[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, 30 insertions, 0 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
index 7b9b494..957a713 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
@@ -317,6 +317,18 @@ static bool IsSafeToMove(const MachineInstr *Def, const MachineInstr *Insert,
                          AliasAnalysis &AA, const MachineRegisterInfo &MRI) {
   assert(Def->getParent() == Insert->getParent());
 
+  // 'catch' and 'extract_exception' should be the first instruction of a BB and
+  // cannot move.
+  if (Def->getOpcode() == WebAssembly::CATCH ||
+      Def->getOpcode() == WebAssembly::EXTRACT_EXCEPTION_I32) {
+    const MachineBasicBlock *MBB = Def->getParent();
+    auto NextI = std::next(MachineBasicBlock::const_iterator(Def));
+    for (auto E = MBB->end(); NextI != E && NextI->isDebugInstr(); ++NextI)
+      ;
+    if (NextI != Insert)
+      return false;
+  }
+
   // Check for register dependencies.
   SmallVector<unsigned, 4> MutableRegisters;
   for (const MachineOperand &MO : Def->operands()) {
@@ -819,6 +831,24 @@ bool WebAssemblyRegStackify::runOnMachineFunction(MachineFunction &MF) {
         if (WebAssembly::isArgument(*Def))
           continue;
 
+        // Currently catch's return value register cannot be stackified, because
+        // the wasm LLVM backend currently does not support live-in values
+        // entering blocks, which is a part of multi-value proposal.
+        //
+        // Once we support live-in values of wasm blocks, this can be:
+        // catch                           ; push except_ref value onto stack
+        // block except_ref -> i32
+        // br_on_exn $__cpp_exception      ; pop the except_ref value
+        // end_block
+        //
+        // But because we don't support it yet, the catch instruction's dst
+        // register should be assigned to a local to be propagated across
+        // 'block' boundary now.
+        //
+        // TODO Fix this once we support the multi-value proposal.
+        if (Def->getOpcode() == WebAssembly::CATCH)
+          continue;
+
         // Decide which strategy to take. Prefer to move a single-use value
         // over cloning it, and prefer cloning over introducing a tee.
         // For moving, we require the def to be in the same block as the use;