[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
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 @@
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 @@
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;