llvm/llvm/lib/CodeGen/WasmEHPrepare.cpp

//===-- WasmEHPrepare - Prepare excepton handling for WebAssembly --------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This transformation is designed for use by code generators which use
// WebAssembly exception handling scheme. This currently supports C++
// exceptions.
//
// WebAssembly exception handling uses Windows exception IR for the middle level
// representation. This pass does the following transformation for every
// catchpad block:
// (In C-style pseudocode)
//
// - Before:
//   catchpad ...
//   exn = wasm.get.exception();
//   selector = wasm.get.selector();
//   ...
//
// - After:
//   catchpad ...
//   exn = wasm.catch(WebAssembly::CPP_EXCEPTION);
//   // Only add below in case it's not a single catch (...)
//   wasm.landingpad.index(index);
//   __wasm_lpad_context.lpad_index = index;
//   __wasm_lpad_context.lsda = wasm.lsda();
//   _Unwind_CallPersonality(exn);
//   selector = __wasm_lpad_context.selector;
//   ...
//
//
// * Background: Direct personality function call
// In WebAssembly EH, the VM is responsible for unwinding the stack once an
// exception is thrown. After the stack is unwound, the control flow is
// transfered to WebAssembly 'catch' instruction.
//
// Unwinding the stack is not done by libunwind but the VM, so the personality
// function in libcxxabi cannot be called from libunwind during the unwinding
// process. So after a catch instruction, we insert a call to a wrapper function
// in libunwind that in turn calls the real personality function.
//
// In Itanium EH, if the personality function decides there is no matching catch
// clause in a call frame and no cleanup action to perform, the unwinder doesn't
// stop there and continues unwinding. But in Wasm EH, the unwinder stops at
// every call frame with a catch intruction, after which the personality
// function is called from the compiler-generated user code here.
//
// In libunwind, we have this struct that serves as a communincation channel
// between the compiler-generated user code and the personality function in
// libcxxabi.
//
// struct _Unwind_LandingPadContext {
//   uintptr_t lpad_index;
//   uintptr_t lsda;
//   uintptr_t selector;
// };
// struct _Unwind_LandingPadContext __wasm_lpad_context = ...;
//
// And this wrapper in libunwind calls the personality function.
//
// _Unwind_Reason_Code _Unwind_CallPersonality(void *exception_ptr) {
//   struct _Unwind_Exception *exception_obj =
//       (struct _Unwind_Exception *)exception_ptr;
//   _Unwind_Reason_Code ret = __gxx_personality_v0(
//       1, _UA_CLEANUP_PHASE, exception_obj->exception_class, exception_obj,
//       (struct _Unwind_Context *)__wasm_lpad_context);
//   return ret;
// }
//
// We pass a landing pad index, and the address of LSDA for the current function
// to the wrapper function _Unwind_CallPersonality in libunwind, and we retrieve
// the selector after it returns.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/WasmEHPrepare.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/WasmEHFuncInfo.h"
#include "llvm/IR/EHPersonalities.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/IntrinsicsWebAssembly.h"
#include "llvm/IR/Module.h"
#include "llvm/InitializePasses.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"

usingnamespacellvm;

#define DEBUG_TYPE …

namespace {
class WasmEHPrepareImpl { … };

class WasmEHPrepare : public FunctionPass { … };

} // end anonymous namespace

PreservedAnalyses WasmEHPreparePass::run(Function &F,
                                         FunctionAnalysisManager &) { … }

char WasmEHPrepare::ID = …;
INITIALIZE_PASS_BEGIN(WasmEHPrepare, DEBUG_TYPE,
                      "Prepare WebAssembly exceptions", false, false)
INITIALIZE_PASS_END(WasmEHPrepare, DEBUG_TYPE, "Prepare WebAssembly exceptions",
                    false, false)

FunctionPass *llvm::createWasmEHPass() { … }

bool WasmEHPrepare::doInitialization(Module &M) { … }

// 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 &BBs) { … }

bool WasmEHPrepareImpl::runOnFunction(Function &F) { … }

bool WasmEHPrepareImpl::prepareThrows(Function &F) { … }

bool WasmEHPrepareImpl::prepareEHPads(Function &F) { … }

// Prepare an EH pad for Wasm EH handling. If NeedPersonality is false, Index is
// ignored.
void WasmEHPrepareImpl::prepareEHPad(BasicBlock *BB, bool NeedPersonality,
                                     unsigned Index) { … }

void llvm::calculateWasmEHInfo(const Function *F, WasmEHFuncInfo &EHInfo) { … }