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//===-- 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]
#include "WebAssembly.h"
#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
#include "WebAssemblySubtarget.h"
#include "llvm/ADT/SCCIterator.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstrBuilder.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-stackify"
namespace {
class WebAssemblyCFGStackify final : public MachineFunctionPass {
const char *getPassName() const override {
return "WebAssembly CFG Stackify";
void getAnalysisUsage(AnalysisUsage &AU) const override {
bool runOnMachineFunction(MachineFunction &MF) override;
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)
InSet.insert(CurrentSCC.begin(), CurrentSCC.end());
MachineBasicBlock *Header = nullptr;
for (MachineBasicBlock *MBB : CurrentSCC) {
for (MachineBasicBlock *Pred : MBB->predecessors()) {
if (InSet.count(Pred))
if (!Header) {
Header = MBB;
// TODO: Implement multiple-entry loops.
report_fatal_error("multiple-entry loops are not supported yet");
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
static bool LoopContains(const MachineLoop *Loop,
const MachineBasicBlock *MBB) {
return Loop ? Loop->contains(MBB) : true;
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;
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.
bool LoopContainsA = LoopContains(Loop, A);
bool LoopContainsB = LoopContains(Loop, B);
if (LoopContainsA && !LoopContainsB)
return true;
if (!LoopContainsA && LoopContainsB)
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;
/// Return the "bottom" block of a loop. This differs from
/// MachineLoop::getBottomBlock in that it works even if the loop is
/// discontiguous.
static MachineBasicBlock *LoopBottom(const MachineLoop *Loop) {
MachineBasicBlock *Bottom = Loop->getHeader();
for (MachineBasicBlock *MBB : Loop->blocks())
if (MBB->getNumber() > Bottom->getNumber())
Bottom = MBB;
return Bottom;
/// Sort the blocks in RPO, taking special care to make sure that loops are
/// contiguous even in the case of split backedges.
/// TODO: Determine whether RPO is actually worthwhile, or whether we should
/// move to just a stable-topological-sort-based approach that would preserve
/// more of the original order.
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 *EntryBlock = &*MF.begin();
Stack.push_back(POStackEntry(EntryBlock, 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));
// Put the block in its position in the MachineFunction.
MachineBasicBlock &MBB = *Entry.MBB;
// 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() &&
"Non-branch terminator with fallthrough cannot yet be rewritten");
if (MBB.empty() || !MBB.back().isTerminator() || MBB.back().isBranch())
if (Stack.empty())
// Now that we've sorted the blocks in RPO, renumber them.
#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.
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())
(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()))
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.");
/// Test whether Pred has any terminators explicitly branching to MBB, as
/// opposed to falling through. Note that it's possible (eg. in unoptimized
/// code) for a branch instruction to both branch to a block and fallthrough
/// to it, so we check the actual branch operands to see if there are any
/// explicit mentions.
static bool ExplicitlyBranchesTo(MachineBasicBlock *Pred,
MachineBasicBlock *MBB) {
for (MachineInstr &MI : Pred->terminators())
for (MachineOperand &MO : MI.explicit_operands())
if (MO.isMBB() && MO.getMBB() == MBB)
return true;
return false;
/// Insert a BLOCK marker for branches to MBB (if needed).
static void PlaceBlockMarker(MachineBasicBlock &MBB, MachineFunction &MF,
SmallVectorImpl<MachineBasicBlock *> &ScopeTops,
const WebAssemblyInstrInfo &TII,
const MachineLoopInfo &MLI,
MachineDominatorTree &MDT) {
// First compute the nearest common dominator of all forward non-fallthrough
// predecessors so that we minimize the time that the BLOCK is on the stack,
// which reduces overall stack height.
MachineBasicBlock *Header = nullptr;
bool IsBranchedTo = false;
int MBBNumber = MBB.getNumber();
for (MachineBasicBlock *Pred : MBB.predecessors())
if (Pred->getNumber() < MBBNumber) {
Header = Header ? MDT.findNearestCommonDominator(Header, Pred) : Pred;
if (ExplicitlyBranchesTo(Pred, &MBB))
IsBranchedTo = true;
if (!Header)
if (!IsBranchedTo)
assert(&MBB != &MF.front() && "Header blocks shouldn't have predecessors");
MachineBasicBlock *LayoutPred = &*prev(MachineFunction::iterator(&MBB));
// If the nearest common dominator is inside a more deeply nested context,
// walk out to the nearest scope which isn't more deeply nested.
for (MachineFunction::iterator I(LayoutPred), E(Header); I != E; --I) {
if (MachineBasicBlock *ScopeTop = ScopeTops[I->getNumber()]) {
if (ScopeTop->getNumber() > Header->getNumber()) {
// Skip over an intervening scope.
I = next(MachineFunction::iterator(ScopeTop));
} else {
// We found a scope level at an appropriate depth.
Header = ScopeTop;
// If there's a loop which ends just before MBB which contains Header, we can
// reuse its label instead of inserting a new BLOCK.
for (MachineLoop *Loop = MLI.getLoopFor(LayoutPred);
Loop && Loop->contains(LayoutPred); Loop = Loop->getParentLoop())
if (Loop && LoopBottom(Loop) == LayoutPred && Loop->contains(Header))
// Decide where in Header to put the BLOCK.
MachineBasicBlock::iterator InsertPos;
MachineLoop *HeaderLoop = MLI.getLoopFor(Header);
if (HeaderLoop && MBB.getNumber() > LoopBottom(HeaderLoop)->getNumber()) {
// Header is the header of a loop that does not lexically contain MBB, so
// the BLOCK needs to be above the LOOP.
InsertPos = Header->begin();
} else {
// Otherwise, insert the BLOCK as late in Header as we can, but before the
// beginning of the local expression tree and any nested BLOCKs.
InsertPos = Header->getFirstTerminator();
while (InsertPos != Header->begin() &&
prev(InsertPos)->definesRegister(WebAssembly::EXPR_STACK) &&
prev(InsertPos)->getOpcode() != WebAssembly::LOOP &&
prev(InsertPos)->getOpcode() != WebAssembly::END_BLOCK &&
prev(InsertPos)->getOpcode() != WebAssembly::END_LOOP)
// Add the BLOCK.
BuildMI(*Header, InsertPos, DebugLoc(), TII.get(WebAssembly::BLOCK));
// Mark the end of the block.
InsertPos = MBB.begin();
while (InsertPos != MBB.end() &&
InsertPos->getOpcode() == WebAssembly::END_LOOP)
BuildMI(MBB, InsertPos, DebugLoc(), TII.get(WebAssembly::END_BLOCK));
// Track the farthest-spanning scope that ends at this point.
int Number = MBB.getNumber();
if (!ScopeTops[Number] ||
ScopeTops[Number]->getNumber() > Header->getNumber())
ScopeTops[Number] = Header;
/// Insert a LOOP marker for a loop starting at MBB (if it's a loop header).
static void PlaceLoopMarker(
MachineBasicBlock &MBB, MachineFunction &MF,
SmallVectorImpl<MachineBasicBlock *> &ScopeTops,
DenseMap<const MachineInstr *, const MachineBasicBlock *> &LoopTops,
const WebAssemblyInstrInfo &TII, const MachineLoopInfo &MLI) {
MachineLoop *Loop = MLI.getLoopFor(&MBB);
if (!Loop || Loop->getHeader() != &MBB)
// The operand of a LOOP is the first block after the loop. If the loop is the
// bottom of the function, insert a dummy block at the end.
MachineBasicBlock *Bottom = LoopBottom(Loop);
auto Iter = next(MachineFunction::iterator(Bottom));
if (Iter == MF.end()) {
MachineBasicBlock *Label = MF.CreateMachineBasicBlock();
// Give it a fake predecessor so that AsmPrinter prints its label.
Iter = next(MachineFunction::iterator(Bottom));
MachineBasicBlock *AfterLoop = &*Iter;
// Mark the beginning of the loop (after the end of any existing loop that
// ends here).
auto InsertPos = MBB.begin();
while (InsertPos != MBB.end() &&
InsertPos->getOpcode() == WebAssembly::END_LOOP)
BuildMI(MBB, InsertPos, DebugLoc(), TII.get(WebAssembly::LOOP));
// Mark the end of the loop.
MachineInstr *End = BuildMI(*AfterLoop, AfterLoop->begin(), DebugLoc(),
LoopTops[End] = &MBB;
assert((!ScopeTops[AfterLoop->getNumber()] ||
ScopeTops[AfterLoop->getNumber()]->getNumber() < MBB.getNumber()) &&
"With RPO we should visit the outer-most loop for a block first.");
if (!ScopeTops[AfterLoop->getNumber()])
ScopeTops[AfterLoop->getNumber()] = &MBB;
static unsigned
GetDepth(const SmallVectorImpl<const MachineBasicBlock *> &Stack,
const MachineBasicBlock *MBB) {
unsigned Depth = 0;
for (auto X : reverse(Stack)) {
if (X == MBB)
assert(Depth < Stack.size() && "Branch destination should be in scope");
return Depth;
/// Insert LOOP and BLOCK markers at appropriate places.
static void PlaceMarkers(MachineFunction &MF, const MachineLoopInfo &MLI,
const WebAssemblyInstrInfo &TII,
MachineDominatorTree &MDT) {
// For each block whose label represents the end of a scope, record the block
// which holds the beginning of the scope. This will allow us to quickly skip
// over scoped regions when walking blocks. We allocate one more than the
// number of blocks in the function to accommodate for the possible fake block
// we may insert at the end.
SmallVector<MachineBasicBlock *, 8> ScopeTops(MF.getNumBlockIDs() + 1);
// For eacn LOOP_END, the corresponding LOOP.
DenseMap<const MachineInstr *, const MachineBasicBlock *> LoopTops;
for (auto &MBB : MF) {
// Place the LOOP for MBB if MBB is the header of a loop.
PlaceLoopMarker(MBB, MF, ScopeTops, LoopTops, TII, MLI);
// Place the BLOCK for MBB if MBB is branched to from above.
PlaceBlockMarker(MBB, MF, ScopeTops, TII, MLI, MDT);
// Now rewrite references to basic blocks to be depth immediates.
SmallVector<const MachineBasicBlock *, 8> Stack;
for (auto &MBB : reverse(MF)) {
for (auto &MI : reverse(MBB)) {
switch (MI.getOpcode()) {
case WebAssembly::BLOCK:
assert(ScopeTops[Stack.back()->getNumber()] == &MBB &&
"Block should be balanced");
case WebAssembly::LOOP:
assert(Stack.back() == &MBB && "Loop top should be balanced");
case WebAssembly::END_BLOCK:
case WebAssembly::END_LOOP:
if (MI.isTerminator()) {
// Rewrite MBB operands to be depth immediates.
SmallVector<MachineOperand, 4> Ops(MI.operands());
while (MI.getNumOperands() > 0)
MI.RemoveOperand(MI.getNumOperands() - 1);
for (auto MO : Ops) {
if (MO.isMBB())
MO = MachineOperand::CreateImm(GetDepth(Stack, MO.getMBB()));
MI.addOperand(MF, MO);
assert(Stack.empty() && "Control flow should be balanced");
bool WebAssemblyCFGStackify::runOnMachineFunction(MachineFunction &MF) {
DEBUG(dbgs() << "********** CFG Stackifying **********\n"
"********** Function: "
<< MF.getName() << '\n');
const auto &MLI = getAnalysis<MachineLoopInfo>();
auto &MDT = getAnalysis<MachineDominatorTree>();
// Liveness is not tracked for EXPR_STACK physreg.
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, MDT);
return true;