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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
/// This file implements a CFG stacking pass.
/// This pass 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.
#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
#include "WebAssembly.h"
#include "WebAssemblyMachineFunctionInfo.h"
#include "WebAssemblySubtarget.h"
#include "WebAssemblyUtilities.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 {
StringRef 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;
"Insert BLOCK and LOOP markers for WebAssembly scopes",
false, false)
FunctionPass *llvm::createWebAssemblyCFGStackify() {
return new WebAssemblyCFGStackify();
/// 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,
DenseMap<const MachineInstr *, MachineInstr *> &BlockTops,
DenseMap<const MachineInstr *, MachineInstr *> &LoopTops,
const WebAssemblyInstrInfo &TII,
const MachineLoopInfo &MLI,
MachineDominatorTree &MDT,
WebAssemblyFunctionInfo &MFI) {
// 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 = &*std::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 = std::next(MachineFunction::iterator(ScopeTop));
} else {
// We found a scope level at an appropriate depth.
Header = ScopeTop;
// Decide where in Header to put the BLOCK.
MachineBasicBlock::iterator InsertPos;
MachineLoop *HeaderLoop = MLI.getLoopFor(Header);
if (HeaderLoop &&
MBB.getNumber() > WebAssembly::getBottom(HeaderLoop)->getNumber()) {
// Header is the header of a loop that does not lexically contain MBB, so
// the BLOCK needs to be above the LOOP, after any END constructs.
InsertPos = Header->begin();
while (InsertPos->getOpcode() == WebAssembly::END_BLOCK ||
InsertPos->getOpcode() == WebAssembly::END_LOOP)
} 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() &&
WebAssembly::isChild(*std::prev(InsertPos), MFI) &&
std::prev(InsertPos)->getOpcode() != WebAssembly::LOOP &&
std::prev(InsertPos)->getOpcode() != WebAssembly::END_BLOCK &&
std::prev(InsertPos)->getOpcode() != WebAssembly::END_LOOP)
// Add the BLOCK.
MachineInstr *Begin =
BuildMI(*Header, InsertPos, Header->findDebugLoc(InsertPos),
// Mark the end of the block.
InsertPos = MBB.begin();
while (InsertPos != MBB.end() &&
InsertPos->getOpcode() == WebAssembly::END_LOOP &&
LoopTops[&*InsertPos]->getParent()->getNumber() >= Header->getNumber())
MachineInstr *End = BuildMI(MBB, InsertPos, MBB.findPrevDebugLoc(InsertPos),
BlockTops[End] = Begin;
// 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 *, MachineInstr *> &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 = WebAssembly::getBottom(Loop);
auto Iter = std::next(MachineFunction::iterator(Bottom));
if (Iter == MF.end()) {
MachineBasicBlock *Label = MF.CreateMachineBasicBlock();
// Give it a fake predecessor so that AsmPrinter prints its label.
Iter = std::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)
MachineInstr *Begin = BuildMI(MBB, InsertPos, MBB.findDebugLoc(InsertPos),
// Mark the end of the loop (using arbitrary debug location that branched
// to the loop end as its location).
DebugLoc EndDL = (*AfterLoop->pred_rbegin())->findBranchDebugLoc();
MachineInstr *End = BuildMI(*AfterLoop, AfterLoop->begin(), EndDL,
LoopTops[End] = Begin;
assert((!ScopeTops[AfterLoop->getNumber()] ||
ScopeTops[AfterLoop->getNumber()]->getNumber() < MBB.getNumber()) &&
"With block sorting the outermost loop for a block should be 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;
/// In normal assembly languages, when the end of a function is unreachable,
/// because the function ends in an infinite loop or a noreturn call or similar,
/// it isn't necessary to worry about the function return type at the end of
/// the function, because it's never reached. However, in WebAssembly, blocks
/// that end at the function end need to have a return type signature that
/// matches the function signature, even though it's unreachable. This function
/// checks for such cases and fixes up the signatures.
static void FixEndsAtEndOfFunction(
MachineFunction &MF,
const WebAssemblyFunctionInfo &MFI,
DenseMap<const MachineInstr *, MachineInstr *> &BlockTops,
DenseMap<const MachineInstr *, MachineInstr *> &LoopTops) {
assert(MFI.getResults().size() <= 1);
if (MFI.getResults().empty())
WebAssembly::ExprType retType;
switch (MFI.getResults().front().SimpleTy) {
case MVT::i32: retType = WebAssembly::ExprType::I32; break;
case MVT::i64: retType = WebAssembly::ExprType::I64; break;
case MVT::f32: retType = WebAssembly::ExprType::F32; break;
case MVT::f64: retType = WebAssembly::ExprType::F64; break;
case MVT::v16i8: retType = WebAssembly::ExprType::I8x16; break;
case MVT::v8i16: retType = WebAssembly::ExprType::I16x8; break;
case MVT::v4i32: retType = WebAssembly::ExprType::I32x4; break;
case MVT::v4f32: retType = WebAssembly::ExprType::F32x4; break;
case MVT::ExceptRef: retType = WebAssembly::ExprType::ExceptRef; break;
default: llvm_unreachable("unexpected return type");
for (MachineBasicBlock &MBB : reverse(MF)) {
for (MachineInstr &MI : reverse(MBB)) {
if (MI.isPosition() || MI.isDebugInstr())
if (MI.getOpcode() == WebAssembly::END_BLOCK) {
if (MI.getOpcode() == WebAssembly::END_LOOP) {
// Something other than an `end`. We're done.
// WebAssembly functions end with an end instruction, as if the function body
// were a block.
static void AppendEndToFunction(
MachineFunction &MF,
const WebAssemblyInstrInfo &TII) {
BuildMI(MF.back(), MF.back().end(),
/// Insert LOOP and BLOCK markers at appropriate places.
static void PlaceMarkers(MachineFunction &MF, const MachineLoopInfo &MLI,
const WebAssemblyInstrInfo &TII,
MachineDominatorTree &MDT,
WebAssemblyFunctionInfo &MFI) {
// 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 each LOOP_END, the corresponding LOOP.
DenseMap<const MachineInstr *, MachineInstr *> LoopTops;
// For each END_BLOCK, the corresponding BLOCK.
DenseMap<const MachineInstr *, MachineInstr *> BlockTops;
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, BlockTops, LoopTops, TII, MLI, MDT, MFI);
// 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()]->getNumber() <= MBB.getNumber() &&
"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");
// Fix up block/loop signatures at the end of the function to conform to
// WebAssembly's rules.
FixEndsAtEndOfFunction(MF, MFI, BlockTops, LoopTops);
// Add an end instruction at the end of the function body.
AppendEndToFunction(MF, TII);
bool WebAssemblyCFGStackify::runOnMachineFunction(MachineFunction &MF) {
LLVM_DEBUG(dbgs() << "********** CFG Stackifying **********\n"
"********** Function: "
<< MF.getName() << '\n');
const auto &MLI = getAnalysis<MachineLoopInfo>();
auto &MDT = getAnalysis<MachineDominatorTree>();
// Liveness is not tracked for VALUE_STACK physreg.
const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
WebAssemblyFunctionInfo &MFI = *MF.getInfo<WebAssemblyFunctionInfo>();
// Place the BLOCK and LOOP markers to indicate the beginnings of scopes.
PlaceMarkers(MF, MLI, TII, MDT, MFI);
return true;