[WebAssembly] Add Wasm exception handling prepare pass

Summary:
This adds a pass that transforms a program to be prepared for Wasm
exception handling. This is using Windows EH instructions and based on
the previous Wasm EH proposal.
(https://github.com/WebAssembly/exception-handling/blob/master/proposals/Exceptions.md)

Reviewers: dschuff, majnemer

Subscribers: jfb, mgorny, sbc100, jgravelle-google, JDevlieghere, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D43746

llvm-svn: 333696

1 files changed, 331 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/WasmEHPrepare.cpp b/llvm/lib/CodeGen/WasmEHPrepare.cpp
new file mode 100644
index 0000000..c2c7bf6
--- /dev/null
+++ b/llvm/lib/CodeGen/WasmEHPrepare.cpp
@@ -0,0 +1,331 @@
+//===-- WasmEHPrepare - Prepare excepton handling for WebAssembly --------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This transformation is designed for use by code generators which use
+// WebAssembly exception handling scheme.
+//
+// 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(0); // 0 is a tag for C++
+//   wasm.landingpad.index(index);
+//   // Only add below in case it's not a single catch (...)
+//   __wasm_lpad_context.lpad_index = index;
+//   __wasm_lpad_context.lsda = wasm.lsda();
+//   _Unwind_CallPersonality(exn);
+//   int selector = __wasm.landingpad_context.selector;
+//   ...
+//
+// Also, does the following for a cleanuppad block with a call to
+// __clang_call_terminate():
+// - Before:
+//   cleanuppad ...
+//   exn = wasm.get.exception();
+//   __clang_call_terminate(exn);
+//
+// - After:
+//   cleanuppad ...
+//   exn = wasm.catch(0); // 0 is a tag for C++
+//   __clang_call_terminate(exn);
+//
+//
+// * Background: WebAssembly EH instructions
+// WebAssembly's try and catch instructions are structured as follows:
+// try
+//   instruction*
+// catch (C++ tag)
+//   instruction*
+// ...
+// catch_all
+//   instruction*
+// try_end
+//
+// A catch instruction in WebAssembly does not correspond to a C++ catch clause.
+// In WebAssembly, there is a single catch instruction for all C++ exceptions.
+// There can be more catch instructions for exceptions in other languages, but
+// they are not generated for now. catch_all catches all exceptions including
+// foreign exceptions (e.g. JavaScript). We turn catchpads into catch (C++ tag)
+// and cleanuppads into catch_all, with one exception: cleanuppad with a call to
+// __clang_call_terminate should be both in catch (C++ tag) and catch_all.
+//
+//
+// * 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, which returns a caught
+// exception object.
+//
+// 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/ADT/SetVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/Triple.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/TargetLowering.h"
+#include "llvm/CodeGen/TargetSubtargetInfo.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/Intrinsics.h"
+#include "llvm/Pass.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "wasmehprepare"
+
+namespace {
+class WasmEHPrepare : public FunctionPass {
+  Type *LPadContextTy = nullptr; // type of 'struct _Unwind_LandingPadContext'
+  GlobalVariable *LPadContextGV = nullptr; // __wasm_lpad_context
+
+  // Field addresses of struct _Unwind_LandingPadContext
+  Value *LPadIndexField = nullptr; // lpad_index field
+  Value *LSDAField = nullptr;      // lsda field
+  Value *SelectorField = nullptr;  // selector
+
+  Function *CatchF = nullptr;           // wasm.catch.extract() intrinsic
+  Function *LPadIndexF = nullptr;       // wasm.landingpad.index() intrinsic
+  Function *LSDAF = nullptr;            // wasm.lsda() intrinsic
+  Function *GetExnF = nullptr;          // wasm.get.exception() intrinsic
+  Function *GetSelectorF = nullptr;     // wasm.get.ehselector() intrinsic
+  Function *CallPersonalityF = nullptr; // _Unwind_CallPersonality() wrapper
+  Function *ClangCallTermF = nullptr;   // __clang_call_terminate() function
+
+  void prepareEHPad(BasicBlock *BB, unsigned Index);
+  void prepareTerminateCleanupPad(BasicBlock *BB);
+
+public:
+  static char ID; // Pass identification, replacement for typeid
+
+  WasmEHPrepare() : FunctionPass(ID) {}
+
+  bool doInitialization(Module &M) override;
+  bool runOnFunction(Function &F) override;
+
+  StringRef getPassName() const override {
+    return "WebAssembly Exception handling preparation";
+  }
+};
+} // end anonymous namespace
+
+char WasmEHPrepare::ID = 0;
+INITIALIZE_PASS(WasmEHPrepare, DEBUG_TYPE, "Prepare WebAssembly exceptions",
+                false, false);
+
+FunctionPass *llvm::createWasmEHPass() { return new WasmEHPrepare(); }
+
+bool WasmEHPrepare::doInitialization(Module &M) {
+  IRBuilder<> IRB(M.getContext());
+  LPadContextTy = StructType::get(IRB.getInt32Ty(),   // lpad_index
+                                  IRB.getInt8PtrTy(), // lsda
+                                  IRB.getInt32Ty()    // selector
+  );
+  return false;
+}
+
+bool WasmEHPrepare::runOnFunction(Function &F) {
+  SmallVector<BasicBlock *, 16> CatchPads;
+  SmallVector<BasicBlock *, 16> CleanupPads;
+  for (BasicBlock &BB : F) {
+    if (!BB.isEHPad())
+      continue;
+    auto *Pad = BB.getFirstNonPHI();
+    if (isa<CatchPadInst>(Pad))
+      CatchPads.push_back(&BB);
+    else if (isa<CleanupPadInst>(Pad))
+      CleanupPads.push_back(&BB);
+  }
+
+  if (CatchPads.empty() && CleanupPads.empty())
+    return false;
+  assert(F.hasPersonalityFn() && "Personality function not found");
+
+  Module &M = *F.getParent();
+  IRBuilder<> IRB(F.getContext());
+
+  // __wasm_lpad_context global variable
+  LPadContextGV = cast<GlobalVariable>(
+      M.getOrInsertGlobal("__wasm_lpad_context", LPadContextTy));
+  LPadIndexField = IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 0,
+                                          "lpad_index_gep");
+  LSDAField =
+      IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 1, "lsda_gep");
+  SelectorField = IRB.CreateConstGEP2_32(LPadContextTy, LPadContextGV, 0, 2,
+                                         "selector_gep");
+
+  // wasm.catch() intinsic, which will be lowered to wasm 'catch' instruction.
+  CatchF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_catch);
+  // wasm.landingpad.index() intrinsic, which is to specify landingpad index
+  LPadIndexF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_landingpad_index);
+  // wasm.lsda() intrinsic. Returns the address of LSDA table for the current
+  // function.
+  LSDAF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_lsda);
+  // wasm.get.exception() and wasm.get.ehselector() intrinsics. Calls to these
+  // are generated in clang.
+  GetExnF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_get_exception);
+  GetSelectorF = Intrinsic::getDeclaration(&M, Intrinsic::wasm_get_ehselector);
+
+  // _Unwind_CallPersonality() wrapper function, which calls the personality
+  CallPersonalityF = cast<Function>(M.getOrInsertFunction(
+      "_Unwind_CallPersonality", IRB.getInt32Ty(), IRB.getInt8PtrTy()));
+  CallPersonalityF->setDoesNotThrow();
+
+  // __clang_call_terminate() function, which is inserted by clang in case a
+  // cleanup throws
+  ClangCallTermF = M.getFunction("__clang_call_terminate");
+
+  unsigned Index = 0;
+  for (auto *BB : CatchPads) {
+    auto *CPI = cast<CatchPadInst>(BB->getFirstNonPHI());
+    // In case of a single catch (...), we don't need to emit LSDA
+    if (CPI->getNumArgOperands() == 1 &&
+        cast<Constant>(CPI->getArgOperand(0))->isNullValue())
+      prepareEHPad(BB, -1);
+    else
+      prepareEHPad(BB, Index++);
+  }
+
+  if (!ClangCallTermF)
+    return !CatchPads.empty();
+
+  // Cleanuppads will turn into catch_all later, but cleanuppads with a call to
+  // __clang_call_terminate() is a special case. __clang_call_terminate() takes
+  // an exception object, so we have to duplicate call in both 'catch <C++ tag>'
+  // and 'catch_all' clauses. Here we only insert a call to catch; the
+  // duplication will be done later. In catch_all, the exception object will be
+  // set to null.
+  for (auto *BB : CleanupPads)
+    for (auto &I : *BB)
+      if (auto *CI = dyn_cast<CallInst>(&I))
+        if (CI->getCalledValue() == ClangCallTermF)
+          prepareEHPad(BB, -1);
+
+  return true;
+}
+
+void WasmEHPrepare::prepareEHPad(BasicBlock *BB, unsigned Index) {
+  assert(BB->isEHPad() && "BB is not an EHPad!");
+  IRBuilder<> IRB(BB->getContext());
+
+  IRB.SetInsertPoint(&*BB->getFirstInsertionPt());
+  // The argument to wasm.catch() is the tag for C++ exceptions, which we set to
+  // 0 for this module.
+  // Pseudocode: void *exn = wasm.catch(0);
+  Instruction *Exn = IRB.CreateCall(CatchF, IRB.getInt32(0), "exn");
+  // Replace the return value of wasm.get.exception() with the return value from
+  // wasm.catch().
+  auto *FPI = cast<FuncletPadInst>(BB->getFirstNonPHI());
+  Instruction *GetExnCI = nullptr, *GetSelectorCI = nullptr;
+  for (auto &U : FPI->uses()) {
+    if (auto *CI = dyn_cast<CallInst>(U.getUser())) {
+      if (CI->getCalledValue() == GetExnF)
+        GetExnCI = CI;
+      else if (CI->getCalledValue() == GetSelectorF)
+        GetSelectorCI = CI;
+    }
+  }
+
+  assert(GetExnCI && "wasm.get.exception() call does not exist");
+  GetExnCI->replaceAllUsesWith(Exn);
+  GetExnCI->eraseFromParent();
+
+  // In case it is a catchpad with single catch (...) or a cleanuppad, we don't
+  // need to call personality function because we don't need a selector.
+  if (FPI->getNumArgOperands() == 0 ||
+      (FPI->getNumArgOperands() == 1 &&
+       cast<Constant>(FPI->getArgOperand(0))->isNullValue())) {
+    if (GetSelectorCI) {
+      assert(GetSelectorCI->use_empty() &&
+             "wasm.get.ehselector() still has uses!");
+      GetSelectorCI->eraseFromParent();
+    }
+    return;
+  }
+  IRB.SetInsertPoint(Exn->getNextNode());
+
+  // This is to create a map of <landingpad EH label, landingpad index> in
+  // SelectionDAGISel, which is to be used in EHStreamer to emit LSDA tables.
+  // Pseudocode: wasm.landingpad.index(Index);
+  IRB.CreateCall(LPadIndexF, IRB.getInt32(Index));
+
+  // Pseudocode: __wasm_lpad_context.lpad_index = index;
+  IRB.CreateStore(IRB.getInt32(Index), LPadIndexField);
+
+  // Store LSDA address only if this catchpad belongs to a top-level
+  // catchswitch. If there is another catchpad that dominates this pad, we don't
+  // need to store LSDA address again, because they are the same throughout the
+  // function and have been already stored before.
+  // TODO Can we not store LSDA address in user function but make libcxxabi
+  // compute it?
+  auto *CPI = cast<CatchPadInst>(FPI);
+  if (isa<ConstantTokenNone>(CPI->getCatchSwitch()->getParentPad()))
+    // Pseudocode: __wasm_lpad_context.lsda = wasm.lsda();
+    IRB.CreateStore(IRB.CreateCall(LSDAF), LSDAField);
+
+  // Pseudocode: _Unwind_CallPersonality(exn);
+  CallInst *PersCI =
+      IRB.CreateCall(CallPersonalityF, Exn, OperandBundleDef("funclet", CPI));
+  PersCI->setDoesNotThrow();
+
+  // Pseudocode: int selector = __wasm.landingpad_context.selector;
+  Instruction *Selector = IRB.CreateLoad(SelectorField, "selector");
+
+  // Replace the return value from wasm.get.ehselector() with the selector value
+  // loaded from __wasm_lpad_context.selector.
+  assert(GetSelectorCI && "wasm.get.ehselector() call does not exist");
+  GetSelectorCI->replaceAllUsesWith(Selector);
+  GetSelectorCI->eraseFromParent();
+}