wasm: add recover support

Add unwinding and recover support for wasm using WebAssembly exception
handling. This still has a few gotchas:

  * Many WASI systems don't support exception handling yet.
    For example, see:
    https://github.com/bytecodealliance/wasmtime/issues/2049
  * Asyncify doesn't support wasm exception handling:
    https://github.com/WebAssembly/binaryen/issues/4470
    This means it's not possible to use goroutines together with
    panic/recover.
  * The current way that exceptions are implemented pretend to be C++
    exceptions, but work slightly differently. If C++ code is called
    (for example through CGo) that raises an exception, that exception
    will be eaten by TinyGo and not be propagated. This is fixable, it
    just hasn't been implemented (because we don't actually support C++
    right now).

I hope that these issues will be resolved over time. At least for now,
people who need `recover()` have a way to use it.

compiler/calls.go

@@ -70,21 +70,25 @@ func (b *builder) createRuntimeInvoke(fnName string, args []llvm.Value, name str
 // createCall creates a call to the given function with the arguments possibly
 // expanded.
 func (b *builder) createCall(fnType llvm.Type, fn llvm.Value, args []llvm.Value, name string) llvm.Value {
-	expanded := make([]llvm.Value, 0, len(args))
-	for _, arg := range args {
-		fragments := b.expandFormalParam(arg)
-		expanded = append(expanded, fragments...)
-	}
-	return b.CreateCall(fnType, fn, expanded, name)
+	return b.CreateCall(fnType, fn, b.expandFormalParams(args), name)
 }
 
 // createInvoke is like createCall but continues execution at the landing pad if
 // the call resulted in a panic.
 func (b *builder) createInvoke(fnType llvm.Type, fn llvm.Value, args []llvm.Value, name string) llvm.Value {
-	if b.hasDeferFrame() {
+	switch b.deferFrameType() {
+	case recoverInlineAsm:
 		b.createInvokeCheckpoint()
+		return b.createCall(fnType, fn, args, name)
+	case recoverWasmEH:
+		continueBB := b.insertBasicBlock("invoke.cont")
+		call := b.CreateInvoke(fnType, fn, b.expandFormalParams(args), continueBB, b.landingpad, name)
+		b.SetInsertPointAtEnd(continueBB)
+		b.blockExits[b.currentBlock] = continueBB
+		return call
+	default:
+		return b.createCall(fnType, fn, args, name)
 	}
-	return b.createCall(fnType, fn, args, name)
 }
 
 // Expand an argument type to a list that can be used in a function call
@@ -123,6 +127,17 @@ func (b *builder) expandFormalParamOffsets(t llvm.Type) []uint64 {
 	}
 }
 
+// expandFormalParams expands every param in the params slice like
+// expandFormalParam.
+func (b *builder) expandFormalParams(params []llvm.Value) []llvm.Value {
+	expanded := make([]llvm.Value, 0, len(params))
+	for _, arg := range params {
+		fragments := b.expandFormalParam(arg)
+		expanded = append(expanded, fragments...)
+	}
+	return expanded
+}
+
 // expandFormalParam splits a formal param value into pieces, so it can be
 // passed directly as part of a function call. For example, it splits up small
 // structs into individual fields. It is the equivalent of expandFormalParamType