[WebAssembly] Check in an initial CFG Stackifier pass

This pass implements a simple algorithm for conversion from CFG to
wasm's structured control flow. It doesn't yet handle multiple-entry
loops; that will be added in a future patch.

It also adds initial support for switch statements.

Differential Revision: http://reviews.llvm.org/D12735

llvm-svn: 247818

1 files changed, 278 insertions, 0 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
new file mode 100644
index 0000000..2769896
--- /dev/null
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
@@ -0,0 +1,278 @@
+//===-- WebAssemblyCFGStackify.cpp - CFG Stackification -------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// \brief This file implements a CFG stacking pass.
+///
+/// This pass reorders the blocks in a function to put them into a reverse
+/// post-order [0], with special care to keep the order as similar as possible
+/// to the original order, and to keep loops contiguous even in the case of
+/// split backedges.
+///
+/// Then, it inserts BLOCK and LOOP markers to mark the start of scopes, since
+/// scope boundaries serve as the labels for WebAssembly's control transfers.
+///
+/// This is sufficient to convert arbitrary CFGs into a form that works on
+/// WebAssembly, provided that all loops are single-entry.
+///
+/// [0] https://en.wikipedia.org/wiki/Depth-first_search#Vertex_orderings
+///
+//===----------------------------------------------------------------------===//
+
+#include "WebAssembly.h"
+#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
+#include "WebAssemblySubtarget.h"
+#include "llvm/ADT/SCCIterator.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineLoopInfo.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-stackify"
+
+namespace {
+class WebAssemblyCFGStackify final : public MachineFunctionPass {
+  const char *getPassName() const override {
+    return "WebAssembly CFG Stackify";
+  }
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.setPreservesCFG();
+    AU.addRequired<MachineLoopInfo>();
+    AU.addPreserved<MachineLoopInfo>();
+    MachineFunctionPass::getAnalysisUsage(AU);
+  }
+
+  bool runOnMachineFunction(MachineFunction &MF) override;
+
+public:
+  static char ID; // Pass identification, replacement for typeid
+  WebAssemblyCFGStackify() : MachineFunctionPass(ID) {}
+};
+} // end anonymous namespace
+
+char WebAssemblyCFGStackify::ID = 0;
+FunctionPass *llvm::createWebAssemblyCFGStackify() {
+  return new WebAssemblyCFGStackify();
+}
+
+static void EliminateMultipleEntryLoops(MachineFunction &MF,
+                                        const MachineLoopInfo &MLI) {
+  SmallPtrSet<MachineBasicBlock *, 8> InSet;
+  for (scc_iterator<MachineFunction *> I = scc_begin(&MF), E = scc_end(&MF);
+       I != E; ++I) {
+    const std::vector<MachineBasicBlock *> &CurrentSCC = *I;
+
+    // Skip trivial SCCs.
+    if (CurrentSCC.size() == 1)
+      continue;
+
+    InSet.insert(CurrentSCC.begin(), CurrentSCC.end());
+    MachineBasicBlock *Header = nullptr;
+    for (MachineBasicBlock *MBB : CurrentSCC) {
+      for (MachineBasicBlock *Pred : MBB->predecessors()) {
+        if (InSet.count(Pred))
+          continue;
+        if (!Header) {
+          Header = MBB;
+          break;
+        }
+        // TODO: Implement multiple-entry loops.
+        report_fatal_error("multiple-entry loops are not supported yet");
+      }
+    }
+    assert(MLI.isLoopHeader(Header));
+
+    InSet.clear();
+  }
+}
+
+namespace {
+/// Post-order traversal stack entry.
+struct POStackEntry {
+  MachineBasicBlock *MBB;
+  SmallVector<MachineBasicBlock *, 0> Succs;
+
+  POStackEntry(MachineBasicBlock *MBB, MachineFunction &MF,
+               const MachineLoopInfo &MLI);
+};
+} // end anonymous namespace
+
+POStackEntry::POStackEntry(MachineBasicBlock *MBB, MachineFunction &MF,
+                           const MachineLoopInfo &MLI)
+    : MBB(MBB), Succs(MBB->successors()) {
+  // RPO is not a unique form, since at every basic block with multiple
+  // successors, the DFS has to pick which order to visit the successors in.
+  // Sort them strategically (see below).
+  MachineLoop *Loop = MLI.getLoopFor(MBB);
+  MachineFunction::iterator Next = next(MachineFunction::iterator(MBB));
+  MachineBasicBlock *LayoutSucc = Next == MF.end() ? nullptr : &*Next;
+  std::stable_sort(
+      Succs.begin(), Succs.end(),
+      [=, &MLI](const MachineBasicBlock *A, const MachineBasicBlock *B) {
+        if (A == B)
+          return false;
+
+        // Keep loops contiguous by preferring the block that's in the same
+        // loop.
+        MachineLoop *LoopA = MLI.getLoopFor(A);
+        MachineLoop *LoopB = MLI.getLoopFor(B);
+        if (LoopA == Loop && LoopB != Loop)
+          return true;
+        if (LoopA != Loop && LoopB == Loop)
+          return false;
+
+        // Minimize perturbation by preferring the block which is the immediate
+        // layout successor.
+        if (A == LayoutSucc)
+          return true;
+        if (B == LayoutSucc)
+          return false;
+
+        // TODO: More sophisticated orderings may be profitable here.
+
+        return false;
+      });
+}
+
+/// Sort the blocks in RPO, taking special care to make sure that loops are
+/// contiguous even in the case of split backedges.
+static void SortBlocks(MachineFunction &MF, const MachineLoopInfo &MLI) {
+  // Note that we do our own RPO rather than using
+  // "llvm/ADT/PostOrderIterator.h" because we want control over the order that
+  // successors are visited in (see above). Also, we can sort the blocks in the
+  // MachineFunction as we go.
+  SmallPtrSet<MachineBasicBlock *, 16> Visited;
+  SmallVector<POStackEntry, 16> Stack;
+
+  MachineBasicBlock *Entry = MF.begin();
+  Visited.insert(Entry);
+  Stack.push_back(POStackEntry(Entry, MF, MLI));
+
+  for (;;) {
+    POStackEntry &Entry = Stack.back();
+    SmallVectorImpl<MachineBasicBlock *> &Succs = Entry.Succs;
+    if (!Succs.empty()) {
+      MachineBasicBlock *Succ = Succs.pop_back_val();
+      if (Visited.insert(Succ).second)
+        Stack.push_back(POStackEntry(Succ, MF, MLI));
+      continue;
+    }
+
+    // Put the block in its position in the MachineFunction.
+    MachineBasicBlock &MBB = *Entry.MBB;
+    MBB.moveBefore(MF.begin());
+
+    // Branch instructions may utilize a fallthrough, so update them if a
+    // fallthrough has been added or removed.
+    if (!MBB.empty() && MBB.back().isTerminator() && !MBB.back().isBranch() &&
+        !MBB.back().isBarrier())
+      report_fatal_error(
+          "Non-branch terminator with fallthrough cannot yet be rewritten");
+    if (MBB.empty() || !MBB.back().isTerminator() || MBB.back().isBranch())
+      MBB.updateTerminator();
+
+    Stack.pop_back();
+    if (Stack.empty())
+      break;
+  }
+
+  // Now that we've sorted the blocks in RPO, renumber them.
+  MF.RenumberBlocks();
+
+#ifndef NDEBUG
+  for (auto &MBB : MF)
+    if (MachineLoop *Loop = MLI.getLoopFor(&MBB)) {
+      // Assert that loops are contiguous.
+      assert(Loop->getHeader() == Loop->getTopBlock());
+      assert(Loop->getHeader() == &MBB ||
+             MLI.getLoopFor(prev(MachineFunction::iterator(&MBB))) == Loop);
+    } else {
+      // Assert that non-loops have no backedge predecessors.
+      for (auto Pred : MBB.predecessors())
+        assert(Pred->getNumber() < MBB.getNumber() &&
+               "CFG still has multiple-entry loops");
+    }
+#endif
+}
+
+/// Insert BLOCK markers at appropriate places.
+static void PlaceBlockMarkers(MachineBasicBlock &MBB, MachineBasicBlock &Succ,
+                              MachineFunction &MF, const MachineLoopInfo &MLI,
+                              const WebAssemblyInstrInfo &TII) {
+  // Backward branches are loop backedges, and we place the LOOP markers
+  // separately. So only consider forward branches here.
+  if (Succ.getNumber() <= MBB.getNumber())
+    return;
+
+  // Place the BLOCK for a forward branch. For simplicity, we just insert
+  // blocks immediately inside loop boundaries.
+  MachineLoop *Loop = MLI.getLoopFor(&Succ);
+  MachineBasicBlock &Header = *(Loop ? Loop->getHeader() : &MF.front());
+  MachineBasicBlock::iterator InsertPos = Header.begin(), End = Header.end();
+  if (InsertPos != End) {
+    if (InsertPos->getOpcode() == WebAssembly::LOOP)
+      ++InsertPos;
+    int SuccNumber = Succ.getNumber();
+    // Position the BLOCK in nesting order.
+    for (; InsertPos != End && InsertPos->getOpcode() == WebAssembly::BLOCK;
+         ++InsertPos) {
+      int N = InsertPos->getOperand(0).getMBB()->getNumber();
+      if (N < SuccNumber)
+        break;
+      // If there's already a BLOCK for Succ, we don't need another.
+      if (N == SuccNumber)
+        return;
+    }
+  }
+
+  BuildMI(Header, InsertPos, DebugLoc(), TII.get(WebAssembly::BLOCK))
+      .addMBB(&Succ);
+}
+
+/// Insert LOOP and BLOCK markers at appropriate places.
+static void PlaceMarkers(MachineFunction &MF, const MachineLoopInfo &MLI,
+                         const WebAssemblyInstrInfo &TII) {
+  for (auto &MBB : MF) {
+    // Place the LOOP for loops.
+    if (MachineLoop *Loop = MLI.getLoopFor(&MBB))
+      if (Loop->getHeader() == &MBB)
+        BuildMI(MBB, MBB.begin(), DebugLoc(), TII.get(WebAssembly::LOOP))
+            .addMBB(Loop->getBottomBlock());
+
+    // Check for forward branches and switches that need BLOCKS placed.
+    for (auto &Term : MBB.terminators())
+      for (auto &MO : Term.operands())
+        if (MO.isMBB())
+          PlaceBlockMarkers(MBB, *MO.getMBB(), MF, MLI, TII);
+  }
+}
+
+bool WebAssemblyCFGStackify::runOnMachineFunction(MachineFunction &MF) {
+  DEBUG(dbgs() << "********** CFG Stackifying **********\n"
+                  "********** Function: "
+               << MF.getName() << '\n');
+
+  const auto &MLI = getAnalysis<MachineLoopInfo>();
+  const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
+
+  // RPO sorting needs all loops to be single-entry.
+  EliminateMultipleEntryLoops(MF, MLI);
+
+  // Sort the blocks in RPO, with contiguous loops.
+  SortBlocks(MF, MLI);
+
+  // Place the BLOCK and LOOP markers to indicate the beginnings of scopes.
+  PlaceMarkers(MF, MLI, TII);
+
+  return true;
+}