all: complete the implementation of interface asserts

Because a few things were left unimplemented it only happened to kind-of
work before in my test cases.
This commit should complete interface-to-interface type asserts.

compiler.go

@@ -446,12 +446,14 @@ func (c *Compiler) Parse(mainPath string, buildTags []string) error {
 	}
 
 	interfaceTypes := c.ir.AllInterfaces()
+	interfaceIndex := make([]llvm.Value, len(interfaceTypes))
 	interfaceLengths := make([]llvm.Value, len(interfaceTypes))
 	interfaceMethods := make([]llvm.Value, 0)
 	for i, itfType := range interfaceTypes {
 		if itfType.Type.NumMethods() > 0xff {
 			return errors.New("too many methods for interface " + itfType.Type.String())
 		}
+		interfaceIndex[i] = llvm.ConstInt(llvm.Int16Type(), uint64(i), false)
 		interfaceLengths[i] = llvm.ConstInt(llvm.Int8Type(), uint64(itfType.Type.NumMethods()), false)
 		funcs := make([]*types.Func, itfType.Type.NumMethods())
 		for i := range funcs {
@@ -493,6 +495,14 @@ func (c *Compiler) Parse(mainPath string, buildTags []string) error {
 	signatureArrayOldGlobal.ReplaceAllUsesWith(llvm.ConstBitCast(signatureArrayNewGlobal, signatureArrayOldGlobal.Type()))
 	signatureArrayOldGlobal.EraseFromParentAsGlobal()
 	signatureArrayNewGlobal.SetName("runtime.methodSetSignatures")
+	interfaceIndexArray := llvm.ConstArray(llvm.Int16Type(), interfaceIndex)
+	interfaceIndexArrayNewGlobal := llvm.AddGlobal(c.mod, interfaceIndexArray.Type(), "runtime.interfaceIndex.tmp")
+	interfaceIndexArrayNewGlobal.SetInitializer(interfaceIndexArray)
+	interfaceIndexArrayNewGlobal.SetLinkage(llvm.InternalLinkage)
+	interfaceIndexArrayOldGlobal := c.mod.NamedGlobal("runtime.interfaceIndex")
+	interfaceIndexArrayOldGlobal.ReplaceAllUsesWith(llvm.ConstBitCast(interfaceIndexArrayNewGlobal, interfaceIndexArrayOldGlobal.Type()))
+	interfaceIndexArrayOldGlobal.EraseFromParentAsGlobal()
+	interfaceIndexArrayNewGlobal.SetName("runtime.interfaceIndex")
 	interfaceLengthsArray := llvm.ConstArray(llvm.Int8Type(), interfaceLengths)
 	interfaceLengthsArrayNewGlobal := llvm.AddGlobal(c.mod, interfaceLengthsArray.Type(), "runtime.interfaceLengths.tmp")
 	interfaceLengthsArrayNewGlobal.SetInitializer(interfaceLengthsArray)

src/runtime/interface.go

@@ -37,7 +37,7 @@ type methodSetRange struct {
 // in.
 var (
 	firstTypeWithMethods uint16            // the lowest typecode that has at least one method
-	methodSetRanges      [0]methodSetRange // indexes into methodSetSignatures and methodSetFunctions
+	methodSetRanges      [0]methodSetRange // indices into methodSetSignatures and methodSetFunctions
 	methodSetSignatures  [0]uint16         // uniqued method ID
 	methodSetFunctions   [0]*uint8         // function pointer of method
 	interfaceIndex       [0]uint16         // mapping from interface ID to an index in interfaceMethods
@@ -81,15 +81,28 @@ func interfaceEqual(x, y _interface) bool {
 // This is a compiler intrinsic.
 //go:nobounds
 func interfaceImplements(typecode, interfaceNum uint16) bool {
-	// method set indexes of the concrete type
+	// method set indices of the interface
+	itfIndex := interfaceIndex[interfaceNum]
+	itfIndexEnd := itfIndex + uint16(interfaceLengths[interfaceNum])
+
+	if itfIndex == itfIndexEnd {
+		// This interface has no methods, so it satisfies all types.
+		// TODO: this should be figured out at compile time (as it is known at
+		// compile time), so that this check is unnecessary at runtime.
+		return true
+	}
+
+	if typecode < firstTypeWithMethods {
+		// Type has no methods while the interface has (checked above), so this
+		// type does not satisfy this interface.
+		return false
+	}
+
+	// method set indices of the concrete type
 	methodSet := methodSetRanges[typecode-firstTypeWithMethods]
 	methodIndex := methodSet.index
 	methodIndexEnd := methodSet.index + methodSet.length
 
-	// method set indexes of the interface
-	itfIndex := interfaceIndex[interfaceNum]
-	itfIndexEnd := itfIndex + uint16(interfaceLengths[interfaceNum])
-
 	// Iterate over all methods of the interface:
 	for itfIndex < itfIndexEnd {
 		methodId := interfaceMethods[itfIndex]