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//=== WebAssemblyLateEHPrepare.cpp - WebAssembly Exception Preparation -===//
//
// 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
/// \brief Does various transformations for exception handling.
///
//===----------------------------------------------------------------------===//
#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
#include "WebAssembly.h"
#include "WebAssemblySubtarget.h"
#include "WebAssemblyUtilities.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/WasmEHFuncInfo.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/Support/Debug.h"
using namespace llvm;
#define DEBUG_TYPE "wasm-late-eh-prepare"
namespace {
class WebAssemblyLateEHPrepare final : public MachineFunctionPass {
StringRef getPassName() const override {
return "WebAssembly Late Prepare Exception";
}
bool runOnMachineFunction(MachineFunction &MF) override;
bool addCatches(MachineFunction &MF);
bool replaceFuncletReturns(MachineFunction &MF);
bool removeUnnecessaryUnreachables(MachineFunction &MF);
bool addExceptionExtraction(MachineFunction &MF);
bool restoreStackPointer(MachineFunction &MF);
public:
static char ID; // Pass identification, replacement for typeid
WebAssemblyLateEHPrepare() : MachineFunctionPass(ID) {}
};
} // end anonymous namespace
char WebAssemblyLateEHPrepare::ID = 0;
INITIALIZE_PASS(WebAssemblyLateEHPrepare, DEBUG_TYPE,
"WebAssembly Late Exception Preparation", false, false)
FunctionPass *llvm::createWebAssemblyLateEHPrepare() {
return new WebAssemblyLateEHPrepare();
}
// Returns the nearest EH pad that dominates this instruction. This does not use
// dominator analysis; it just does BFS on its predecessors until arriving at an
// EH pad. This assumes valid EH scopes so the first EH pad it arrives in all
// possible search paths should be the same.
// Returns nullptr in case it does not find any EH pad in the search, or finds
// multiple different EH pads.
static MachineBasicBlock *getMatchingEHPad(MachineInstr *MI) {
MachineFunction *MF = MI->getParent()->getParent();
SmallVector<MachineBasicBlock *, 2> WL;
SmallPtrSet<MachineBasicBlock *, 2> Visited;
WL.push_back(MI->getParent());
MachineBasicBlock *EHPad = nullptr;
while (!WL.empty()) {
MachineBasicBlock *MBB = WL.pop_back_val();
if (Visited.count(MBB))
continue;
Visited.insert(MBB);
if (MBB->isEHPad()) {
if (EHPad && EHPad != MBB)
return nullptr;
EHPad = MBB;
continue;
}
if (MBB == &MF->front())
return nullptr;
WL.append(MBB->pred_begin(), MBB->pred_end());
}
return EHPad;
}
// Erase the specified BBs if the BB does not have any remaining predecessors,
// and also all its dead children.
template <typename Container>
static void eraseDeadBBsAndChildren(const Container &MBBs) {
SmallVector<MachineBasicBlock *, 8> WL(MBBs.begin(), MBBs.end());
while (!WL.empty()) {
MachineBasicBlock *MBB = WL.pop_back_val();
if (!MBB->pred_empty())
continue;
SmallVector<MachineBasicBlock *, 4> Succs(MBB->succ_begin(),
MBB->succ_end());
WL.append(MBB->succ_begin(), MBB->succ_end());
for (auto *Succ : Succs)
MBB->removeSuccessor(Succ);
MBB->eraseFromParent();
}
}
bool WebAssemblyLateEHPrepare::runOnMachineFunction(MachineFunction &MF) {
LLVM_DEBUG(dbgs() << "********** Late EH Prepare **********\n"
"********** Function: "
<< MF.getName() << '\n');
if (MF.getTarget().getMCAsmInfo()->getExceptionHandlingType() !=
ExceptionHandling::Wasm)
return false;
bool Changed = false;
if (MF.getFunction().hasPersonalityFn()) {
Changed |= addCatches(MF);
Changed |= replaceFuncletReturns(MF);
}
Changed |= removeUnnecessaryUnreachables(MF);
if (MF.getFunction().hasPersonalityFn()) {
Changed |= addExceptionExtraction(MF);
Changed |= restoreStackPointer(MF);
}
return Changed;
}
// Add catch instruction to beginning of catchpads and cleanuppads.
bool WebAssemblyLateEHPrepare::addCatches(MachineFunction &MF) {
bool Changed = false;
const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
MachineRegisterInfo &MRI = MF.getRegInfo();
for (auto &MBB : MF) {
if (MBB.isEHPad()) {
Changed = true;
auto InsertPos = MBB.begin();
if (InsertPos->isEHLabel()) // EH pad starts with an EH label
++InsertPos;
Register DstReg = MRI.createVirtualRegister(&WebAssembly::EXNREFRegClass);
BuildMI(MBB, InsertPos, MBB.begin()->getDebugLoc(),
TII.get(WebAssembly::CATCH), DstReg);
}
}
return Changed;
}
bool WebAssemblyLateEHPrepare::replaceFuncletReturns(MachineFunction &MF) {
bool Changed = false;
const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
for (auto &MBB : MF) {
auto Pos = MBB.getFirstTerminator();
if (Pos == MBB.end())
continue;
MachineInstr *TI = &*Pos;
switch (TI->getOpcode()) {
case WebAssembly::CATCHRET: {
// Replace a catchret with a branch
MachineBasicBlock *TBB = TI->getOperand(0).getMBB();
if (!MBB.isLayoutSuccessor(TBB))
BuildMI(MBB, TI, TI->getDebugLoc(), TII.get(WebAssembly::BR))
.addMBB(TBB);
TI->eraseFromParent();
Changed = true;
break;
}
case WebAssembly::CLEANUPRET:
case WebAssembly::RETHROW_IN_CATCH: {
// Replace a cleanupret/rethrow_in_catch with a rethrow
auto *EHPad = getMatchingEHPad(TI);
auto CatchPos = EHPad->begin();
if (CatchPos->isEHLabel()) // EH pad starts with an EH label
++CatchPos;
MachineInstr *Catch = &*CatchPos;
Register ExnReg = Catch->getOperand(0).getReg();
BuildMI(MBB, TI, TI->getDebugLoc(), TII.get(WebAssembly::RETHROW))
.addReg(ExnReg);
TI->eraseFromParent();
Changed = true;
break;
}
}
}
return Changed;
}
bool WebAssemblyLateEHPrepare::removeUnnecessaryUnreachables(
MachineFunction &MF) {
bool Changed = false;
for (auto &MBB : MF) {
for (auto &MI : MBB) {
if (MI.getOpcode() != WebAssembly::THROW &&
MI.getOpcode() != WebAssembly::RETHROW)
continue;
Changed = true;
// The instruction after the throw should be an unreachable or a branch to
// another BB that should eventually lead to an unreachable. Delete it
// because throw itself is a terminator, and also delete successors if
// any.
MBB.erase(std::next(MI.getIterator()), MBB.end());
SmallVector<MachineBasicBlock *, 8> Succs(MBB.succ_begin(),
MBB.succ_end());
for (auto *Succ : Succs)
if (!Succ->isEHPad())
MBB.removeSuccessor(Succ);
eraseDeadBBsAndChildren(Succs);
}
}
return Changed;
}
// Wasm uses 'br_on_exn' instruction to check the tag of an exception. It takes
// exnref type object returned by 'catch', and branches to the destination if it
// matches a given tag. We currently use __cpp_exception symbol to represent the
// tag for all C++ exceptions.
//
// block $l (result i32)
// ...
// ;; exnref $e is on the stack at this point
// br_on_exn $l $e ;; branch to $l with $e's arguments
// ...
// end
// ;; Here we expect the extracted values are on top of the wasm value stack
// ... Handle exception using values ...
//
// br_on_exn takes an exnref object and branches if it matches the given tag.
// There can be multiple br_on_exn instructions if we want to match for another
// tag, but for now we only test for __cpp_exception tag, and if it does not
// match, i.e., it is a foreign exception, we rethrow it.
//
// In the destination BB that's the target of br_on_exn, extracted exception
// values (in C++'s case a single i32, which represents an exception pointer)
// are placed on top of the wasm stack. Because we can't model wasm stack in
// LLVM instruction, we use 'extract_exception' pseudo instruction to retrieve
// it. The pseudo instruction will be deleted later.
bool WebAssemblyLateEHPrepare::addExceptionExtraction(MachineFunction &MF) {
const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
MachineRegisterInfo &MRI = MF.getRegInfo();
auto *EHInfo = MF.getWasmEHFuncInfo();
SmallVector<MachineInstr *, 16> ExtractInstrs;
SmallVector<MachineInstr *, 8> ToDelete;
for (auto &MBB : MF) {
for (auto &MI : MBB) {
if (MI.getOpcode() == WebAssembly::EXTRACT_EXCEPTION_I32) {
if (MI.getOperand(0).isDead())
ToDelete.push_back(&MI);
else
ExtractInstrs.push_back(&MI);
}
}
}
bool Changed = !ToDelete.empty() || !ExtractInstrs.empty();
for (auto *MI : ToDelete)
MI->eraseFromParent();
if (ExtractInstrs.empty())
return Changed;
// Find terminate pads.
SmallSet<MachineBasicBlock *, 8> TerminatePads;
for (auto &MBB : MF) {
for (auto &MI : MBB) {
if (MI.isCall()) {
const MachineOperand &CalleeOp = MI.getOperand(0);
if (CalleeOp.isGlobal() && CalleeOp.getGlobal()->getName() ==
WebAssembly::ClangCallTerminateFn)
TerminatePads.insert(getMatchingEHPad(&MI));
}
}
}
for (auto *Extract : ExtractInstrs) {
MachineBasicBlock *EHPad = getMatchingEHPad(Extract);
assert(EHPad && "No matching EH pad for extract_exception");
auto CatchPos = EHPad->begin();
if (CatchPos->isEHLabel()) // EH pad starts with an EH label
++CatchPos;
MachineInstr *Catch = &*CatchPos;
if (Catch->getNextNode() != Extract)
EHPad->insert(Catch->getNextNode(), Extract->removeFromParent());
// - Before:
// ehpad:
// %exnref:exnref = catch
// %exn:i32 = extract_exception
// ... use exn ...
//
// - After:
// ehpad:
// %exnref:exnref = catch
// br_on_exn %thenbb, $__cpp_exception, %exnref
// br %elsebb
// elsebb:
// rethrow
// thenbb:
// %exn:i32 = extract_exception
// ... use exn ...
Register ExnReg = Catch->getOperand(0).getReg();
auto *ThenMBB = MF.CreateMachineBasicBlock();
auto *ElseMBB = MF.CreateMachineBasicBlock();
MF.insert(std::next(MachineFunction::iterator(EHPad)), ElseMBB);
MF.insert(std::next(MachineFunction::iterator(ElseMBB)), ThenMBB);
ThenMBB->splice(ThenMBB->end(), EHPad, Extract, EHPad->end());
ThenMBB->transferSuccessors(EHPad);
EHPad->addSuccessor(ThenMBB);
EHPad->addSuccessor(ElseMBB);
DebugLoc DL = Extract->getDebugLoc();
const char *CPPExnSymbol = MF.createExternalSymbolName("__cpp_exception");
BuildMI(EHPad, DL, TII.get(WebAssembly::BR_ON_EXN))
.addMBB(ThenMBB)
.addExternalSymbol(CPPExnSymbol)
.addReg(ExnReg);
BuildMI(EHPad, DL, TII.get(WebAssembly::BR)).addMBB(ElseMBB);
// When this is a terminate pad with __clang_call_terminate() call, we don't
// rethrow it anymore and call __clang_call_terminate() with a nullptr
// argument, which will call std::terminate().
//
// - Before:
// ehpad:
// %exnref:exnref = catch
// %exn:i32 = extract_exception
// call @__clang_call_terminate(%exn)
// unreachable
//
// - After:
// ehpad:
// %exnref:exnref = catch
// br_on_exn %thenbb, $__cpp_exception, %exnref
// br %elsebb
// elsebb:
// call @__clang_call_terminate(0)
// unreachable
// thenbb:
// %exn:i32 = extract_exception
// call @__clang_call_terminate(%exn)
// unreachable
if (TerminatePads.count(EHPad)) {
Function *ClangCallTerminateFn =
MF.getFunction().getParent()->getFunction(
WebAssembly::ClangCallTerminateFn);
assert(ClangCallTerminateFn &&
"There is no __clang_call_terminate() function");
Register Reg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
BuildMI(ElseMBB, DL, TII.get(WebAssembly::CONST_I32), Reg).addImm(0);
BuildMI(ElseMBB, DL, TII.get(WebAssembly::CALL_VOID))
.addGlobalAddress(ClangCallTerminateFn)
.addReg(Reg);
BuildMI(ElseMBB, DL, TII.get(WebAssembly::UNREACHABLE));
} else {
BuildMI(ElseMBB, DL, TII.get(WebAssembly::RETHROW)).addReg(ExnReg);
if (EHInfo->hasEHPadUnwindDest(EHPad))
ElseMBB->addSuccessor(EHInfo->getEHPadUnwindDest(EHPad));
}
}
return true;
}
// After the stack is unwound due to a thrown exception, the __stack_pointer
// global can point to an invalid address. This inserts instructions that
// restore __stack_pointer global.
bool WebAssemblyLateEHPrepare::restoreStackPointer(MachineFunction &MF) {
const auto *FrameLowering = static_cast<const WebAssemblyFrameLowering *>(
MF.getSubtarget().getFrameLowering());
if (!FrameLowering->needsPrologForEH(MF))
return false;
bool Changed = false;
for (auto &MBB : MF) {
if (!MBB.isEHPad())
continue;
Changed = true;
// Insert __stack_pointer restoring instructions at the beginning of each EH
// pad, after the catch instruction. Here it is safe to assume that SP32
// holds the latest value of __stack_pointer, because the only exception for
// this case is when a function uses the red zone, but that only happens
// with leaf functions, and we don't restore __stack_pointer in leaf
// functions anyway.
auto InsertPos = MBB.begin();
if (InsertPos->isEHLabel()) // EH pad starts with an EH label
++InsertPos;
if (InsertPos->getOpcode() == WebAssembly::CATCH)
++InsertPos;
FrameLowering->writeSPToGlobal(WebAssembly::SP32, MF, MBB, InsertPos,
MBB.begin()->getDebugLoc());
}
return Changed;
}