lib/Target/WebAssembly/WebAssemblyCFGSort.cpp - toolchain/llvm - Git at Google

 //===-- WebAssemblyCFGSort.cpp - CFG Sorting ------------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
 /// This file implements a CFG sorting pass.
 ///
 /// This pass reorders the blocks in a function to put them into topological
 /// order, ignoring loop backedges, and without any loop being interrupted
 /// by a block not dominated by the loop header, with special care to keep the
 /// order as similar as possible to the original order.
 ///
 ////===----------------------------------------------------------------------===//

 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
 #include "WebAssembly.h"
 #include "WebAssemblySubtarget.h"
 #include "WebAssemblyUtilities.h"
 #include "llvm/ADT/PriorityQueue.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/CodeGen/MachineDominators.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineLoopInfo.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;

 #define DEBUG_TYPE "wasm-cfg-sort"

 namespace {
 class WebAssemblyCFGSort final : public MachineFunctionPass {
   StringRef getPassName() const override { return "WebAssembly CFG Sort"; }

   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.setPreservesCFG();
     AU.addRequired<MachineDominatorTree>();
     AU.addPreserved<MachineDominatorTree>();
     AU.addRequired<MachineLoopInfo>();
     AU.addPreserved<MachineLoopInfo>();
     MachineFunctionPass::getAnalysisUsage(AU);
   }

   bool runOnMachineFunction(MachineFunction &MF) override;

 public:
   static char ID; // Pass identification, replacement for typeid
   WebAssemblyCFGSort() : MachineFunctionPass(ID) {}
 };
 } // end anonymous namespace

 char WebAssemblyCFGSort::ID = 0;
 INITIALIZE_PASS(WebAssemblyCFGSort, DEBUG_TYPE,
                 "Reorders blocks in topological order", false, false)

 FunctionPass *llvm::createWebAssemblyCFGSort() {
   return new WebAssemblyCFGSort();
 }

 static void MaybeUpdateTerminator(MachineBasicBlock *MBB) {
 #ifndef NDEBUG
   bool AnyBarrier = false;
 #endif
   bool AllAnalyzable = true;
   for (const MachineInstr &Term : MBB->terminators()) {
 #ifndef NDEBUG
     AnyBarrier |= Term.isBarrier();
 #endif
     AllAnalyzable &= Term.isBranch() && !Term.isIndirectBranch();
   }
   assert((AnyBarrier || AllAnalyzable) &&
          "AnalyzeBranch needs to analyze any block with a fallthrough");
   if (AllAnalyzable)
     MBB->updateTerminator();
 }

 namespace {
 /// Sort blocks by their number.
 struct CompareBlockNumbers {
   bool operator()(const MachineBasicBlock *A,
                   const MachineBasicBlock *B) const {
     return A->getNumber() > B->getNumber();
   }
 };
 /// Sort blocks by their number in the opposite order..
 struct CompareBlockNumbersBackwards {
   bool operator()(const MachineBasicBlock *A,
                   const MachineBasicBlock *B) const {
     return A->getNumber() < B->getNumber();
   }
 };
 /// Bookkeeping for a loop to help ensure that we don't mix blocks not dominated
 /// by the loop header among the loop's blocks.
 struct Entry {
   const MachineLoop *Loop;
   unsigned NumBlocksLeft;

   /// List of blocks not dominated by Loop's header that are deferred until
   /// after all of Loop's blocks have been seen.
   std::vector<MachineBasicBlock *> Deferred;

   explicit Entry(const MachineLoop *L)
       : Loop(L), NumBlocksLeft(L->getNumBlocks()) {}
 };
 } // end anonymous namespace

 /// Sort the blocks, taking special care to make sure that loops are not
 /// interrupted by blocks not dominated by their header.
 /// TODO: There are many opportunities for improving the heuristics here.
 /// Explore them.
 static void SortBlocks(MachineFunction &MF, const MachineLoopInfo &MLI,
                        const MachineDominatorTree &MDT) {
   // Prepare for a topological sort: Record the number of predecessors each
   // block has, ignoring loop backedges.
   MF.RenumberBlocks();
   SmallVector<unsigned, 16> NumPredsLeft(MF.getNumBlockIDs(), 0);
   for (MachineBasicBlock &MBB : MF) {
     unsigned N = MBB.pred_size();
     if (MachineLoop *L = MLI.getLoopFor(&MBB))
       if (L->getHeader() == &MBB)
         for (const MachineBasicBlock *Pred : MBB.predecessors())
           if (L->contains(Pred))
             --N;
     NumPredsLeft[MBB.getNumber()] = N;
   }

   // Topological sort the CFG, with additional constraints:
   //  - Between a loop header and the last block in the loop, there can be
   //    no blocks not dominated by the loop header.
   //  - It's desirable to preserve the original block order when possible.
   // We use two ready lists; Preferred and Ready. Preferred has recently
   // processed successors, to help preserve block sequences from the original
   // order. Ready has the remaining ready blocks.
   PriorityQueue<MachineBasicBlock *, std::vector<MachineBasicBlock *>,
                 CompareBlockNumbers>
       Preferred;
   PriorityQueue<MachineBasicBlock *, std::vector<MachineBasicBlock *>,
                 CompareBlockNumbersBackwards>
       Ready;
   SmallVector<Entry, 4> Loops;
   for (MachineBasicBlock *MBB = &MF.front();;) {
     const MachineLoop *L = MLI.getLoopFor(MBB);
     if (L) {
       // If MBB is a loop header, add it to the active loop list. We can't put
       // any blocks that it doesn't dominate until we see the end of the loop.
       if (L->getHeader() == MBB)
         Loops.push_back(Entry(L));
       // For each active loop the block is in, decrement the count. If MBB is
       // the last block in an active loop, take it off the list and pick up any
       // blocks deferred because the header didn't dominate them.
       for (Entry &E : Loops)
         if (E.Loop->contains(MBB) && --E.NumBlocksLeft == 0)
           for (auto DeferredBlock : E.Deferred)
             Ready.push(DeferredBlock);
       while (!Loops.empty() && Loops.back().NumBlocksLeft == 0)
         Loops.pop_back();
     }
     // The main topological sort logic.
     for (MachineBasicBlock *Succ : MBB->successors()) {
       // Ignore backedges.
       if (MachineLoop *SuccL = MLI.getLoopFor(Succ))
         if (SuccL->getHeader() == Succ && SuccL->contains(MBB))
           continue;
       // Decrement the predecessor count. If it's now zero, it's ready.
       if (--NumPredsLeft[Succ->getNumber()] == 0)
         Preferred.push(Succ);
     }
     // Determine the block to follow MBB. First try to find a preferred block,
     // to preserve the original block order when possible.
     MachineBasicBlock *Next = nullptr;
     while (!Preferred.empty()) {
       Next = Preferred.top();
       Preferred.pop();
       // If X isn't dominated by the top active loop header, defer it until that
       // loop is done.
       if (!Loops.empty() &&
           !MDT.dominates(Loops.back().Loop->getHeader(), Next)) {
         Loops.back().Deferred.push_back(Next);
         Next = nullptr;
         continue;
       }
       // If Next was originally ordered before MBB, and it isn't because it was
       // loop-rotated above the header, it's not preferred.
       if (Next->getNumber() < MBB->getNumber() &&
           (!L || !L->contains(Next) ||
            L->getHeader()->getNumber() < Next->getNumber())) {
         Ready.push(Next);
         Next = nullptr;
         continue;
       }
       break;
     }
     // If we didn't find a suitable block in the Preferred list, check the
     // general Ready list.
     if (!Next) {
       // If there are no more blocks to process, we're done.
       if (Ready.empty()) {
         MaybeUpdateTerminator(MBB);
         break;
       }
       for (;;) {
         Next = Ready.top();
         Ready.pop();
         // If Next isn't dominated by the top active loop header, defer it until
         // that loop is done.
         if (!Loops.empty() &&
             !MDT.dominates(Loops.back().Loop->getHeader(), Next)) {
           Loops.back().Deferred.push_back(Next);
           continue;
         }
         break;
       }
     }
     // Move the next block into place and iterate.
     Next->moveAfter(MBB);
     MaybeUpdateTerminator(MBB);
     MBB = Next;
   }
   assert(Loops.empty() && "Active loop list not finished");
   MF.RenumberBlocks();

 #ifndef NDEBUG
   SmallSetVector<MachineLoop *, 8> OnStack;

   // Insert a sentinel representing the degenerate loop that starts at the
   // function entry block and includes the entire function as a "loop" that
   // executes once.
   OnStack.insert(nullptr);

   for (auto &MBB : MF) {
     assert(MBB.getNumber() >= 0 && "Renumbered blocks should be non-negative.");

     MachineLoop *Loop = MLI.getLoopFor(&MBB);
     if (Loop && &MBB == Loop->getHeader()) {
       // Loop header. The loop predecessor should be sorted above, and the other
       // predecessors should be backedges below.
       for (auto Pred : MBB.predecessors())
         assert(
             (Pred->getNumber() < MBB.getNumber() || Loop->contains(Pred)) &&
             "Loop header predecessors must be loop predecessors or backedges");
       assert(OnStack.insert(Loop) && "Loops should be declared at most once.");
     } else {
       // Not a loop header. All predecessors should be sorted above.
       for (auto Pred : MBB.predecessors())
         assert(Pred->getNumber() < MBB.getNumber() &&
                "Non-loop-header predecessors should be topologically sorted");
       assert(OnStack.count(MLI.getLoopFor(&MBB)) &&
              "Blocks must be nested in their loops");
     }
     while (OnStack.size() > 1 && &MBB == LoopBottom(OnStack.back()))
       OnStack.pop_back();
   }
   assert(OnStack.pop_back_val() == nullptr &&
          "The function entry block shouldn't actually be a loop header");
   assert(OnStack.empty() &&
          "Control flow stack pushes and pops should be balanced.");
 #endif
 }

 bool WebAssemblyCFGSort::runOnMachineFunction(MachineFunction &MF) {
   DEBUG(dbgs() << "********** CFG Sorting **********\n"
                   "********** Function: "
                << MF.getName() << '\n');

   const auto &MLI = getAnalysis<MachineLoopInfo>();
   auto &MDT = getAnalysis<MachineDominatorTree>();
   // Liveness is not tracked for VALUE_STACK physreg.
   MF.getRegInfo().invalidateLiveness();

   // Sort the blocks, with contiguous loops.
   SortBlocks(MF, MLI, MDT);

   return true;
 }
	//===-- WebAssemblyCFGSort.cpp - CFG Sorting ------------------------------===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	///
	/// \file
	/// This file implements a CFG sorting pass.
	///
	/// This pass reorders the blocks in a function to put them into topological
	/// order, ignoring loop backedges, and without any loop being interrupted
	/// by a block not dominated by the loop header, with special care to keep the
	/// order as similar as possible to the original order.
	///
	////===----------------------------------------------------------------------===//

	#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
	#include "WebAssembly.h"
	#include "WebAssemblySubtarget.h"
	#include "WebAssemblyUtilities.h"
	#include "llvm/ADT/PriorityQueue.h"
	#include "llvm/ADT/SetVector.h"
	#include "llvm/CodeGen/MachineDominators.h"
	#include "llvm/CodeGen/MachineFunction.h"
	#include "llvm/CodeGen/MachineLoopInfo.h"
	#include "llvm/CodeGen/MachineRegisterInfo.h"
	#include "llvm/CodeGen/Passes.h"
	#include "llvm/Support/Debug.h"
	#include "llvm/Support/raw_ostream.h"
	using namespace llvm;

	#define DEBUG_TYPE "wasm-cfg-sort"

	namespace {
	class WebAssemblyCFGSort final : public MachineFunctionPass {
	StringRef getPassName() const override { return "WebAssembly CFG Sort"; }

	void getAnalysisUsage(AnalysisUsage &AU) const override {
	AU.setPreservesCFG();
	AU.addRequired<MachineDominatorTree>();
	AU.addPreserved<MachineDominatorTree>();
	AU.addRequired<MachineLoopInfo>();
	AU.addPreserved<MachineLoopInfo>();
	MachineFunctionPass::getAnalysisUsage(AU);
	}

	bool runOnMachineFunction(MachineFunction &MF) override;

	public:
	static char ID; // Pass identification, replacement for typeid
	WebAssemblyCFGSort() : MachineFunctionPass(ID) {}
	};
	} // end anonymous namespace

	char WebAssemblyCFGSort::ID = 0;
	INITIALIZE_PASS(WebAssemblyCFGSort, DEBUG_TYPE,
	"Reorders blocks in topological order", false, false)

	FunctionPass *llvm::createWebAssemblyCFGSort() {
	return new WebAssemblyCFGSort();
	}

	static void MaybeUpdateTerminator(MachineBasicBlock *MBB) {
	#ifndef NDEBUG
	bool AnyBarrier = false;
	#endif
	bool AllAnalyzable = true;
	for (const MachineInstr &Term : MBB->terminators()) {
	#ifndef NDEBUG
	AnyBarrier \|= Term.isBarrier();
	#endif
	AllAnalyzable &= Term.isBranch() && !Term.isIndirectBranch();
	}
	assert((AnyBarrier \|\| AllAnalyzable) &&
	"AnalyzeBranch needs to analyze any block with a fallthrough");
	if (AllAnalyzable)
	MBB->updateTerminator();
	}

	namespace {
	/// Sort blocks by their number.
	struct CompareBlockNumbers {
	bool operator()(const MachineBasicBlock *A,
	const MachineBasicBlock *B) const {
	return A->getNumber() > B->getNumber();
	}
	};
	/// Sort blocks by their number in the opposite order..
	struct CompareBlockNumbersBackwards {
	bool operator()(const MachineBasicBlock *A,
	const MachineBasicBlock *B) const {
	return A->getNumber() < B->getNumber();
	}
	};
	/// Bookkeeping for a loop to help ensure that we don't mix blocks not dominated
	/// by the loop header among the loop's blocks.
	struct Entry {
	const MachineLoop *Loop;
	unsigned NumBlocksLeft;

	/// List of blocks not dominated by Loop's header that are deferred until
	/// after all of Loop's blocks have been seen.
	std::vector<MachineBasicBlock *> Deferred;

	explicit Entry(const MachineLoop *L)
	: Loop(L), NumBlocksLeft(L->getNumBlocks()) {}
	};
	} // end anonymous namespace

	/// Sort the blocks, taking special care to make sure that loops are not
	/// interrupted by blocks not dominated by their header.
	/// TODO: There are many opportunities for improving the heuristics here.
	/// Explore them.
	static void SortBlocks(MachineFunction &MF, const MachineLoopInfo &MLI,
	const MachineDominatorTree &MDT) {
	// Prepare for a topological sort: Record the number of predecessors each
	// block has, ignoring loop backedges.
	MF.RenumberBlocks();
	SmallVector<unsigned, 16> NumPredsLeft(MF.getNumBlockIDs(), 0);
	for (MachineBasicBlock &MBB : MF) {
	unsigned N = MBB.pred_size();
	if (MachineLoop *L = MLI.getLoopFor(&MBB))
	if (L->getHeader() == &MBB)
	for (const MachineBasicBlock *Pred : MBB.predecessors())
	if (L->contains(Pred))
	--N;
	NumPredsLeft[MBB.getNumber()] = N;
	}

	// Topological sort the CFG, with additional constraints:
	// - Between a loop header and the last block in the loop, there can be
	// no blocks not dominated by the loop header.
	// - It's desirable to preserve the original block order when possible.
	// We use two ready lists; Preferred and Ready. Preferred has recently
	// processed successors, to help preserve block sequences from the original
	// order. Ready has the remaining ready blocks.
	PriorityQueue<MachineBasicBlock , std::vector<MachineBasicBlock >,
	CompareBlockNumbers>
	Preferred;
	PriorityQueue<MachineBasicBlock , std::vector<MachineBasicBlock >,
	CompareBlockNumbersBackwards>
	Ready;
	SmallVector<Entry, 4> Loops;
	for (MachineBasicBlock *MBB = &MF.front();;) {
	const MachineLoop *L = MLI.getLoopFor(MBB);
	if (L) {
	// If MBB is a loop header, add it to the active loop list. We can't put
	// any blocks that it doesn't dominate until we see the end of the loop.
	if (L->getHeader() == MBB)
	Loops.push_back(Entry(L));
	// For each active loop the block is in, decrement the count. If MBB is
	// the last block in an active loop, take it off the list and pick up any
	// blocks deferred because the header didn't dominate them.
	for (Entry &E : Loops)
	if (E.Loop->contains(MBB) && --E.NumBlocksLeft == 0)
	for (auto DeferredBlock : E.Deferred)
	Ready.push(DeferredBlock);
	while (!Loops.empty() && Loops.back().NumBlocksLeft == 0)
	Loops.pop_back();
	}
	// The main topological sort logic.
	for (MachineBasicBlock *Succ : MBB->successors()) {
	// Ignore backedges.
	if (MachineLoop *SuccL = MLI.getLoopFor(Succ))
	if (SuccL->getHeader() == Succ && SuccL->contains(MBB))
	continue;
	// Decrement the predecessor count. If it's now zero, it's ready.
	if (--NumPredsLeft[Succ->getNumber()] == 0)
	Preferred.push(Succ);
	}
	// Determine the block to follow MBB. First try to find a preferred block,
	// to preserve the original block order when possible.
	MachineBasicBlock *Next = nullptr;
	while (!Preferred.empty()) {
	Next = Preferred.top();
	Preferred.pop();
	// If X isn't dominated by the top active loop header, defer it until that
	// loop is done.
	if (!Loops.empty() &&
	!MDT.dominates(Loops.back().Loop->getHeader(), Next)) {
	Loops.back().Deferred.push_back(Next);
	Next = nullptr;
	continue;
	}
	// If Next was originally ordered before MBB, and it isn't because it was
	// loop-rotated above the header, it's not preferred.
	if (Next->getNumber() < MBB->getNumber() &&
	(!L \|\| !L->contains(Next) \|\|
	L->getHeader()->getNumber() < Next->getNumber())) {
	Ready.push(Next);
	Next = nullptr;
	continue;
	}
	break;
	}
	// If we didn't find a suitable block in the Preferred list, check the
	// general Ready list.
	if (!Next) {
	// If there are no more blocks to process, we're done.
	if (Ready.empty()) {
	MaybeUpdateTerminator(MBB);
	break;
	}
	for (;;) {
	Next = Ready.top();
	Ready.pop();
	// If Next isn't dominated by the top active loop header, defer it until
	// that loop is done.
	if (!Loops.empty() &&
	!MDT.dominates(Loops.back().Loop->getHeader(), Next)) {
	Loops.back().Deferred.push_back(Next);
	continue;
	}
	break;
	}
	}
	// Move the next block into place and iterate.
	Next->moveAfter(MBB);
	MaybeUpdateTerminator(MBB);
	MBB = Next;
	}
	assert(Loops.empty() && "Active loop list not finished");
	MF.RenumberBlocks();

	#ifndef NDEBUG
	SmallSetVector<MachineLoop *, 8> OnStack;

	// Insert a sentinel representing the degenerate loop that starts at the
	// function entry block and includes the entire function as a "loop" that
	// executes once.
	OnStack.insert(nullptr);

	for (auto &MBB : MF) {
	assert(MBB.getNumber() >= 0 && "Renumbered blocks should be non-negative.");

	MachineLoop *Loop = MLI.getLoopFor(&MBB);
	if (Loop && &MBB == Loop->getHeader()) {
	// Loop header. The loop predecessor should be sorted above, and the other
	// predecessors should be backedges below.
	for (auto Pred : MBB.predecessors())
	assert(
	(Pred->getNumber() < MBB.getNumber() \|\| Loop->contains(Pred)) &&
	"Loop header predecessors must be loop predecessors or backedges");
	assert(OnStack.insert(Loop) && "Loops should be declared at most once.");
	} else {
	// Not a loop header. All predecessors should be sorted above.
	for (auto Pred : MBB.predecessors())
	assert(Pred->getNumber() < MBB.getNumber() &&
	"Non-loop-header predecessors should be topologically sorted");
	assert(OnStack.count(MLI.getLoopFor(&MBB)) &&
	"Blocks must be nested in their loops");
	}
	while (OnStack.size() > 1 && &MBB == LoopBottom(OnStack.back()))
	OnStack.pop_back();
	}
	assert(OnStack.pop_back_val() == nullptr &&
	"The function entry block shouldn't actually be a loop header");
	assert(OnStack.empty() &&
	"Control flow stack pushes and pops should be balanced.");
	#endif
	}

	bool WebAssemblyCFGSort::runOnMachineFunction(MachineFunction &MF) {
	DEBUG(dbgs() << "******** CFG Sorting ********\n"
	"********** Function: "
	<< MF.getName() << '\n');

	const auto &MLI = getAnalysis<MachineLoopInfo>();
	auto &MDT = getAnalysis<MachineDominatorTree>();
	// Liveness is not tracked for VALUE_STACK physreg.
	MF.getRegInfo().invalidateLiveness();

	// Sort the blocks, with contiguous loops.
	SortBlocks(MF, MLI, MDT);

	return true;
	}