   // Gather up the blocks that we're going to extract.
   std::vector<BasicBlock *> ToExtract;
   ToExtract.push_back(NewNonReturnBlock);
   for (BasicBlock &BB : *DuplicateFunction)
-    if (&BB != NewEntryBlock && &BB != NewReturnBlock &&
-        &BB != NewNonReturnBlock)
+    if (!ToBeInlined(&BB) && &BB != NewNonReturnBlock)
       ToExtract.push_back(&BB);
 
   // The CodeExtractor needs a dominator tree.

test/Transforms/CodeExtractor/MultiConditions.ll

+; RUN: opt < %s -partial-inliner -S | FileCheck %s
+; RUN: opt < %s -passes=partial-inliner -S | FileCheck %s
+
+define i32 @bar(i32) local_unnamed_addr #0 {
+  %2 = icmp slt i32 %0, 0
+  br i1 %2, label %6, label %3
+
+; <label>:3:                                      ; preds = %1
+  %4 = tail call i32 (...) @channels() #1
+  %5 = icmp slt i32 %4, %0
+  br i1 %5, label %6, label %7
+
+; <label>:6:                                      ; preds = %3, %1
+  tail call void (...) @foo() #1
+  tail call void (...) @foo() #1
+  tail call void (...) @foo() #1
+  tail call void (...) @foo() #1
+  tail call void (...) @foo() #1
+  tail call void (...) @foo() #1
+  tail call void (...) @foo() #1
+  tail call void (...) @foo() #1
+  tail call void (...) @foo() #1
+  br label %7
+
+; <label>:7:                                      ; preds = %3, %6
+  %.0 = phi i32 [ 0, %6 ], [ 1, %3 ]
+  ret i32 %.0
+}
+
+declare i32 @channels(...) local_unnamed_addr
+
+declare void @foo(...) local_unnamed_addr
+
+; Function Attrs: noinline nounwind uwtable
+define i32 @dummy_caller(i32) local_unnamed_addr #0 {
+; CHECK-LABEL: @dummy_caller
+; CHECK: br i1
+; CHECK: call void @bar.1_
+  %2 = tail call i32 @bar(i32 %0)
+  ret i32 %2
+}
+
+attributes #0 = { noinline nounwind uwtable }
+attributes #1 = { nounwind }
+
+!llvm.ident = !{!0}
+
+!0 = !{!"clang version 5.0.0 (trunk 300576)"}