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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
/// 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"
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 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;
// <LOOP|TRY marker, Loop/exception bottom BB> map
DenseMap<const MachineInstr *, MachineBasicBlock *> BeginToBottom;
// 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);
MachineBasicBlock *getBottom(const 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())
// Even if a rethrow takes a BB argument, it is not a branch
if (!WebAssembly::isRethrow(MI))
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);
}
MachineBasicBlock *Bottom = BeginToBottom.lookup(Begin);
if (Bottom)
BeginToBottom.erase(Begin);
}
// Given a LOOP/TRY marker, returns its bottom BB. Use cached information if any
// to prevent recomputation.
MachineBasicBlock *
WebAssemblyCFGStackify::getBottom(const MachineInstr *Begin) {
const auto &MLI = getAnalysis<MachineLoopInfo>();
const auto &WEI = getAnalysis<WebAssemblyExceptionInfo>();
if (BeginToBottom.count(Begin))
return BeginToBottom[Begin];
if (Begin->getOpcode() == WebAssembly::LOOP) {
MachineLoop *L = MLI.getLoopFor(Begin->getParent());
assert(L);
BeginToBottom[Begin] = WebAssembly::getBottom(L);
} else if (Begin->getOpcode() == WebAssembly::TRY) {
WebAssemblyException *WE = WEI.getExceptionFor(TryToEHPad[Begin]);
assert(WE);
BeginToBottom[Begin] = WebAssembly::getBottom(WE);
} else
assert(false);
return BeginToBottom[Begin];
}
/// Insert a BLOCK marker for branches to MBB (if needed).
void WebAssemblyCFGStackify::placeBlockMarker(MachineBasicBlock &MBB) {
// This should have been handled in placeTryMarker.
if (MBB.isEHPad())
return;
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;
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.
// 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/TRY marker and the bottom block of
// the loop/exception is above MBB, it should be after the BLOCK, because
// the loop/exception is nested in this block. Otherwise it should be before
// the BLOCK.
if (MI.getOpcode() == WebAssembly::LOOP ||
MI.getOpcode() == WebAssembly::TRY) {
if (MBB.getNumber() > getBottom(&MI)->getNumber())
AfterSet.insert(&MI);
#ifndef NDEBUG
else
BeforeSet.insert(&MI);
#endif
}
// All previously inserted BLOCK markers should be after the BLOCK because
// they are all nested blocks.
if (MI.getOpcode() == WebAssembly::BLOCK)
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.
auto InsertPos = GetLatestInsertPos(Header, BeforeSet, AfterSet);
MachineInstr *Begin =
BuildMI(*Header, InsertPos, Header->findDebugLoc(InsertPos),
TII.get(WebAssembly::BLOCK))
.addImm(int64_t(WebAssembly::ExprType::Void));
// 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(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;
// 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) {
if (!MBB.isEHPad())
return;
// catch_all terminate pad is grouped together with catch terminate pad and
// does not need a separate TRY and END_TRY marker.
if (WebAssembly::isCatchAllTerminatePad(MBB))
return;
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(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 *AfterTry = &*Iter;
assert(AfterTry != &MF.front());
MachineBasicBlock *LayoutPred =
&*std::prev(MachineFunction::iterator(AfterTry));
// 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 TRY.
// 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, the LOOP 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) {
if (MBB.getNumber() > Bottom->getNumber())
AfterSet.insert(&MI);
#ifndef NDEBUG
else
BeforeSet.insert(&MI);
#endif
}
// All previously inserted TRY markers should be after the TRY because they
// are all nested trys.
if (MI.getOpcode() == WebAssembly::TRY)
AfterSet.insert(&MI);
#ifndef NDEBUG
// All END_(LOOP/TRY) markers should be before the TRY.
if (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() || !WebAssembly::isRethrow(*TermPos)) {
for (const auto &MI : reverse(*Header)) {
if (MI.isCall()) {
AfterSet.insert(&MI);
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 : *AfterTry) {
#ifndef NDEBUG
// END_TRY should precede existing LOOP markers.
if (MI.getOpcode() == WebAssembly::LOOP)
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) {
if (EndToBegin[&MI]->getParent()->getNumber() >= Header->getNumber())
BeforeSet.insert(&MI);
#ifndef NDEBUG
else
AfterSet.insert(&MI);
#endif
}
}
// Mark the end of the TRY.
InsertPos = GetEarliestInsertPos(AfterTry, BeforeSet, AfterSet);
MachineInstr *End =
BuildMI(*AfterTry, InsertPos, Bottom->findBranchDebugLoc(),
TII.get(WebAssembly::END_TRY));
registerTryScope(Begin, End, &MBB);
// Track the farthest-spanning scope that ends at this point.
int Number = AfterTry->getNumber();
if (!ScopeTops[Number] ||
ScopeTops[Number]->getNumber() > Header->getNumber())
ScopeTops[Number] = Header;
}
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) {
const MCAsmInfo *MCAI = MF.getTarget().getMCAsmInfo();
// 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);
// Place the TRY for MBB if MBB is the EH pad of an exception.
if (MCAI->getExceptionHandlingType() == ExceptionHandling::Wasm &&
MF.getFunction().hasPersonalityFn())
for (auto &MBB : MF)
placeTryMarker(MBB);
// Place the BLOCK for MBB if MBB is branched to from above.
for (auto &MBB : MF)
placeBlockMarker(MBB);
}
void WebAssemblyCFGStackify::rewriteDepthImmediates(MachineFunction &MF) {
const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
// Now rewrite references to basic blocks to be depth immediates.
// We need two stacks: one for normal scopes and the other for EH pad scopes.
// EH pad stack is used to rewrite depths in rethrow instructions.
SmallVector<const MachineBasicBlock *, 8> Stack;
SmallVector<const MachineBasicBlock *, 8> EHPadStack;
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:
assert(ScopeTops[Stack.back()->getNumber()]->getNumber() <=
MBB.getNumber() &&
"Block/try should be balanced");
Stack.pop_back();
break;
case WebAssembly::TRY:
assert(ScopeTops[Stack.back()->getNumber()]->getNumber() <=
MBB.getNumber() &&
"Block/try marker should be balanced");
Stack.pop_back();
EHPadStack.pop_back();
break;
case WebAssembly::CATCH_I32:
case WebAssembly::CATCH_I64:
case WebAssembly::CATCH_ALL:
// Currently the only case there are more than one catch for a try is
// for catch terminate pad, in the form of
// try
// catch
// call @__clang_call_terminate
// unreachable
// catch_all
// call @std::terminate
// unreachable
// end
// So we shouldn't push the current BB for the second catch_all block
// here.
if (!WebAssembly::isCatchAllTerminatePad(MBB))
EHPadStack.push_back(&MBB);
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;
case WebAssembly::RETHROW: {
// Rewrite MBB operands to be depth immediates.
unsigned EHPadDepth = GetDepth(EHPadStack, MI.getOperand(0).getMBB());
MI.RemoveOperand(0);
MI.addOperand(MF, MachineOperand::CreateImm(EHPadDepth));
break;
}
case WebAssembly::RETHROW_TO_CALLER: {
MachineInstr *Rethrow =
BuildMI(MBB, MI, MI.getDebugLoc(), TII.get(WebAssembly::RETHROW))
.addImm(EHPadStack.size());
MI.eraseFromParent();
I = MachineBasicBlock::reverse_iterator(Rethrow);
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();
BeginToBottom.clear();
}
bool WebAssemblyCFGStackify::runOnMachineFunction(MachineFunction &MF) {
LLVM_DEBUG(dbgs() << "********** CFG Stackifying **********\n"
"********** Function: "
<< MF.getName() << '\n');
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);
// 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);
return true;
}
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