compiler: support function pointers outside of addrspace 0

In LLVM 8, the AVR backend has moved all function pointers to address
space 1 by default. Much of the code still assumes function pointers
live in address space 0, leading to assertion failures.

This commit fixes this problem by autodetecting function pointers and
avoiding them in interface pseudo-calls.

compiler/compiler.go

@@ -57,6 +57,7 @@ type Compiler struct {
 	targetData              llvm.TargetData
 	intType                 llvm.Type
 	i8ptrType               llvm.Type // for convenience
+	funcPtrAddrSpace        int
 	uintptrType             llvm.Type
 	initFuncs               []llvm.Value
 	interfaceInvokeWrappers []interfaceInvokeWrapper
@@ -125,6 +126,11 @@ func NewCompiler(pkgName string, config Config) (*Compiler, error) {
 	}
 	c.i8ptrType = llvm.PointerType(c.ctx.Int8Type(), 0)
 
+	dummyFuncType := llvm.FunctionType(c.ctx.VoidType(), nil, false)
+	dummyFunc := llvm.AddFunction(c.mod, "tinygo.dummy", dummyFuncType)
+	c.funcPtrAddrSpace = dummyFunc.Type().PointerAddressSpace()
+	dummyFunc.EraseFromParentAsFunction()
+
 	return c, nil
 }
 
@@ -462,7 +468,7 @@ func (c *Compiler) getLLVMType(goType types.Type) (llvm.Type, error) {
 		// {context, funcptr}
 		paramTypes = append(paramTypes, c.i8ptrType) // context
 		paramTypes = append(paramTypes, c.i8ptrType) // parent coroutine
-		ptr := llvm.PointerType(llvm.FunctionType(returnType, paramTypes, false), 0)
+		ptr := llvm.PointerType(llvm.FunctionType(returnType, paramTypes, false), c.funcPtrAddrSpace)
 		ptr = c.ctx.StructType([]llvm.Type{c.i8ptrType, ptr}, false)
 		return ptr, nil
 	case *types.Slice: