[PATCH] D44931: [WebAssembly] Use Windows EH instructions for Wasm EH

[Heejin Ahn via Phabricator via cfe-commits]

aheejin added a comment.

Thank you for the reviews!



================
Comment at: lib/CodeGen/CGException.cpp:1173
+        cast<llvm::CatchPadInst>(CatchStartBlock->getFirstNonPHI());
+    CurrentFuncletPad = CPI;
+  }
----------------
majnemer wrote:
> Hmm, why is this done? Won't RestoreCurrentFuncletPad undo this?
Isn't `RestoreCurrentFuncletPad` outside of `if (EHPersonality::get(*this).isWasmPersonality())`? Isn't this supposed to stay until this function finishes?


================
Comment at: lib/CodeGen/CGException.cpp:1241-1245
+    while (llvm::TerminatorInst *TI = RethrowBlock->getTerminator()) {
+      llvm::BranchInst *BI = cast<llvm::BranchInst>(TI);
+      assert(BI->isConditional());
+      RethrowBlock = BI->getSuccessor(1);
+    }
----------------
majnemer wrote:
> This seems pretty fragile, why is this guaranteed to work? Could we maintain a map from CatchSwitchInst to catch-all block?
The function call sequence here is `CodeGenFunction::ExitCXXTryStmt` -> `emitCatchDispatchBlock` (static) -> `emitWasmCatchDispatchBlock` (static) and `emitCatchDispatchBlock` also has other callers, so it is a little cumbersome to pass a map to those functions to be filled in. (We have to make a parameter that's only gonna be used for wasm to both `emitCatchDispatchBlock` and `emitWasmCatchDispatchBlock`)

The other way is also change those static `emit` functions into `CodeGenFunction` class's member functions and make the map as a member variable.

But first, in which case do you think this will be fragile? `emitWasmCatchDispatchBlock` follows the structure of the landingpad model, so for a C++ code like this
```
try {
  ...
} catch (int) {
  ...
} catch (float) {
  ...
}
```
the BB structure that starts from wasm's `catch.start` block will look like
```
catch.dispatch:                                   ; preds = %entry
  %0 = catchswitch within none [label %catch.start] unwind to caller

catch.start:                                      ; preds = %catch.dispatch
  %1 = catchpad within %0 [i8* bitcast (i8** @_ZTIi to i8*), i8* bitcast (i8** @_ZTIf to i8*)]
  %2 = call i8* @llvm.wasm.get.exception()
  %3 = call i32 @llvm.wasm.get.ehselector()
  %4 = call i32 @llvm.eh.typeid.for(i8* bitcast (i8** @_ZTIi to i8*)) #2
  %matches = icmp eq i32 %3, %4
  br i1 %matches, label %catch12, label %catch.fallthrough

catch12:                                          ; preds = %catch.start
  body of catch (int)

catch.fallthrough:                                ; preds = %catch.start
  %8 = call i32 @llvm.eh.typeid.for(i8* bitcast (i8** @_ZTIf to i8*)) #2
  %matches1 = icmp eq i32 %3, %8
  br i1 %matches1, label %catch, label %rethrow

catch:                                            ; preds = %catch.fallthrough
  body of catch (float)

rethrow:                                          ; preds = %catch.fallthrough
  call void @__cxa_rethrow() #5 [ "funclet"(token %1) ]
  unreachable
```

So to me it looks like, no matter how the bodies of `catch (int)` or `catch (float)` are complicated, there should always be blocks like `catch.start` and `catch.fallthrough`, which compares typeids and divide control flow depending on the typeid comparison. I could very well be mistaken, so please let me know if so.


Repository:
  rC Clang

https://reviews.llvm.org/D44931