[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/WebAssemblyInstrInfo.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyInstrInfo.cpp
index 9592130..995dc88 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyInstrInfo.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyInstrInfo.cpp
@@ -134,6 +134,17 @@
else
FBB = MI.getOperand(0).getMBB();
break;
+ case WebAssembly::BR_ON_EXN:
+ if (HaveCond)
+ return true;
+ // If we're running after CFGStackify, we can't optimize further.
+ if (!MI.getOperand(0).isMBB())
+ return true;
+ Cond.push_back(MachineOperand::CreateImm(true));
+ Cond.push_back(MI.getOperand(2));
+ TBB = MI.getOperand(0).getMBB();
+ HaveCond = true;
+ break;
}
if (MI.isBarrier())
break;
@@ -179,9 +190,22 @@
assert(Cond.size() == 2 && "Expected a flag and a successor block");
+ MachineFunction &MF = *MBB.getParent();
+ auto &MRI = MF.getRegInfo();
+ bool IsBrOnExn = Cond[1].isReg() && MRI.getRegClass(Cond[1].getReg()) ==
+ &WebAssembly::EXCEPT_REFRegClass;
+
if (Cond[0].getImm()) {
- BuildMI(&MBB, DL, get(WebAssembly::BR_IF)).addMBB(TBB).add(Cond[1]);
+ if (IsBrOnExn) {
+ const char *CPPExnSymbol = MF.createExternalSymbolName("__cpp_exception");
+ BuildMI(&MBB, DL, get(WebAssembly::BR_ON_EXN))
+ .addMBB(TBB)
+ .addExternalSymbol(CPPExnSymbol, WebAssemblyII::MO_SYMBOL_EVENT)
+ .add(Cond[1]);
+ } else
+ BuildMI(&MBB, DL, get(WebAssembly::BR_IF)).addMBB(TBB).add(Cond[1]);
} else {
+ assert(!IsBrOnExn && "br_on_exn does not have a reversed condition");
BuildMI(&MBB, DL, get(WebAssembly::BR_UNLESS)).addMBB(TBB).add(Cond[1]);
}
if (!FBB)
@@ -193,7 +217,15 @@
bool WebAssemblyInstrInfo::reverseBranchCondition(
SmallVectorImpl<MachineOperand> &Cond) const {
- assert(Cond.size() == 2 && "Expected a flag and a successor block");
+ assert(Cond.size() == 2 && "Expected a flag and a condition expression");
+
+ // br_on_exn's condition cannot be reversed
+ MachineFunction &MF = *Cond[1].getParent()->getParent()->getParent();
+ auto &MRI = MF.getRegInfo();
+ if (Cond[1].isReg() &&
+ MRI.getRegClass(Cond[1].getReg()) == &WebAssembly::EXCEPT_REFRegClass)
+ return true;
+
Cond.front() = MachineOperand::CreateImm(!Cond.front().getImm());
return false;
}