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

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//===-- WebAssemblyCFGStackify.cpp - CFG Stackification -------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
///
/// \file
/// This file implements a CFG stacking pass.
///
/// This pass inserts BLOCK, LOOP, and TRY 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.
///
/// In case we use exceptions, this pass also fixes mismatches in unwind
/// destinations created during transforming CFG into wasm structured format.
///
//===----------------------------------------------------------------------===//
#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
#include "WebAssembly.h"
#include "WebAssemblyExceptionInfo.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/CodeGen/WasmEHFuncInfo.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <cstring>
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.addRequired<MachineDominatorTree>();
AU.addRequired<MachineLoopInfo>();
AU.addRequired<WebAssemblyExceptionInfo>();
MachineFunctionPass::getAnalysisUsage(AU);
}
bool runOnMachineFunction(MachineFunction &MF) override;
// 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.
SmallVector<MachineBasicBlock *, 8> ScopeTops;
void placeMarkers(MachineFunction &MF);
void placeBlockMarker(MachineBasicBlock &MBB);
void placeLoopMarker(MachineBasicBlock &MBB);
void placeTryMarker(MachineBasicBlock &MBB);
void removeUnnecessaryInstrs(MachineFunction &MF);
void rewriteDepthImmediates(MachineFunction &MF);
void fixEndsAtEndOfFunction(MachineFunction &MF);
// For each BLOCK|LOOP|TRY, the corresponding END_(BLOCK|LOOP|TRY).
DenseMap<const MachineInstr *, MachineInstr *> BeginToEnd;
// For each END_(BLOCK|LOOP|TRY), the corresponding BLOCK|LOOP|TRY.
DenseMap<const MachineInstr *, MachineInstr *> EndToBegin;
// <TRY marker, EH pad> map
DenseMap<const MachineInstr *, MachineBasicBlock *> TryToEHPad;
// <EH pad, TRY marker> map
DenseMap<const MachineBasicBlock *, MachineInstr *> EHPadToTry;
// Helper functions to register / unregister scope information created by
// marker instructions.
void registerScope(MachineInstr *Begin, MachineInstr *End);
void registerTryScope(MachineInstr *Begin, MachineInstr *End,
MachineBasicBlock *EHPad);
void unregisterScope(MachineInstr *Begin);
public:
static char ID; // Pass identification, replacement for typeid
WebAssemblyCFGStackify() : MachineFunctionPass(ID) {}
~WebAssemblyCFGStackify() override { releaseMemory(); }
void releaseMemory() override;
};
} // end anonymous namespace
char WebAssemblyCFGStackify::ID = 0;
INITIALIZE_PASS(WebAssemblyCFGStackify, DEBUG_TYPE,
"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;
}
// Returns an iterator to the earliest position possible within the MBB,
// satisfying the restrictions given by BeforeSet and AfterSet. BeforeSet
// contains instructions that should go before the marker, and AfterSet contains
// ones that should go after the marker. In this function, AfterSet is only
// used for sanity checking.
static MachineBasicBlock::iterator
getEarliestInsertPos(MachineBasicBlock *MBB,
const SmallPtrSet<const MachineInstr *, 4> &BeforeSet,
const SmallPtrSet<const MachineInstr *, 4> &AfterSet) {
auto InsertPos = MBB->end();
while (InsertPos != MBB->begin()) {
if (BeforeSet.count(&*std::prev(InsertPos))) {
#ifndef NDEBUG
// Sanity check
for (auto Pos = InsertPos, E = MBB->begin(); Pos != E; --Pos)
assert(!AfterSet.count(&*std::prev(Pos)));
#endif
break;
}
--InsertPos;
}
return InsertPos;
}
// Returns an iterator to the latest position possible within the MBB,
// satisfying the restrictions given by BeforeSet and AfterSet. BeforeSet
// contains instructions that should go before the marker, and AfterSet contains
// ones that should go after the marker. In this function, BeforeSet is only
// used for sanity checking.
static MachineBasicBlock::iterator
getLatestInsertPos(MachineBasicBlock *MBB,
const SmallPtrSet<const MachineInstr *, 4> &BeforeSet,
const SmallPtrSet<const MachineInstr *, 4> &AfterSet) {
auto InsertPos = MBB->begin();
while (InsertPos != MBB->end()) {
if (AfterSet.count(&*InsertPos)) {
#ifndef NDEBUG
// Sanity check
for (auto Pos = InsertPos, E = MBB->end(); Pos != E; ++Pos)
assert(!BeforeSet.count(&*Pos));
#endif
break;
}
++InsertPos;
}
return InsertPos;
}
void WebAssemblyCFGStackify::registerScope(MachineInstr *Begin,
MachineInstr *End) {
BeginToEnd[Begin] = End;
EndToBegin[End] = Begin;
}
void WebAssemblyCFGStackify::registerTryScope(MachineInstr *Begin,
MachineInstr *End,
MachineBasicBlock *EHPad) {
registerScope(Begin, End);
TryToEHPad[Begin] = EHPad;
EHPadToTry[EHPad] = Begin;
}
void WebAssemblyCFGStackify::unregisterScope(MachineInstr *Begin) {
assert(BeginToEnd.count(Begin));
MachineInstr *End = BeginToEnd[Begin];
assert(EndToBegin.count(End));
BeginToEnd.erase(Begin);
EndToBegin.erase(End);
MachineBasicBlock *EHPad = TryToEHPad.lookup(Begin);
if (EHPad) {
assert(EHPadToTry.count(EHPad));
TryToEHPad.erase(Begin);
EHPadToTry.erase(EHPad);
}
}
/// Insert a BLOCK marker for branches to MBB (if needed).
void WebAssemblyCFGStackify::placeBlockMarker(MachineBasicBlock &MBB) {
assert(!MBB.isEHPad());
MachineFunction &MF = *MBB.getParent();
auto &MDT = getAnalysis<MachineDominatorTree>();
const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
const auto &MFI = *MF.getInfo<WebAssemblyFunctionInfo>();
// 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;
bool IsBrOnExn = false;
MachineInstr *BrOnExn = nullptr;
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 (Pred->getFirstTerminator()->getOpcode() == WebAssembly::BR_ON_EXN) {
IsBrOnExn = true;
assert(!BrOnExn && "There should be only one br_on_exn per block");
BrOnExn = &*Pred->getFirstTerminator();
}
}
}
}
if (!Header)
return;
if (!IsBranchedTo)
return;
assert(&MBB != &MF.front() && "Header blocks shouldn't have predecessors");
MachineBasicBlock *LayoutPred = MBB.getPrevNode();
// 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(ScopeTop->getIterator());
} else {
// We found a scope level at an appropriate depth.
Header = ScopeTop;
break;
}
}
}
// Decide where in Header to put the BLOCK.
// Instructions that should go before the BLOCK.
SmallPtrSet<const MachineInstr *, 4> BeforeSet;
// Instructions that should go after the BLOCK.
SmallPtrSet<const MachineInstr *, 4> AfterSet;
for (const auto &MI : *Header) {
// If there is a previously placed LOOP marker and the bottom block of the
// loop is above MBB, it should be after the BLOCK, because the loop is
// nested in this BLOCK. Otherwise it should be before the BLOCK.
if (MI.getOpcode() == WebAssembly::LOOP) {
auto *LoopBottom = BeginToEnd[&MI]->getParent()->getPrevNode();
if (MBB.getNumber() > LoopBottom->getNumber())
AfterSet.insert(&MI);
#ifndef NDEBUG
else
BeforeSet.insert(&MI);
#endif
}
// All previously inserted BLOCK/TRY markers should be after the BLOCK
// because they are all nested blocks.
if (MI.getOpcode() == WebAssembly::BLOCK ||
MI.getOpcode() == WebAssembly::TRY)
AfterSet.insert(&MI);
#ifndef NDEBUG
// All END_(BLOCK|LOOP|TRY) markers should be before the BLOCK.
if (MI.getOpcode() == WebAssembly::END_BLOCK ||
MI.getOpcode() == WebAssembly::END_LOOP ||
MI.getOpcode() == WebAssembly::END_TRY)
BeforeSet.insert(&MI);
#endif
// Terminators should go after the BLOCK.
if (MI.isTerminator())
AfterSet.insert(&MI);
}
// Local expression tree should go after the BLOCK.
for (auto I = Header->getFirstTerminator(), E = Header->begin(); I != E;
--I) {
if (std::prev(I)->isDebugInstr() || std::prev(I)->isPosition())
continue;
if (WebAssembly::isChild(*std::prev(I), MFI))
AfterSet.insert(&*std::prev(I));
else
break;
}
// Add the BLOCK.
// 'br_on_exn' extracts except_ref object and pushes variable number of values
// depending on its tag. For C++ exception, its a single i32 value, and the
// generated code will be in the form of:
// block i32
// br_on_exn 0, $__cpp_exception
// rethrow
// end_block
WebAssembly::ExprType ReturnType = WebAssembly::ExprType::Void;
if (IsBrOnExn) {
const char *TagName = BrOnExn->getOperand(1).getSymbolName();
if (std::strcmp(TagName, "__cpp_exception") != 0)
llvm_unreachable("Only C++ exception is supported");
ReturnType = WebAssembly::ExprType::I32;
}
auto InsertPos = getLatestInsertPos(Header, BeforeSet, AfterSet);
MachineInstr *Begin =
BuildMI(*Header, InsertPos, Header->findDebugLoc(InsertPos),
TII.get(WebAssembly::BLOCK))
.addImm(int64_t(ReturnType));
// Decide where in Header to put the END_BLOCK.
BeforeSet.clear();
AfterSet.clear();
for (auto &MI : MBB) {
#ifndef NDEBUG
// END_BLOCK should precede existing LOOP and TRY markers.
if (MI.getOpcode() == WebAssembly::LOOP ||
MI.getOpcode() == WebAssembly::TRY)
AfterSet.insert(&MI);
#endif
// If there is a previously placed END_LOOP marker and the header of the
// loop is above this block's header, the END_LOOP should be placed after
// the BLOCK, because the loop contains this block. Otherwise the END_LOOP
// should be placed before the BLOCK. The same for END_TRY.
if (MI.getOpcode() == WebAssembly::END_LOOP ||
MI.getOpcode() == WebAssembly::END_TRY) {
if (EndToBegin[&MI]->getParent()->getNumber() >= Header->getNumber())
BeforeSet.insert(&MI);
#ifndef NDEBUG
else
AfterSet.insert(&MI);
#endif
}
}
// Mark the end of the block.
InsertPos = getEarliestInsertPos(&MBB, BeforeSet, AfterSet);
MachineInstr *End = BuildMI(MBB, InsertPos, MBB.findPrevDebugLoc(InsertPos),
TII.get(WebAssembly::END_BLOCK));
registerScope(Begin, End);
// 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).
void WebAssemblyCFGStackify::placeLoopMarker(MachineBasicBlock &MBB) {
MachineFunction &MF = *MBB.getParent();
const auto &MLI = getAnalysis<MachineLoopInfo>();
const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
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 = WebAssembly::getBottom(Loop);
auto Iter = std::next(Bottom->getIterator());
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(Bottom->getIterator());
}
MachineBasicBlock *AfterLoop = &*Iter;
// Decide where in Header to put the LOOP.
SmallPtrSet<const MachineInstr *, 4> BeforeSet;
SmallPtrSet<const MachineInstr *, 4> AfterSet;
for (const auto &MI : MBB) {
// LOOP marker should be after any existing loop that ends here. Otherwise
// we assume the instruction belongs to the loop.
if (MI.getOpcode() == WebAssembly::END_LOOP)
BeforeSet.insert(&MI);
#ifndef NDEBUG
else
AfterSet.insert(&MI);
#endif
}
// Mark the beginning of the loop.
auto InsertPos = getEarliestInsertPos(&MBB, BeforeSet, AfterSet);
MachineInstr *Begin = BuildMI(MBB, InsertPos, MBB.findDebugLoc(InsertPos),
TII.get(WebAssembly::LOOP))
.addImm(int64_t(WebAssembly::ExprType::Void));
// Decide where in Header to put the END_LOOP.
BeforeSet.clear();
AfterSet.clear();
#ifndef NDEBUG
for (const auto &MI : MBB)
// Existing END_LOOP markers belong to parent loops of this loop
if (MI.getOpcode() == WebAssembly::END_LOOP)
AfterSet.insert(&MI);
#endif
// Mark the end of the loop (using arbitrary debug location that branched to
// the loop end as its location).
InsertPos = getEarliestInsertPos(AfterLoop, BeforeSet, AfterSet);
DebugLoc EndDL = (*AfterLoop->pred_rbegin())->findBranchDebugLoc();
MachineInstr *End =
BuildMI(*AfterLoop, InsertPos, EndDL, TII.get(WebAssembly::END_LOOP));
registerScope(Begin, End);
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;
}
void WebAssemblyCFGStackify::placeTryMarker(MachineBasicBlock &MBB) {
assert(MBB.isEHPad());
MachineFunction &MF = *MBB.getParent();
auto &MDT = getAnalysis<MachineDominatorTree>();
const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
const auto &WEI = getAnalysis<WebAssemblyExceptionInfo>();
const auto &MFI = *MF.getInfo<WebAssemblyFunctionInfo>();
// Compute the nearest common dominator of all unwind predecessors
MachineBasicBlock *Header = nullptr;
int MBBNumber = MBB.getNumber();
for (auto *Pred : MBB.predecessors()) {
if (Pred->getNumber() < MBBNumber) {
Header = Header ? MDT.findNearestCommonDominator(Header, Pred) : Pred;
assert(!explicitlyBranchesTo(Pred, &MBB) &&
"Explicit branch to an EH pad!");
}
}
if (!Header)
return;
// If this try is at the bottom of the function, insert a dummy block at the
// end.
WebAssemblyException *WE = WEI.getExceptionFor(&MBB);
assert(WE);
MachineBasicBlock *Bottom = WebAssembly::getBottom(WE);
auto Iter = std::next(Bottom->getIterator());
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(Bottom->getIterator());
}
MachineBasicBlock *Cont = &*Iter;
assert(Cont != &MF.front());
MachineBasicBlock *LayoutPred = Cont->getPrevNode();
// 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(ScopeTop->getIterator());
} else {
// We found a scope level at an appropriate depth.
Header = ScopeTop;
break;
}
}
}
// Decide where in Header to put the TRY.
// Instructions that should go before the TRY.
SmallPtrSet<const MachineInstr *, 4> BeforeSet;
// Instructions that should go after the TRY.
SmallPtrSet<const MachineInstr *, 4> AfterSet;
for (const auto &MI : *Header) {
// If there is a previously placed LOOP marker and the bottom block of the
// loop is above MBB, it should be after the TRY, because the loop is nested
// in this TRY. Otherwise it should be before the TRY.
if (MI.getOpcode() == WebAssembly::LOOP) {
auto *LoopBottom = BeginToEnd[&MI]->getParent()->getPrevNode();
if (MBB.getNumber() > LoopBottom->getNumber())
AfterSet.insert(&MI);
#ifndef NDEBUG
else
BeforeSet.insert(&MI);
#endif
}
// All previously inserted BLOCK/TRY markers should be after the TRY because
// they are all nested trys.
if (MI.getOpcode() == WebAssembly::BLOCK ||
MI.getOpcode() == WebAssembly::TRY)
AfterSet.insert(&MI);
#ifndef NDEBUG
// All END_(BLOCK/LOOP/TRY) markers should be before the TRY.
if (MI.getOpcode() == WebAssembly::END_BLOCK ||
MI.getOpcode() == WebAssembly::END_LOOP ||
MI.getOpcode() == WebAssembly::END_TRY)
BeforeSet.insert(&MI);
#endif
// Terminators should go after the TRY.
if (MI.isTerminator())
AfterSet.insert(&MI);
}
// Local expression tree should go after the TRY.
for (auto I = Header->getFirstTerminator(), E = Header->begin(); I != E;
--I) {
if (std::prev(I)->isDebugInstr() || std::prev(I)->isPosition())
continue;
if (WebAssembly::isChild(*std::prev(I), MFI))
AfterSet.insert(&*std::prev(I));
else
break;
}
// If Header unwinds to MBB (= Header contains 'invoke'), the try block should
// contain the call within it. So the call should go after the TRY. The
// exception is when the header's terminator is a rethrow instruction, in
// which case that instruction, not a call instruction before it, is gonna
// throw.
if (MBB.isPredecessor(Header)) {
auto TermPos = Header->getFirstTerminator();
if (TermPos == Header->end() ||
TermPos->getOpcode() != WebAssembly::RETHROW) {
for (const auto &MI : reverse(*Header)) {
if (MI.isCall()) {
AfterSet.insert(&MI);
// Possibly throwing calls are usually wrapped by EH_LABEL
// instructions. We don't want to split them and the call.
if (MI.getIterator() != Header->begin() &&
std::prev(MI.getIterator())->isEHLabel())
AfterSet.insert(&*std::prev(MI.getIterator()));
break;
}
}
}
}
// Add the TRY.
auto InsertPos = getLatestInsertPos(Header, BeforeSet, AfterSet);
MachineInstr *Begin =
BuildMI(*Header, InsertPos, Header->findDebugLoc(InsertPos),
TII.get(WebAssembly::TRY))
.addImm(int64_t(WebAssembly::ExprType::Void));
// Decide where in Header to put the END_TRY.
BeforeSet.clear();
AfterSet.clear();
for (const auto &MI : *Cont) {
#ifndef NDEBUG
// END_TRY should precede existing LOOP and BLOCK markers.
if (MI.getOpcode() == WebAssembly::LOOP ||
MI.getOpcode() == WebAssembly::BLOCK)
AfterSet.insert(&MI);
// All END_TRY markers placed earlier belong to exceptions that contains
// this one.
if (MI.getOpcode() == WebAssembly::END_TRY)
AfterSet.insert(&MI);
#endif
// If there is a previously placed END_LOOP marker and its header is after
// where TRY marker is, this loop is contained within the 'catch' part, so
// the END_TRY marker should go after that. Otherwise, the whole try-catch
// is contained within this loop, so the END_TRY should go before that.
if (MI.getOpcode() == WebAssembly::END_LOOP) {
// For a LOOP to be after TRY, LOOP's BB should be after TRY's BB; if they
// are in the same BB, LOOP is always before TRY.
if (EndToBegin[&MI]->getParent()->getNumber() > Header->getNumber())
BeforeSet.insert(&MI);
#ifndef NDEBUG
else
AfterSet.insert(&MI);
#endif
}
// It is not possible for an END_BLOCK to be already in this block.
}
// Mark the end of the TRY.
InsertPos = getEarliestInsertPos(Cont, BeforeSet, AfterSet);
MachineInstr *End =
BuildMI(*Cont, InsertPos, Bottom->findBranchDebugLoc(),
TII.get(WebAssembly::END_TRY));
registerTryScope(Begin, End, &MBB);
// Track the farthest-spanning scope that ends at this point. We create two
// mappings: (BB with 'end_try' -> BB with 'try') and (BB with 'catch' -> BB
// with 'try'). We need to create 'catch' -> 'try' mapping here too because
// markers should not span across 'catch'. For example, this should not
// happen:
//
// try
// block --| (X)
// catch |
// end_block --|
// end_try
for (int Number : {Cont->getNumber(), MBB.getNumber()}) {
if (!ScopeTops[Number] ||
ScopeTops[Number]->getNumber() > Header->getNumber())
ScopeTops[Number] = Header;
}
}
void WebAssemblyCFGStackify::removeUnnecessaryInstrs(MachineFunction &MF) {
const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
// When there is an unconditional branch right before a catch instruction and
// it branches to the end of end_try marker, we don't need the branch, because
// it there is no exception, the control flow transfers to that point anyway.
// bb0:
// try
// ...
// br bb2 <- Not necessary
// bb1:
// catch
// ...
// bb2:
// end
for (auto &MBB : MF) {
if (!MBB.isEHPad())
continue;
MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
SmallVector<MachineOperand, 4> Cond;
MachineBasicBlock *EHPadLayoutPred = MBB.getPrevNode();
MachineBasicBlock *Cont = BeginToEnd[EHPadToTry[&MBB]]->getParent();
bool Analyzable = !TII.analyzeBranch(*EHPadLayoutPred, TBB, FBB, Cond);
if (Analyzable && ((Cond.empty() && TBB && TBB == Cont) ||
(!Cond.empty() && FBB && FBB == Cont)))
TII.removeBranch(*EHPadLayoutPred);
}
// When there are block / end_block markers that overlap with try / end_try
// markers, and the block and try markers' return types are the same, the
// block /end_block markers are not necessary, because try / end_try markers
// also can serve as boundaries for branches.
// block <- Not necessary
// try
// ...
// catch
// ...
// end
// end <- Not necessary
SmallVector<MachineInstr *, 32> ToDelete;
for (auto &MBB : MF) {
for (auto &MI : MBB) {
if (MI.getOpcode() != WebAssembly::TRY)
continue;
MachineInstr *Try = &MI, *EndTry = BeginToEnd[Try];
MachineBasicBlock *TryBB = Try->getParent();
MachineBasicBlock *Cont = EndTry->getParent();
int64_t RetType = Try->getOperand(0).getImm();
for (auto B = Try->getIterator(), E = std::next(EndTry->getIterator());
B != TryBB->begin() && E != Cont->end() &&
std::prev(B)->getOpcode() == WebAssembly::BLOCK &&
E->getOpcode() == WebAssembly::END_BLOCK &&
std::prev(B)->getOperand(0).getImm() == RetType;
--B, ++E) {
ToDelete.push_back(&*std::prev(B));
ToDelete.push_back(&*E);
}
}
}
for (auto *MI : ToDelete) {
if (MI->getOpcode() == WebAssembly::BLOCK)
unregisterScope(MI);
MI->eraseFromParent();
}
}
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.
void WebAssemblyCFGStackify::fixEndsAtEndOfFunction(MachineFunction &MF) {
const auto &MFI = *MF.getInfo<WebAssemblyFunctionInfo>();
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:
case MVT::v8i16:
case MVT::v4i32:
case MVT::v2i64:
case MVT::v4f32:
case MVT::v2f64:
RetType = WebAssembly::ExprType::V128;
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())
continue;
if (MI.getOpcode() == WebAssembly::END_BLOCK) {
EndToBegin[&MI]->getOperand(0).setImm(int32_t(RetType));
continue;
}
if (MI.getOpcode() == WebAssembly::END_LOOP) {
EndToBegin[&MI]->getOperand(0).setImm(int32_t(RetType));
continue;
}
// Something other than an `end`. We're done.
return;
}
}
}
// 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(),
MF.back().findPrevDebugLoc(MF.back().end()),
TII.get(WebAssembly::END_FUNCTION));
}
/// Insert LOOP/TRY/BLOCK markers at appropriate places.
void WebAssemblyCFGStackify::placeMarkers(MachineFunction &MF) {
// 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.
ScopeTops.resize(MF.getNumBlockIDs() + 1);
// Place the LOOP for MBB if MBB is the header of a loop.
for (auto &MBB : MF)
placeLoopMarker(MBB);
const MCAsmInfo *MCAI = MF.getTarget().getMCAsmInfo();
for (auto &MBB : MF) {
if (MBB.isEHPad()) {
// Place the TRY for MBB if MBB is the EH pad of an exception.
if (MCAI->getExceptionHandlingType() == ExceptionHandling::Wasm &&
MF.getFunction().hasPersonalityFn())
placeTryMarker(MBB);
} else {
// Place the BLOCK for MBB if MBB is branched to from above.
placeBlockMarker(MBB);
}
}
}
void WebAssemblyCFGStackify::rewriteDepthImmediates(MachineFunction &MF) {
// Now rewrite references to basic blocks to be depth immediates.
SmallVector<const MachineBasicBlock *, 8> Stack;
for (auto &MBB : reverse(MF)) {
for (auto I = MBB.rbegin(), E = MBB.rend(); I != E; ++I) {
MachineInstr &MI = *I;
switch (MI.getOpcode()) {
case WebAssembly::BLOCK:
case WebAssembly::TRY:
assert(ScopeTops[Stack.back()->getNumber()]->getNumber() <=
MBB.getNumber() &&
"Block/try marker 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:
case WebAssembly::END_TRY:
Stack.push_back(&MBB);
break;
case WebAssembly::END_LOOP:
Stack.push_back(EndToBegin[&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");
}
void WebAssemblyCFGStackify::releaseMemory() {
ScopeTops.clear();
BeginToEnd.clear();
EndToBegin.clear();
TryToEHPad.clear();
EHPadToTry.clear();
}
bool WebAssemblyCFGStackify::runOnMachineFunction(MachineFunction &MF) {
LLVM_DEBUG(dbgs() << "********** CFG Stackifying **********\n"
"********** Function: "
<< MF.getName() << '\n');
const MCAsmInfo *MCAI = MF.getTarget().getMCAsmInfo();
releaseMemory();
// Liveness is not tracked for VALUE_STACK physreg.
MF.getRegInfo().invalidateLiveness();
// Place the BLOCK/LOOP/TRY markers to indicate the beginnings of scopes.
placeMarkers(MF);
if (MCAI->getExceptionHandlingType() == ExceptionHandling::Wasm &&
MF.getFunction().hasPersonalityFn())
// Remove unnecessary instructions.
removeUnnecessaryInstrs(MF);
// Convert MBB operands in terminators to relative depth immediates.
rewriteDepthImmediates(MF);
// Fix up block/loop/try signatures at the end of the function to conform to
// WebAssembly's rules.
fixEndsAtEndOfFunction(MF);
// Add an end instruction at the end of the function body.
const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
if (!MF.getSubtarget<WebAssemblySubtarget>()
.getTargetTriple()
.isOSBinFormatELF())
appendEndToFunction(MF, TII);
MF.getInfo<WebAssemblyFunctionInfo>()->setCFGStackified();
return true;
}
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