[WebAssembly] Exception handling: Switch to the new proposal

Summary:
This switches the EH implementation to the new proposal:
https://github.com/WebAssembly/exception-handling/blob/master/proposals/Exceptions.md
(The previous proposal was
 https://github.com/WebAssembly/exception-handling/blob/master/proposals/old/Exceptions.md)

- Instruction changes
  - Now we have one single `catch` instruction that returns a except_ref
    value
  - `throw` now can take variable number of operations
  - `rethrow` does not have 'depth' argument anymore
  - `br_on_exn` queries an except_ref to see if it matches the tag and
    branches to the given label if true.
  - `extract_exception` is a pseudo instruction that simulates popping
    values from wasm stack. This is to make `br_on_exn`, a very special
    instruction, work: `br_on_exn` puts values onto the stack only if it
    is taken, and the # of values can vay depending on the tag.

- Now there's only one `catch` per `try`, this patch removes all special
  handling for terminate pad with a call to `__clang_call_terminate`.
  Before it was the only case there are two catch clauses (a normal
  `catch` and `catch_all` per `try`).

- Make `rethrow` act as a terminator like `throw`. This splits BB after
  `rethrow` in WasmEHPrepare, and deletes an unnecessary `unreachable`
  after `rethrow` in LateEHPrepare.

- Now we stop at all catchpads (because we add wasm `catch` instruction
  that catches all exceptions), this creates new
  `findWasmUnwindDestinations` function in SelectionDAGBuilder.

- Now we use `br_on_exn` instrution to figure out if an except_ref
  matches the current tag or not, LateEHPrepare generates this sequence
  for catch pads:
```
  catch
  block i32
  br_on_exn $__cpp_exception
  end_block
  extract_exception
```

- Branch analysis for `br_on_exn` in WebAssemblyInstrInfo

- Other various misc. changes to switch to the new proposal.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D57134

llvm-svn: 352598

1 files changed, 36 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 9849725ad1f..155d65f127e 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -1456,6 +1456,36 @@ void SelectionDAGBuilder::visitCleanupPad(const CleanupPadInst &CPI) {
   }
 }
 
+// For wasm, there's alwyas a single catch pad attached to a catchswitch, and
+// the control flow always stops at the single catch pad, as it does for a
+// cleanup pad. In case the exception caught is not of the types the catch pad
+// catches, it will be rethrown by a rethrow.
+static void findWasmUnwindDestinations(
+    FunctionLoweringInfo &FuncInfo, const BasicBlock *EHPadBB,
+    BranchProbability Prob,
+    SmallVectorImpl<std::pair<MachineBasicBlock *, BranchProbability>>
+        &UnwindDests) {
+  while (EHPadBB) {
+    const Instruction *Pad = EHPadBB->getFirstNonPHI();
+    if (isa<CleanupPadInst>(Pad)) {
+      // Stop on cleanup pads.
+      UnwindDests.emplace_back(FuncInfo.MBBMap[EHPadBB], Prob);
+      UnwindDests.back().first->setIsEHScopeEntry();
+      break;
+    } else if (auto *CatchSwitch = dyn_cast<CatchSwitchInst>(Pad)) {
+      // Add the catchpad handlers to the possible destinations. We don't
+      // continue to the unwind destination of the catchswitch for wasm.
+      for (const BasicBlock *CatchPadBB : CatchSwitch->handlers()) {
+        UnwindDests.emplace_back(FuncInfo.MBBMap[CatchPadBB], Prob);
+        UnwindDests.back().first->setIsEHScopeEntry();
+      }
+      break;
+    } else {
+      continue;
+    }
+  }
+}
+
 /// When an invoke or a cleanupret unwinds to the next EH pad, there are
 /// many places it could ultimately go. In the IR, we have a single unwind
 /// destination, but in the machine CFG, we enumerate all the possible blocks.
@@ -1476,6 +1506,11 @@ static void findUnwindDestinations(
   bool IsWasmCXX = Personality == EHPersonality::Wasm_CXX;
   bool IsSEH = isAsynchronousEHPersonality(Personality);
 
+  if (IsWasmCXX) {
+    findWasmUnwindDestinations(FuncInfo, EHPadBB, Prob, UnwindDests);
+    return;
+  }
+
   while (EHPadBB) {
     const Instruction *Pad = EHPadBB->getFirstNonPHI();
     BasicBlock *NewEHPadBB = nullptr;
@@ -1488,8 +1523,7 @@ static void findUnwindDestinations(
       // personalities.
       UnwindDests.emplace_back(FuncInfo.MBBMap[EHPadBB], Prob);
       UnwindDests.back().first->setIsEHScopeEntry();
-      if (!IsWasmCXX)
-        UnwindDests.back().first->setIsEHFuncletEntry();
+      UnwindDests.back().first->setIsEHFuncletEntry();
       break;
     } else if (auto *CatchSwitch = dyn_cast<CatchSwitchInst>(Pad)) {
       // Add the catchpad handlers to the possible destinations.