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llvm-project/llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp

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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 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.
///
/// 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.
///
//===----------------------------------------------------------------------===//
#include "WebAssembly.h"
#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
#include "WebAssemblyMachineFunctionInfo.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/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 {
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
WebAssemblyCFGStackify() : MachineFunctionPass(ID) {}
};
} // end anonymous namespace
char WebAssemblyCFGStackify::ID = 0;
FunctionPass *llvm::createWebAssemblyCFGStackify() {
return new WebAssemblyCFGStackify();
}
/// 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;
}
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()) {}
};
}
/// 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 sucessors, 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
}
/// 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)
return;
if (!IsBranchedTo)
return;
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;
break;
}
}
}
// 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, after any END constructs.
InsertPos = Header->begin();
while (InsertPos->getOpcode() == WebAssembly::END_BLOCK ||
InsertPos->getOpcode() == WebAssembly::END_LOOP)
++InsertPos;
} 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)
--InsertPos;
}
// Add the BLOCK.
MachineInstr *Begin = BuildMI(*Header, InsertPos, DebugLoc(),
TII.get(WebAssembly::BLOCK))
.addImm(int64_t(WebAssembly::ExprType::Void));
// Mark the end of the block.
InsertPos = MBB.begin();
while (InsertPos != MBB.end() &&
InsertPos->getOpcode() == WebAssembly::END_LOOP &&
LoopTops[&*InsertPos]->getParent()->getNumber() >= Header->getNumber())
++InsertPos;
MachineInstr *End = BuildMI(MBB, InsertPos, DebugLoc(),
TII.get(WebAssembly::END_BLOCK));
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)
return;
// 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 = std::next(MachineFunction::iterator(Bottom));
if (Iter == MF.end()) {
MachineBasicBlock *Label = MF.CreateMachineBasicBlock();
// Give it a fake predecessor so that AsmPrinter prints its label.
Label->addSuccessor(Label);
MF.push_back(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)
++InsertPos;
MachineInstr *Begin = BuildMI(MBB, InsertPos, DebugLoc(),
TII.get(WebAssembly::LOOP))
.addImm(int64_t(WebAssembly::ExprType::Void));
// Mark the end of the loop.
MachineInstr *End = BuildMI(*AfterLoop, AfterLoop->begin(), DebugLoc(),
TII.get(WebAssembly::END_LOOP));
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)
break;
++Depth;
}
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())
return;
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;
default: llvm_unreachable("unexpected return type");
}
for (MachineBasicBlock &MBB : reverse(MF)) {
for (MachineInstr &MI : reverse(MBB)) {
if (MI.isPosition() || MI.isDebugValue())
continue;
if (MI.getOpcode() == WebAssembly::END_BLOCK) {
BlockTops[&MI]->getOperand(0).setImm(int32_t(retType));
continue;
}
if (MI.getOpcode() == WebAssembly::END_LOOP) {
LoopTops[&MI]->getOperand(0).setImm(int32_t(retType));
continue;
}
// Something other than an `end`. We're done.
return;
}
}
}
/// 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");
Stack.pop_back();
break;
case WebAssembly::LOOP:
assert(Stack.back() == &MBB && "Loop top should be balanced");
Stack.pop_back();
break;
case WebAssembly::END_BLOCK:
Stack.push_back(&MBB);
break;
case WebAssembly::END_LOOP:
Stack.push_back(LoopTops[&MI]->getParent());
break;
default:
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);
}
}
break;
}
}
}
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);
}
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 VALUE_STACK physreg.
const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
WebAssemblyFunctionInfo &MFI = *MF.getInfo<WebAssemblyFunctionInfo>();
MF.getRegInfo().invalidateLiveness();
// Sort the blocks, with contiguous loops.
SortBlocks(MF, MLI, MDT);
// Place the BLOCK and LOOP markers to indicate the beginnings of scopes.
PlaceMarkers(MF, MLI, TII, MDT, MFI);
return true;
}
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