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//===-- 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 {
bool runOnMachineFunction(MachineFunction &MF) override;
static char ID; // Pass identification, replacement for typeid
WebAssemblyCFGSort() : MachineFunctionPass(ID) {}
} // end anonymous namespace
char WebAssemblyCFGSort::ID = 0;
"Reorders blocks in topological order", false, false)
FunctionPass *llvm::createWebAssemblyCFGSort() {
return new WebAssemblyCFGSort();
static void MaybeUpdateTerminator(MachineBasicBlock *MBB) {
#ifndef NDEBUG
bool AnyBarrier = false;
bool AllAnalyzable = true;
for (const MachineInstr &Term : MBB->terminators()) {
#ifndef NDEBUG
AnyBarrier |= Term.isBarrier();
AllAnalyzable &= Term.isBranch() && !Term.isIndirectBranch();
assert((AnyBarrier || AllAnalyzable) &&
"AnalyzeBranch needs to analyze any block with a fallthrough");
if (AllAnalyzable)
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.
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))
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 *>,
PriorityQueue<MachineBasicBlock *, std::vector<MachineBasicBlock *>,
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)
// 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)
while (!Loops.empty() && Loops.back().NumBlocksLeft == 0)
// 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))
// Decrement the predecessor count. If it's now zero, it's ready.
if (--NumPredsLeft[Succ->getNumber()] == 0)
// 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 =;
// 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)) {
Next = nullptr;
// 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())) {
Next = nullptr;
// 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()) {
for (;;) {
Next =;
// 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)) {
// Move the next block into place and iterate.
MBB = Next;
assert(Loops.empty() && "Active loop list not finished");
#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 == WebAssembly::getBottom(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.");
bool WebAssemblyCFGSort::runOnMachineFunction(MachineFunction &MF) {
LLVM_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.
// Sort the blocks, with contiguous loops.
SortBlocks(MF, MLI, MDT);
return true;