compiler: lower interfaces in a separate pass

This commit changes many things:

  * Most interface-related operations are moved into an optimization
    pass for more modularity. IR construction creates pseudo-calls which
    are lowered in this pass.
  * Type codes are assigned in this interface lowering pass, after DCE.
  * Type codes are sorted by usage: types more often used in type
    asserts are assigned lower numbers to ease jump table construction
    during machine code generation.
  * Interface assertions are optimized: they are replaced by constant
    false, comparison against a constant, or a typeswitch with only
    concrete types in the general case.
  * Interface calls are replaced with unreachable, direct calls, or a
    concrete type switch with direct calls depending on the number of
    implementing types. This hopefully makes some interface patterns
    zero-cost.

These changes lead to a ~0.5K reduction in code size on Cortex-M for
testdata/interface.go. It appears that a major cause for this is the
replacement of function pointers with direct calls, which are far more
susceptible to optimization. Also, not having a fixed global array of
function pointers greatly helps dead code elimination.

This change also makes future optimizations easier, like optimizations
on interface value comparisons.

compiler/compiler.go

@@ -43,29 +43,30 @@ type Config struct {
 
 type Compiler struct {
 	Config
-	mod              llvm.Module
-	ctx              llvm.Context
-	builder          llvm.Builder
-	dibuilder        *llvm.DIBuilder
-	cu               llvm.Metadata
-	difiles          map[string]llvm.Metadata
-	ditypes          map[string]llvm.Metadata
-	machine          llvm.TargetMachine
-	targetData       llvm.TargetData
-	intType          llvm.Type
-	i8ptrType        llvm.Type // for convenience
-	uintptrType      llvm.Type
-	coroIdFunc       llvm.Value
-	coroSizeFunc     llvm.Value
-	coroBeginFunc    llvm.Value
-	coroSuspendFunc  llvm.Value
-	coroEndFunc      llvm.Value
-	coroFreeFunc     llvm.Value
-	initFuncs        []llvm.Value
-	deferFuncs       []*ir.Function
-	deferInvokeFuncs []InvokeDeferFunction
-	ctxDeferFuncs    []ContextDeferFunction
-	ir               *ir.Program
+	mod                     llvm.Module
+	ctx                     llvm.Context
+	builder                 llvm.Builder
+	dibuilder               *llvm.DIBuilder
+	cu                      llvm.Metadata
+	difiles                 map[string]llvm.Metadata
+	ditypes                 map[string]llvm.Metadata
+	machine                 llvm.TargetMachine
+	targetData              llvm.TargetData
+	intType                 llvm.Type
+	i8ptrType               llvm.Type // for convenience
+	uintptrType             llvm.Type
+	coroIdFunc              llvm.Value
+	coroSizeFunc            llvm.Value
+	coroBeginFunc           llvm.Value
+	coroSuspendFunc         llvm.Value
+	coroEndFunc             llvm.Value
+	coroFreeFunc            llvm.Value
+	initFuncs               []llvm.Value
+	deferFuncs              []*ir.Function
+	deferInvokeFuncs        []InvokeDeferFunction
+	ctxDeferFuncs           []ContextDeferFunction
+	interfaceInvokeWrappers []interfaceInvokeWrapper
+	ir                      *ir.Program
 }
 
 type Frame struct {
@@ -489,6 +490,15 @@ func (c *Compiler) Compile(mainPath string) error {
 		c.builder.CreateRetVoid()
 	}
 
+	// Define the already declared functions that wrap methods for use in
+	// interfaces.
+	for _, state := range c.interfaceInvokeWrappers {
+		err = c.createInterfaceInvokeWrapper(state)
+		if err != nil {
+			return err
+		}
+	}
+
 	// After all packages are imported, add a synthetic initializer function
 	// that calls the initializer of each package.
 	initFn := c.ir.GetFunction(c.ir.Program.ImportedPackage("runtime").Members["initAll"].(*ssa.Function))
@@ -534,13 +544,6 @@ func (c *Compiler) Compile(mainPath string) error {
 	}
 	c.builder.CreateRetVoid()
 
-	// Add runtime type information for interfaces: interface calls and type
-	// asserts.
-	err = c.createInterfaceRTTI()
-	if err != nil {
-		return err
-	}
-
 	// see: https://reviews.llvm.org/D18355
 	c.mod.AddNamedMetadataOperand("llvm.module.flags",
 		c.ctx.MDNode([]llvm.Metadata{
@@ -995,17 +998,6 @@ func (c *Compiler) getInterpretedValue(prefix string, value ir.Value) (llvm.Valu
 		ptr := llvm.ConstInBoundsGEP(value.Global.LLVMGlobal, []llvm.Value{zero})
 		return ptr, nil
 
-	case *ir.InterfaceValue:
-		underlying := llvm.ConstPointerNull(c.i8ptrType) // could be any 0 value
-		if value.Elem != nil {
-			elem, err := c.getInterpretedValue(prefix, value.Elem)
-			if err != nil {
-				return llvm.Value{}, err
-			}
-			underlying = elem
-		}
-		return c.parseMakeInterface(underlying, value.Type, prefix)
-
 	case *ir.MapValue:
 		// Create initial bucket.
 		firstBucketGlobal, keySize, valueSize, err := c.initMapNewBucket(prefix, value.Type)

compiler/interface-lowering.go

@@ -0,0 +1,715 @@
+package compiler
+
+// This file provides function to lower interface intrinsics to their final LLVM
+// form, optimizing them in the process.
+//
+// During SSA construction, the following pseudo-calls are created:
+//     runtime.makeInterface(typecode, methodSet)
+//     runtime.typeAssert(typecode, assertedType)
+//     runtime.interfaceImplements(typecode, interfaceMethodSet)
+//     runtime.interfaceMethod(typecode, interfaceMethodSet, signature)
+// See src/runtime/interface.go for details.
+// These calls are to declared but not defined functions, so the optimizer will
+// leave them alone.
+//
+// This pass lowers the above functions to their final form:
+//
+// makeInterface:
+//     Replaced with a constant typecode.
+//
+// typeAssert:
+//     Replaced with an icmp instruction so it can be directly used in a type
+//     switch. This is very easy to optimize for LLVM: it will often translate a
+//     type switch into a regular switch statement.
+//     When this type assert is not possible (the type is never used in an
+//     interface with makeInterface), this call is replaced with a constant
+//     false to optimize the type assert away completely.
+//
+// interfaceImplements:
+//     This call is translated into a call that checks whether the underlying
+//     type is one of the types implementing this interface.
+//     When there is only one type implementing this interface, the check is
+//     replaced with a simple icmp instruction, just like a type assert.
+//     When there is no type at all that implements this interface, it is
+//     replaced with a constant false to optimize it completely.
+//
+// interfaceMethod:
+//     This call is replaced with a call to a function that calls the
+//     appropriate method depending on the underlying type.
+//     When there is only one type implementing this interface, this call is
+//     translated into a direct call of that method.
+//     When there is no type implementing this interface, this code is marked
+//     unreachable as there is no way such an interface could be constructed.
+//
+// Note that this way of implementing interfaces is very different from how the
+// main Go compiler implements them. For more details on how the main Go
+// compiler does it: https://research.swtch.com/interfaces
+
+import (
+	"sort"
+	"strings"
+
+	"github.com/aykevl/go-llvm"
+)
+
+// signatureInfo is a Go signature of an interface method. It does not represent
+// any method in particular.
+type signatureInfo struct {
+	name       string
+	methods    []*methodInfo
+	interfaces []*interfaceInfo
+}
+
+// methodName takes a method name like "func String()" and returns only the
+// name, which is "String" in this case.
+func (s *signatureInfo) methodName() string {
+	if !strings.HasPrefix(s.name, "func ") {
+		panic("signature must start with \"func \"")
+	}
+	methodName := s.name[len("func "):]
+	if openingParen := strings.IndexByte(methodName, '('); openingParen < 0 {
+		panic("no opening paren in signature name")
+	} else {
+		return methodName[:openingParen]
+	}
+}
+
+// methodInfo describes a single method on a concrete type.
+type methodInfo struct {
+	*signatureInfo
+	function llvm.Value
+}
+
+// typeInfo describes a single concrete Go type, which can be a basic or a named
+// type. If it is a named type, it may have methods.
+type typeInfo struct {
+	name                string
+	typecode            llvm.Value
+	methodSet           llvm.Value
+	num                 uint64 // the type number after lowering
+	countMakeInterfaces int    // how often this type is used in an interface
+	countTypeAsserts    int    // how often a type assert happens on this method
+	methods             []*methodInfo
+}
+
+// getMethod looks up the method on this type with the given signature and
+// returns it. The method must exist on this type, otherwise getMethod will
+// panic.
+func (t *typeInfo) getMethod(signature *signatureInfo) *methodInfo {
+	for _, method := range t.methods {
+		if method.signatureInfo == signature {
+			return method
+		}
+	}
+	panic("could not find method")
+}
+
+// id returns the fully-qualified type name including import path, removing the
+// $type suffix.
+func (t *typeInfo) id() string {
+	if !strings.HasSuffix(t.name, "$type") {
+		panic("concrete type does not have $type suffix: " + t.name)
+	}
+	return t.name[:len(t.name)-len("$type")]
+}
+
+// typeInfoSlice implements sort.Slice, sorting the most commonly used types
+// first.
+type typeInfoSlice []*typeInfo
+
+func (t typeInfoSlice) Len() int { return len(t) }
+func (t typeInfoSlice) Less(i, j int) bool {
+	// Try to sort the most commonly used types first.
+	if t[i].countTypeAsserts != t[j].countTypeAsserts {
+		return t[i].countTypeAsserts < t[j].countTypeAsserts
+	}
+	if t[i].countMakeInterfaces != t[j].countMakeInterfaces {
+		return t[i].countMakeInterfaces < t[j].countMakeInterfaces
+	}
+	return t[i].name < t[j].name
+}
+func (t typeInfoSlice) Swap(i, j int) { t[i], t[j] = t[j], t[i] }
+
+// interfaceInfo keeps information about a Go interface type, including all
+// methods it has.
+type interfaceInfo struct {
+	name        string                        // name with $interface suffix
+	signatures  []*signatureInfo              // method set
+	types       typeInfoSlice                 // types this interface implements
+	assertFunc  llvm.Value                    // runtime.interfaceImplements replacement
+	methodFuncs map[*signatureInfo]llvm.Value // runtime.interfaceMethod replacements for each signature
+}
+
+// id removes the $interface suffix from the name and returns the clean
+// interface name including import path.
+func (itf *interfaceInfo) id() string {
+	if !strings.HasSuffix(itf.name, "$interface") {
+		panic("interface type does not have $interface suffix: " + itf.name)
+	}
+	return itf.name[:len(itf.name)-len("$interface")]
+}
+
+// lowerInterfacesPass keeps state related to the interface lowering pass. The
+// pass has been implemented as an object type because of its complexity, but
+// should be seen as a regular function call (see LowerInterfaces).
+type lowerInterfacesPass struct {
+	*Compiler
+	types      map[string]*typeInfo
+	signatures map[string]*signatureInfo
+	interfaces map[string]*interfaceInfo
+}
+
+// Lower all interface functions. They are emitted by the compiler as
+// higher-level intrinsics that need some lowering before LLVM can work on them.
+// This is done so that a few cleanup passes can run before assigning the final
+// type codes.
+func (c *Compiler) LowerInterfaces() {
+	p := &lowerInterfacesPass{
+		Compiler:   c,
+		types:      make(map[string]*typeInfo),
+		signatures: make(map[string]*signatureInfo),
+		interfaces: make(map[string]*interfaceInfo),
+	}
+	p.run()
+}
+
+// run runs the pass itself.
+func (p *lowerInterfacesPass) run() {
+	// Count per type how often it is put in an interface. Also, collect all
+	// methods this type has (if it is named).
+	makeInterface := p.mod.NamedFunction("runtime.makeInterface")
+	makeInterfaceUses := getUses(makeInterface)
+	for _, use := range makeInterfaceUses {
+		typecode := use.Operand(0)
+		name := typecode.Name()
+		if t, ok := p.types[name]; !ok {
+			// This is the first time this type has been seen, add it to the
+			// list of types.
+			t = p.addType(typecode)
+			p.addTypeMethods(t, use.Operand(1))
+		} else {
+			p.addTypeMethods(t, use.Operand(1))
+		}
+
+		// Count the number of MakeInterface instructions, for sorting the
+		// typecodes later.
+		p.types[name].countMakeInterfaces++
+	}
+
+	// Count per type how often it is type asserted on (e.g. in a switch
+	// statement).
+	typeAssert := p.mod.NamedFunction("runtime.typeAssert")
+	typeAssertUses := getUses(typeAssert)
+	for _, use := range typeAssertUses {
+		typecode := use.Operand(1)
+		name := typecode.Name()
+		if _, ok := p.types[name]; !ok {
+			p.addType(typecode)
+		}
+		p.types[name].countTypeAsserts++
+	}
+
+	// Find all interface method calls.
+	interfaceMethod := p.mod.NamedFunction("runtime.interfaceMethod")
+	interfaceMethodUses := getUses(interfaceMethod)
+	for _, use := range interfaceMethodUses {
+		methodSet := use.Operand(1).Operand(0)
+		name := methodSet.Name()
+		if _, ok := p.interfaces[name]; !ok {
+			p.addInterface(methodSet)
+		}
+	}
+
+	// Find all interface type asserts.
+	interfaceImplements := p.mod.NamedFunction("runtime.interfaceImplements")
+	interfaceImplementsUses := getUses(interfaceImplements)
+	for _, use := range interfaceImplementsUses {
+		methodSet := use.Operand(1).Operand(0)
+		name := methodSet.Name()
+		if _, ok := p.interfaces[name]; !ok {
+			p.addInterface(methodSet)
+		}
+	}
+
+	// Find all the interfaces that are implemented per type.
+	for _, t := range p.types {
+		// This type has no methods, so don't spend time calculating them.
+		if len(t.methods) == 0 {
+			continue
+		}
+
+		// Pre-calculate a set of signatures that this type has, for easy
+		// lookup/check.
+		typeSignatureSet := make(map[*signatureInfo]struct{})
+		for _, method := range t.methods {
+			typeSignatureSet[method.signatureInfo] = struct{}{}
+		}
+
+		// A set of interfaces, mapped from the name to the info.
+		// When the name maps to a nil pointer, one of the methods of this type
+		// exists in the given interface but not all of them so this type
+		// doesn't implement the interface.
+		satisfiesInterfaces := make(map[string]*interfaceInfo)
+
+		for _, method := range t.methods {
+			for _, itf := range method.interfaces {
+				if _, ok := satisfiesInterfaces[itf.name]; ok {
+					// interface already checked with a different method
+					continue
+				}
+				// check whether this interface satisfies this type
+				satisfies := true
+				for _, itfSignature := range itf.signatures {
+					if _, ok := typeSignatureSet[itfSignature]; !ok {
+						satisfiesInterfaces[itf.name] = nil // does not satisfy
+						satisfies = false
+						break
+					}
+				}
+				if !satisfies {
+					continue
+				}
+				satisfiesInterfaces[itf.name] = itf
+			}
+		}
+
+		// Add this type to all interfaces that satisfy this type.
+		for _, itf := range satisfiesInterfaces {
+			if itf == nil {
+				// Interface does not implement this type, but one of the
+				// methods on this type also exists on the interface.
+				continue
+			}
+			itf.types = append(itf.types, t)
+		}
+	}
+
+	// Sort all types added to the interfaces, to check for more common types
+	// first.
+	for _, itf := range p.interfaces {
+		sort.Sort(itf.types)
+	}
+
+	// Replace all interface methods with their uses, if possible.
+	for _, use := range interfaceMethodUses {
+		typecode := use.Operand(0)
+		signature := p.signatures[use.Operand(2).Name()]
+
+		// If the interface was created in the same function, we can insert a
+		// direct call. This may not happen often but it is an easy
+		// optimization so let's do it anyway.
+		if !typecode.IsACallInst().IsNil() && typecode.CalledValue() == makeInterface {
+			name := typecode.Operand(0).Name()
+			typ := p.types[name]
+			p.replaceInvokeWithCall(use, typ, signature)
+			continue
+		}
+
+		methodSet := use.Operand(1).Operand(0) // global variable
+		itf := p.interfaces[methodSet.Name()]
+		if len(itf.types) == 0 {
+			// This method call is impossible: no type implements this
+			// interface. In fact, the previous type assert that got this
+			// interface value should already have returned false.
+			// Replace the function pointer with undef (which will then be
+			// called), indicating to the optimizer this code is unreachable.
+			use.ReplaceAllUsesWith(llvm.Undef(p.i8ptrType))
+			use.EraseFromParentAsInstruction()
+		} else if len(itf.types) == 1 {
+			// There is only one implementation of the given type.
+			// Call that function directly.
+			p.replaceInvokeWithCall(use, itf.types[0], signature)
+		} else {
+			// There are multiple types implementing this interface, thus there
+			// are multiple possible functions to call. Delegate calling the
+			// right function to a special wrapper function.
+			bitcasts := getUses(use)
+			if len(bitcasts) != 1 || bitcasts[0].IsABitCastInst().IsNil() {
+				panic("expected exactly one bitcast use of runtime.interfaceMethod")
+			}
+			bitcast := bitcasts[0]
+			calls := getUses(bitcast)
+			if len(calls) != 1 || calls[0].IsACallInst().IsNil() {
+				panic("expected exactly one call use of runtime.interfaceMethod")
+			}
+			call := calls[0]
+
+			// Set up parameters for the call. First copy the regular params...
+			params := make([]llvm.Value, call.OperandsCount())
+			paramTypes := make([]llvm.Type, len(params))
+			for i := 0; i < len(params)-1; i++ {
+				params[i] = call.Operand(i)
+				paramTypes[i] = params[i].Type()
+			}
+			// then add the typecode to the end of the list.
+			params[len(params)-1] = typecode
+			paramTypes[len(params)-1] = p.uintptrType
+
+			// Create a function that redirects the call to the destination
+			// call, after selecting the right concrete type.
+			redirector := p.getInterfaceMethodFunc(itf, signature, call.Type(), paramTypes)
+
+			// Replace the old lookup/bitcast/call with the new call.
+			p.builder.SetInsertPointBefore(call)
+			retval := p.builder.CreateCall(redirector, params, "")
+			if retval.Type().TypeKind() != llvm.VoidTypeKind {
+				call.ReplaceAllUsesWith(retval)
+			}
+			call.EraseFromParentAsInstruction()
+			bitcast.EraseFromParentAsInstruction()
+			use.EraseFromParentAsInstruction()
+		}
+	}
+
+	// Replace all typeasserts on interface types with matches on their concrete
+	// types, if possible.
+	for _, use := range interfaceImplementsUses {
+		actualType := use.Operand(0)
+		if !actualType.IsACallInst().IsNil() && actualType.CalledValue() == makeInterface {
+			// Type assert is in the same function that creates the interface
+			// value. This means the underlying type is already known so match
+			// on that.
+			// This may not happen often but it is an easy optimization.
+			name := actualType.Operand(0).Name()
+			typ := p.types[name]
+			p.builder.SetInsertPointBefore(use)
+			assertedType := p.builder.CreatePtrToInt(typ.typecode, p.uintptrType, "typeassert.typecode")
+			commaOk := p.builder.CreateICmp(llvm.IntEQ, assertedType, actualType, "typeassert.ok")
+			use.ReplaceAllUsesWith(commaOk)
+			use.EraseFromParentAsInstruction()
+			continue
+		}
+
+		methodSet := use.Operand(1).Operand(0) // global variable
+		itf := p.interfaces[methodSet.Name()]
+		if len(itf.types) == 0 {
+			// There are no types implementing this interface, so this assert
+			// can never succeed.
+			// Signal this to the optimizer by branching on constant false. It
+			// should remove the "then" block.
+			use.ReplaceAllUsesWith(llvm.ConstInt(p.ctx.Int1Type(), 0, false))
+			use.EraseFromParentAsInstruction()
+		} else if len(itf.types) == 1 {
+			// There is only one type implementing this interface.
+			// Transform this interface assert into comparison against a
+			// constant.
+			p.builder.SetInsertPointBefore(use)
+			assertedType := p.builder.CreatePtrToInt(itf.types[0].typecode, p.uintptrType, "typeassert.typecode")
+			commaOk := p.builder.CreateICmp(llvm.IntEQ, assertedType, actualType, "typeassert.ok")
+			use.ReplaceAllUsesWith(commaOk)
+			use.EraseFromParentAsInstruction()
+		} else {
+			// There are multiple possible types implementing this interface.
+			// Create a function that does a type switch on all available types
+			// that implement this interface.
+			fn := p.getInterfaceImplementsFunc(itf)
+			p.builder.SetInsertPointBefore(use)
+			commaOk := p.builder.CreateCall(fn, []llvm.Value{actualType}, "typeassert.ok")
+			use.ReplaceAllUsesWith(commaOk)
+			use.EraseFromParentAsInstruction()
+		}
+	}
+
+	// Make a slice of types sorted by frequency of use.
+	typeSlice := make(typeInfoSlice, 0, len(p.types))
+	for _, t := range p.types {
+		typeSlice = append(typeSlice, t)
+	}
+	sort.Sort(typeSlice)
+
+	// A type code must fit in 16 bits.
+	if len(typeSlice) >= 1<<16 {
+		panic("typecode does not fit in a uint16: too many types in this program")
+	}
+
+	// Assign a type code for each type.
+	for i, t := range typeSlice {
+		t.num = uint64(i + 1)
+	}
+
+	// Replace each call to runtime.makeInterface with the constant type code.
+	for _, use := range makeInterfaceUses {
+		global := use.Operand(0)
+		t := p.types[global.Name()]
+		use.ReplaceAllUsesWith(llvm.ConstPtrToInt(t.typecode, p.uintptrType))
+		use.EraseFromParentAsInstruction()
+	}
+
+	// Replace each type assert with an actual type comparison or (if the type
+	// assert is impossible) the constant false.
+	for _, use := range typeAssertUses {
+		actualType := use.Operand(0)
+		assertedTypeGlobal := use.Operand(1)
+		t := p.types[assertedTypeGlobal.Name()]
+		var commaOk llvm.Value
+		if t.countMakeInterfaces == 0 {
+			// impossible type assert: optimize accordingly
+			commaOk = llvm.ConstInt(llvm.Int1Type(), 0, false)
+		} else {
+			// regular type assert
+			p.builder.SetInsertPointBefore(use)
+			commaOk = p.builder.CreateICmp(llvm.IntEQ, llvm.ConstPtrToInt(assertedTypeGlobal, p.uintptrType), actualType, "typeassert.ok")
+		}
+		use.ReplaceAllUsesWith(commaOk)
+		use.EraseFromParentAsInstruction()
+	}
+
+	// Fill in each helper function for type asserts on interfaces
+	// (interface-to-interface matches).
+	for _, itf := range p.interfaces {
+		if !itf.assertFunc.IsNil() {
+			p.createInterfaceImplementsFunc(itf)
+		}
+		for signature := range itf.methodFuncs {
+			p.createInterfaceMethodFunc(itf, signature)
+		}
+	}
+
+	// Replace all ptrtoint typecode placeholders with their final type code
+	// numbers.
+	for _, typ := range p.types {
+		for _, use := range getUses(typ.typecode) {
+			if use.IsConstant() && use.Opcode() == llvm.PtrToInt {
+				use.ReplaceAllUsesWith(llvm.ConstInt(p.uintptrType, typ.num, false))
+			}
+		}
+	}
+
+	// Remove method sets of types. Unnecessary, but cleans up the IR for
+	// inspection.
+	for _, typ := range p.types {
+		if !typ.methodSet.IsNil() {
+			typ.methodSet.EraseFromParentAsGlobal()
+			typ.methodSet = llvm.Value{}
+		}
+	}
+}
+
+// addType retrieves Go type information based on a i16 global variable.
+// Only the name of the i16 is relevant, the object itself is const-propagated
+// and discared afterwards.
+func (p *lowerInterfacesPass) addType(typecode llvm.Value) *typeInfo {
+	name := typecode.Name()
+	t := &typeInfo{
+		name:     name,
+		typecode: typecode,
+	}
+	p.types[name] = t
+	return t
+}
+
+// addTypeMethods reads the method set of the given type info struct. It
+// retrieves the signatures and the references to the method functions
+// themselves for later type<->interface matching.
+func (p *lowerInterfacesPass) addTypeMethods(t *typeInfo, methodSet llvm.Value) {
+	if !t.methodSet.IsNil() || methodSet.IsNull() {
+		// no methods or methods already read
+		return
+	}
+	methodSet = methodSet.Operand(0) // get global from GEP
+
+	// This type has methods, collect all methods of this type.
+	t.methodSet = methodSet
+	set := methodSet.Initializer() // get value from global
+	for i := 0; i < set.Type().ArrayLength(); i++ {
+		methodData := llvm.ConstExtractValue(set, []uint32{uint32(i)})
+		signatureName := llvm.ConstExtractValue(methodData, []uint32{0}).Name()
+		function := llvm.ConstExtractValue(methodData, []uint32{1}).Operand(0)
+		signature := p.getSignature(signatureName)
+		method := &methodInfo{
+			function:      function,
+			signatureInfo: signature,
+		}
+		signature.methods = append(signature.methods, method)
+		t.methods = append(t.methods, method)
+	}
+}
+
+// addInterface reads information about an interface, which is the
+// fully-qualified name and the signatures of all methods it has.
+func (p *lowerInterfacesPass) addInterface(methodSet llvm.Value) {
+	name := methodSet.Name()
+	t := &interfaceInfo{
+		name: name,
+	}
+	p.interfaces[name] = t
+	methodSet = methodSet.Initializer() // get global value from getelementptr
+	for i := 0; i < methodSet.Type().ArrayLength(); i++ {
+		signatureName := llvm.ConstExtractValue(methodSet, []uint32{uint32(i)}).Name()
+		signature := p.getSignature(signatureName)
+		signature.interfaces = append(signature.interfaces, t)
+		t.signatures = append(t.signatures, signature)
+	}
+}
+
+// getSignature returns a new *signatureInfo, creating it if it doesn't already
+// exist.
+func (p *lowerInterfacesPass) getSignature(name string) *signatureInfo {
+	if _, ok := p.signatures[name]; !ok {
+		p.signatures[name] = &signatureInfo{
+			name: name,
+		}
+	}
+	return p.signatures[name]
+}
+
+// replaceInvokeWithCall replaces a runtime.interfaceMethod + bitcast with a
+// concrete method. This can be done when only one type implements the
+// interface.
+func (p *lowerInterfacesPass) replaceInvokeWithCall(use llvm.Value, typ *typeInfo, signature *signatureInfo) {
+	bitcasts := getUses(use)
+	if len(bitcasts) != 1 || bitcasts[0].IsABitCastInst().IsNil() {
+		panic("expected exactly one bitcast use of runtime.interfaceMethod")
+	}
+	bitcast := bitcasts[0]
+	function := typ.getMethod(signature).function
+	if bitcast.Type() != function.Type() {
+		p.builder.SetInsertPointBefore(use)
+		function = p.builder.CreateBitCast(function, bitcast.Type(), "")
+	}
+	bitcast.ReplaceAllUsesWith(function)
+	bitcast.EraseFromParentAsInstruction()
+	use.EraseFromParentAsInstruction()
+}
+
+// getInterfaceImplementsFunc returns a function that checks whether a given
+// interface type implements a given interface, by checking all possible types
+// that implement this interface.
+func (p *lowerInterfacesPass) getInterfaceImplementsFunc(itf *interfaceInfo) llvm.Value {
+	if !itf.assertFunc.IsNil() {
+		return itf.assertFunc
+	}
+
+	// Create the function and function signature.
+	// TODO: debug info
+	fnName := itf.id() + "$typeassert"
+	fnType := llvm.FunctionType(p.ctx.Int1Type(), []llvm.Type{p.uintptrType}, false)
+	itf.assertFunc = llvm.AddFunction(p.mod, fnName, fnType)
+	itf.assertFunc.Param(0).SetName("actualType")
+
+	// Type asserts will be made for each type, so increment the counter for
+	// those.
+	for _, typ := range itf.types {
+		typ.countTypeAsserts++
+	}
+
+	return itf.assertFunc
+}
+
+// createInterfaceImplementsFunc finishes the work of
+// getInterfaceImplementsFunc, because it needs to run after types have a type
+// code assigned.
+//
+// The type match is implemented using a big type switch over all possible
+// types.
+func (p *lowerInterfacesPass) createInterfaceImplementsFunc(itf *interfaceInfo) {
+	fn := itf.assertFunc
+	fn.SetLinkage(llvm.InternalLinkage)
+	fn.SetUnnamedAddr(true)
+
+	// TODO: debug info
+
+	// Create all used basic blocks.
+	entry := llvm.AddBasicBlock(fn, "entry")
+	thenBlock := llvm.AddBasicBlock(fn, "then")
+	elseBlock := llvm.AddBasicBlock(fn, "else")
+
+	// Add all possible types as cases.
+	p.builder.SetInsertPointAtEnd(entry)
+	actualType := fn.Param(0)
+	sw := p.builder.CreateSwitch(actualType, elseBlock, len(itf.types))
+	for _, typ := range itf.types {
+		sw.AddCase(llvm.ConstInt(p.uintptrType, typ.num, false), thenBlock)
+	}
+
+	// Fill 'then' block (type assert was successful).
+	p.builder.SetInsertPointAtEnd(thenBlock)
+	p.builder.CreateRet(llvm.ConstInt(p.ctx.Int1Type(), 1, false))
+
+	// Fill 'else' block (type asserted failed).
+	p.builder.SetInsertPointAtEnd(elseBlock)
+	p.builder.CreateRet(llvm.ConstInt(p.ctx.Int1Type(), 0, false))
+}
+
+// getInterfaceMethodFunc return a function that returns a function pointer for
+// calling a method on an interface. It only declares the function,
+// createInterfaceMethodFunc actually defines the function.
+func (p *lowerInterfacesPass) getInterfaceMethodFunc(itf *interfaceInfo, signature *signatureInfo, returnType llvm.Type, params []llvm.Type) llvm.Value {
+	if fn, ok := itf.methodFuncs[signature]; ok {
+		// This function has already been created.
+		return fn
+	}
+	if itf.methodFuncs == nil {
+		// initialize the above map
+		itf.methodFuncs = make(map[*signatureInfo]llvm.Value)
+	}
+
+	// Construct the function name, which is of the form:
+	//     (main.Stringer).String
+	fnName := "(" + itf.id() + ")." + signature.methodName()
+	fnType := llvm.FunctionType(returnType, params, false)
+	fn := llvm.AddFunction(p.mod, fnName, fnType)
+	fn.LastParam().SetName("actualType")
+	itf.methodFuncs[signature] = fn
+	return fn
+}
+
+// createInterfaceMethodFunc finishes the work of getInterfaceMethodFunc,
+// because it needs to run after type codes have been assigned to concrete
+// types.
+//
+// Matching the actual type is implemented using a big type switch over all
+// possible types.
+func (p *lowerInterfacesPass) createInterfaceMethodFunc(itf *interfaceInfo, signature *signatureInfo) {
+	fn := itf.methodFuncs[signature]
+	fn.SetLinkage(llvm.InternalLinkage)
+	fn.SetUnnamedAddr(true)
+
+	// TODO: debug info
+
+	// Create entry block.
+	entry := llvm.AddBasicBlock(fn, "entry")
+
+	// Create default block and make it unreachable (which it is, because all
+	// possible types are checked).
+	defaultBlock := llvm.AddBasicBlock(fn, "default")
+	p.builder.SetInsertPointAtEnd(defaultBlock)
+	p.builder.CreateUnreachable()
+
+	// Create type switch in entry block.
+	p.builder.SetInsertPointAtEnd(entry)
+	actualType := fn.LastParam()
+	sw := p.builder.CreateSwitch(actualType, defaultBlock, len(itf.types))
+
+	// Collect the params that will be passed to the functions to call.
+	// These params exclude the receiver (which may actually consist of multiple
+	// parts).
+	params := make([]llvm.Value, fn.ParamsCount()-2)
+	for i := range params {
+		params[i] = fn.Param(i + 1)
+	}
+
+	// Define all possible functions that can be called.
+	for _, typ := range itf.types {
+		bb := llvm.AddBasicBlock(fn, typ.id())
+		sw.AddCase(llvm.ConstInt(p.uintptrType, typ.num, false), bb)
+
+		// The function we will redirect to when the interface has this type.
+		function := typ.getMethod(signature).function
+
+		p.builder.SetInsertPointAtEnd(bb)
+		receiver := fn.FirstParam()
+		if receiver.Type() != function.FirstParam().Type() {
+			// When the receiver is a pointer, it is not wrapped. This means the
+			// i8* has to be cast to the correct pointer type of the target
+			// function.
+			receiver = p.builder.CreateBitCast(receiver, function.FirstParam().Type(), "")
+		}
+		retval := p.builder.CreateCall(function, append([]llvm.Value{receiver}, params...), "")
+		if retval.Type().TypeKind() == llvm.VoidTypeKind {
+			p.builder.CreateRetVoid()
+		} else {
+			p.builder.CreateRet(retval)
+		}
+	}
+}

compiler/interface.go

@@ -1,5 +1,10 @@
 package compiler
 
+// This file transforms interface-related instructions (*ssa.MakeInterface,
+// *ssa.TypeAssert, calls on interface types) to an intermediate IR form, to be
+// lowered to the final form by the interface lowering pass. See
+// interface-lowering.go for more details.
+
 import (
 	"errors"
 	"go/types"
@@ -32,10 +37,10 @@ func (c *Compiler) parseMakeInterface(val llvm.Value, typ types.Type, global str
 			// Allocate on the heap and put a pointer in the interface.
 			// TODO: escape analysis.
 			sizeValue := llvm.ConstInt(c.uintptrType, size, false)
-			alloc := c.createRuntimeCall("alloc", []llvm.Value{sizeValue}, "")
-			itfValueCast := c.builder.CreateBitCast(alloc, llvm.PointerType(val.Type(), 0), "")
+			alloc := c.createRuntimeCall("alloc", []llvm.Value{sizeValue}, "makeinterface.alloc")
+			itfValueCast := c.builder.CreateBitCast(alloc, llvm.PointerType(val.Type(), 0), "makeinterface.cast.value")
 			c.builder.CreateStore(val, itfValueCast)
-			itfValue = c.builder.CreateBitCast(itfValueCast, c.i8ptrType, "")
+			itfValue = c.builder.CreateBitCast(itfValueCast, c.i8ptrType, "makeinterface.cast.i8ptr")
 		}
 	} else if size == 0 {
 		itfValue = llvm.ConstPointerNull(c.i8ptrType)
@@ -43,38 +48,136 @@ func (c *Compiler) parseMakeInterface(val llvm.Value, typ types.Type, global str
 		// Directly place the value in the interface.
 		switch val.Type().TypeKind() {
 		case llvm.IntegerTypeKind:
-			itfValue = c.builder.CreateIntToPtr(val, c.i8ptrType, "")
+			itfValue = c.builder.CreateIntToPtr(val, c.i8ptrType, "makeinterface.cast.int")
 		case llvm.PointerTypeKind:
-			itfValue = c.builder.CreateBitCast(val, c.i8ptrType, "")
+			itfValue = c.builder.CreateBitCast(val, c.i8ptrType, "makeinterface.cast.ptr")
 		case llvm.StructTypeKind:
 			// A bitcast would be useful here, but bitcast doesn't allow
 			// aggregate types. So we'll bitcast it using an alloca.
 			// Hopefully this will get optimized away.
-			mem := c.builder.CreateAlloca(c.i8ptrType, "")
-			memStructPtr := c.builder.CreateBitCast(mem, llvm.PointerType(val.Type(), 0), "")
+			mem := c.builder.CreateAlloca(c.i8ptrType, "makeinterface.cast.struct")
+			memStructPtr := c.builder.CreateBitCast(mem, llvm.PointerType(val.Type(), 0), "makeinterface.cast.struct.cast")
 			c.builder.CreateStore(val, memStructPtr)
-			itfValue = c.builder.CreateLoad(mem, "")
+			itfValue = c.builder.CreateLoad(mem, "makeinterface.cast.load")
 		default:
 			return llvm.Value{}, errors.New("todo: makeinterface: cast small type to i8*")
 		}
 	}
-	itfTypeNum, _ := c.ir.TypeNum(typ)
-	if itfTypeNum >= 1<<16 {
-		return llvm.Value{}, errors.New("interface typecodes do not fit in a 16-bit integer")
+	itfTypeCodeGlobal := c.getTypeCode(typ)
+	itfMethodSetGlobal, err := c.getTypeMethodSet(typ)
+	if err != nil {
+		return llvm.Value{}, nil
 	}
-	itf := llvm.ConstNamedStruct(c.mod.GetTypeByName("runtime._interface"), []llvm.Value{llvm.ConstInt(c.ctx.Int16Type(), uint64(itfTypeNum), false), llvm.Undef(c.i8ptrType)})
+	itfTypeCode := c.createRuntimeCall("makeInterface", []llvm.Value{itfTypeCodeGlobal, itfMethodSetGlobal}, "makeinterface.typecode")
+	itf := llvm.Undef(c.mod.GetTypeByName("runtime._interface"))
+	itf = c.builder.CreateInsertValue(itf, itfTypeCode, 0, "")
 	itf = c.builder.CreateInsertValue(itf, itfValue, 1, "")
 	return itf, nil
 }
 
+// getTypeCode returns a reference to a type code.
+// It returns a pointer to an external global which should be replaced with the
+// real type in the interface lowering pass.
+func (c *Compiler) getTypeCode(typ types.Type) llvm.Value {
+	global := c.mod.NamedGlobal(typ.String() + "$type")
+	if global.IsNil() {
+		global = llvm.AddGlobal(c.mod, c.ctx.Int8Type(), typ.String()+"$type")
+		global.SetGlobalConstant(true)
+	}
+	return global
+}
+
+// getTypeMethodSet returns a reference (GEP) to a global method set. This
+// method set should be unreferenced after the interface lowering pass.
+func (c *Compiler) getTypeMethodSet(typ types.Type) (llvm.Value, error) {
+	global := c.mod.NamedGlobal(typ.String() + "$methodset")
+	zero := llvm.ConstInt(c.ctx.Int32Type(), 0, false)
+	if !global.IsNil() {
+		// the method set already exists
+		return llvm.ConstGEP(global, []llvm.Value{zero, zero}), nil
+	}
+
+	ms := c.ir.Program.MethodSets.MethodSet(typ)
+	if ms.Len() == 0 {
+		// no methods, so can leave that one out
+		return llvm.ConstPointerNull(llvm.PointerType(c.mod.GetTypeByName("runtime.interfaceMethodInfo"), 0)), nil
+	}
+
+	methods := make([]llvm.Value, ms.Len())
+	interfaceMethodInfoType := c.mod.GetTypeByName("runtime.interfaceMethodInfo")
+	for i := 0; i < ms.Len(); i++ {
+		method := ms.At(i)
+		signatureGlobal := c.getMethodSignature(method.Obj().(*types.Func))
+		f := c.ir.GetFunction(c.ir.Program.MethodValue(method))
+		if f.LLVMFn.IsNil() {
+			// compiler error, so panic
+			panic("cannot find function: " + f.LinkName())
+		}
+		fn, err := c.getInterfaceInvokeWrapper(f)
+		if err != nil {
+			return llvm.Value{}, err
+		}
+		methodInfo := llvm.ConstNamedStruct(interfaceMethodInfoType, []llvm.Value{
+			signatureGlobal,
+			llvm.ConstBitCast(fn, c.i8ptrType),
+		})
+		methods[i] = methodInfo
+	}
+	arrayType := llvm.ArrayType(interfaceMethodInfoType, len(methods))
+	value := llvm.ConstArray(interfaceMethodInfoType, methods)
+	global = llvm.AddGlobal(c.mod, arrayType, typ.String()+"$methodset")
+	global.SetInitializer(value)
+	global.SetGlobalConstant(true)
+	global.SetLinkage(llvm.PrivateLinkage)
+	return llvm.ConstGEP(global, []llvm.Value{zero, zero}), nil
+}
+
+// getInterfaceMethodSet returns a global variable with the method set of the
+// given named interface type. This method set is used by the interface lowering
+// pass.
+func (c *Compiler) getInterfaceMethodSet(typ *types.Named) llvm.Value {
+	global := c.mod.NamedGlobal(typ.String() + "$interface")
+	zero := llvm.ConstInt(c.ctx.Int32Type(), 0, false)
+	if !global.IsNil() {
+		// method set already exist, return it
+		return llvm.ConstGEP(global, []llvm.Value{zero, zero})
+	}
+
+	// Every method is a *i16 reference indicating the signature of this method.
+	methods := make([]llvm.Value, typ.Underlying().(*types.Interface).NumMethods())
+	for i := range methods {
+		method := typ.Underlying().(*types.Interface).Method(i)
+		methods[i] = c.getMethodSignature(method)
+	}
+
+	value := llvm.ConstArray(methods[0].Type(), methods)
+	global = llvm.AddGlobal(c.mod, value.Type(), typ.String()+"$interface")
+	global.SetInitializer(value)
+	global.SetGlobalConstant(true)
+	global.SetLinkage(llvm.PrivateLinkage)
+	return llvm.ConstGEP(global, []llvm.Value{zero, zero})
+}
+
+// getMethodSignature returns a global variable which is a reference to an
+// external *i16 indicating the indicating the signature of this method. It is
+// used during the interface lowering pass.
+func (c *Compiler) getMethodSignature(method *types.Func) llvm.Value {
+	signature := ir.MethodSignature(method)
+	signatureGlobal := c.mod.NamedGlobal("func " + signature)
+	if signatureGlobal.IsNil() {
+		signatureGlobal = llvm.AddGlobal(c.mod, c.ctx.Int8Type(), "func "+signature)
+		signatureGlobal.SetGlobalConstant(true)
+	}
+	return signatureGlobal
+}
+
 // parseTypeAssert will emit the code for a typeassert, used in if statements
-// and in switch statements (Go SSA does not have type switches, only if/else
+// and in type switches (Go SSA does not have type switches, only if/else
 // chains). Note that even though the Go SSA does not contain type switches,
 // LLVM will recognize the pattern and make it a real switch in many cases.
 //
 // Type asserts on concrete types are trivial: just compare type numbers. Type
-// asserts on interfaces are more difficult to implement and so are delegated to
-// a runtime library function.
+// asserts on interfaces are more difficult, see the comments in the function.
 func (c *Compiler) parseTypeAssert(frame *Frame, expr *ssa.TypeAssert) (llvm.Value, error) {
 	itf, err := c.parseExpr(frame, expr.X)
 	if err != nil {
@@ -91,38 +194,24 @@ func (c *Compiler) parseTypeAssert(frame *Frame, expr *ssa.TypeAssert) (llvm.Val
 
 	actualTypeNum := c.builder.CreateExtractValue(itf, 0, "interface.type")
 	commaOk := llvm.Value{}
-	if itf, ok := expr.AssertedType.Underlying().(*types.Interface); ok {
+	if _, ok := expr.AssertedType.Underlying().(*types.Interface); ok {
 		// Type assert on interface type.
-		// This is slightly non-trivial: at runtime the list of methods
-		// needs to be checked to see whether it implements the interface.
-		// At the same time, the interface value itself is unchanged.
-		itfTypeNum := c.ir.InterfaceNum(itf)
-		itfTypeNumValue := llvm.ConstInt(c.ctx.Int16Type(), uint64(itfTypeNum), false)
-		commaOk = c.createRuntimeCall("interfaceImplements", []llvm.Value{actualTypeNum, itfTypeNumValue}, "")
+		// This pseudo call will be lowered in the interface lowering pass to a
+		// real call which checks whether the provided typecode is any of the
+		// concrete types that implements this interface.
+		// This is very different from how interface asserts are implemented in
+		// the main Go compiler, where the runtime checks whether the type
+		// implements each method of the interface. See:
+		// https://research.swtch.com/interfaces
+		methodSet := c.getInterfaceMethodSet(expr.AssertedType.(*types.Named))
+		commaOk = c.createRuntimeCall("interfaceImplements", []llvm.Value{actualTypeNum, methodSet}, "")
 
 	} else {
 		// Type assert on concrete type.
-		// This is easy: just compare the type number.
-		assertedTypeNum, typeExists := c.ir.TypeNum(expr.AssertedType)
-		if !typeExists {
-			// Static analysis has determined this type assert will never apply.
-			// Using undef here so that LLVM knows we'll never get here and
-			// can optimize accordingly.
-			undef := llvm.Undef(assertedType)
-			commaOk := llvm.ConstInt(c.ctx.Int1Type(), 0, false)
-			if expr.CommaOk {
-				return c.ctx.ConstStruct([]llvm.Value{undef, commaOk}, false), nil
-			} else {
-				c.createRuntimeCall("interfaceTypeAssert", []llvm.Value{commaOk}, "")
-				return undef, nil
-			}
-		}
-		if assertedTypeNum >= 1<<16 {
-			return llvm.Value{}, errors.New("interface typecodes do not fit in a 16-bit integer")
-		}
-
-		assertedTypeNumValue := llvm.ConstInt(c.ctx.Int16Type(), uint64(assertedTypeNum), false)
-		commaOk = c.builder.CreateICmp(llvm.IntEQ, assertedTypeNumValue, actualTypeNum, "")
+		// Call runtime.typeAssert, which will be lowered to a simple icmp or
+		// const false in the interface lowering pass.
+		assertedTypeCodeGlobal := c.getTypeCode(expr.AssertedType)
+		commaOk = c.createRuntimeCall("typeAssert", []llvm.Value{actualTypeNum, assertedTypeCodeGlobal}, "typecode")
 	}
 
 	// Add 2 new basic blocks (that should get optimized away): one for the
@@ -171,16 +260,14 @@ func (c *Compiler) parseTypeAssert(frame *Frame, expr *ssa.TypeAssert) (llvm.Val
 				valueOk = c.builder.CreatePtrToInt(valuePtr, assertedType, "typeassert.value.ok")
 			case llvm.PointerTypeKind:
 				valueOk = c.builder.CreateBitCast(valuePtr, assertedType, "typeassert.value.ok")
-			case llvm.StructTypeKind:
+			default: // struct, float, etc.
 				// A bitcast would be useful here, but bitcast doesn't allow
 				// aggregate types. So we'll bitcast it using an alloca.
 				// Hopefully this will get optimized away.
 				mem := c.builder.CreateAlloca(c.i8ptrType, "")
 				c.builder.CreateStore(valuePtr, mem)
-				memStructPtr := c.builder.CreateBitCast(mem, llvm.PointerType(assertedType, 0), "")
-				valueOk = c.builder.CreateLoad(memStructPtr, "typeassert.value.ok")
-			default:
-				return llvm.Value{}, errors.New("todo: typeassert: bitcast small types")
+				memCast := c.builder.CreateBitCast(mem, llvm.PointerType(assertedType, 0), "")
+				valueOk = c.builder.CreateLoad(memCast, "typeassert.value.ok")
 			}
 		}
 	}
@@ -231,7 +318,8 @@ func (c *Compiler) getInvokeCall(frame *Frame, instr *ssa.CallCommon) (llvm.Valu
 	typecode := c.builder.CreateExtractValue(itf, 0, "invoke.typecode")
 	values := []llvm.Value{
 		typecode,
-		llvm.ConstInt(c.ctx.Int16Type(), uint64(c.ir.MethodNum(instr.Method)), false),
+		c.getInterfaceMethodSet(instr.Value.Type().(*types.Named)),
+		c.getMethodSignature(instr.Method),
 	}
 	fn := c.createRuntimeCall("interfaceMethod", values, "invoke.func")
 	fnCast := c.builder.CreateBitCast(fn, llvmFnType, "invoke.func.cast")
@@ -255,142 +343,40 @@ func (c *Compiler) getInvokeCall(frame *Frame, instr *ssa.CallCommon) (llvm.Valu
 	return fnCast, args, nil
 }
 
-// Initialize runtime type information, for interfaces.
-// See src/runtime/interface.go for more details.
-func (c *Compiler) createInterfaceRTTI() error {
-	dynamicTypes := c.ir.AllDynamicTypes()
-	numDynamicTypes := 0
-	for _, meta := range dynamicTypes {
-		numDynamicTypes += len(meta.Methods)
-	}
-	ranges := make([]llvm.Value, 0, len(dynamicTypes))
-	funcPointers := make([]llvm.Value, 0, numDynamicTypes)
-	signatures := make([]llvm.Value, 0, numDynamicTypes)
-	startIndex := 0
-	rangeType := c.mod.GetTypeByName("runtime.methodSetRange")
-	for _, meta := range dynamicTypes {
-		rangeValues := []llvm.Value{
-			llvm.ConstInt(c.ctx.Int16Type(), uint64(startIndex), false),
-			llvm.ConstInt(c.ctx.Int16Type(), uint64(len(meta.Methods)), false),
-		}
-		rangeValue := llvm.ConstNamedStruct(rangeType, rangeValues)
-		ranges = append(ranges, rangeValue)
-		methods := make([]*types.Selection, 0, len(meta.Methods))
-		for _, method := range meta.Methods {
-			methods = append(methods, method)
-		}
-		c.ir.SortMethods(methods)
-		for _, method := range methods {
-			f := c.ir.GetFunction(c.ir.Program.MethodValue(method))
-			if f.LLVMFn.IsNil() {
-				return errors.New("cannot find function: " + f.LinkName())
-			}
-			fn, err := c.wrapInterfaceInvoke(f)
-			if err != nil {
-				return err
-			}
-			fnPtr := llvm.ConstBitCast(fn, c.i8ptrType)
-			funcPointers = append(funcPointers, fnPtr)
-			signatureNum := c.ir.MethodNum(method.Obj().(*types.Func))
-			signature := llvm.ConstInt(c.ctx.Int16Type(), uint64(signatureNum), false)
-			signatures = append(signatures, signature)
-		}
-		startIndex += len(meta.Methods)
-	}
-
-	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(c.ctx.Int16Type(), uint64(i), false)
-		interfaceLengths[i] = llvm.ConstInt(c.ctx.Int8Type(), uint64(itfType.Type.NumMethods()), false)
-		funcs := make([]*types.Func, itfType.Type.NumMethods())
-		for i := range funcs {
-			funcs[i] = itfType.Type.Method(i)
-		}
-		c.ir.SortFuncs(funcs)
-		for _, f := range funcs {
-			id := llvm.ConstInt(c.ctx.Int16Type(), uint64(c.ir.MethodNum(f)), false)
-			interfaceMethods = append(interfaceMethods, id)
-		}
-	}
-
-	if len(ranges) >= 1<<16 {
-		return errors.New("method call numbers do not fit in a 16-bit integer")
-	}
-
-	// Replace the pre-created arrays with the generated arrays.
-	rangeArray := llvm.ConstArray(rangeType, ranges)
-	rangeArrayNewGlobal := llvm.AddGlobal(c.mod, rangeArray.Type(), "runtime.methodSetRanges.tmp")
-	rangeArrayNewGlobal.SetInitializer(rangeArray)
-	rangeArrayNewGlobal.SetLinkage(llvm.InternalLinkage)
-	rangeArrayOldGlobal := c.mod.NamedGlobal("runtime.methodSetRanges")
-	rangeArrayOldGlobal.ReplaceAllUsesWith(llvm.ConstBitCast(rangeArrayNewGlobal, rangeArrayOldGlobal.Type()))
-	rangeArrayOldGlobal.EraseFromParentAsGlobal()
-	rangeArrayNewGlobal.SetName("runtime.methodSetRanges")
-	funcArray := llvm.ConstArray(c.i8ptrType, funcPointers)
-	funcArrayNewGlobal := llvm.AddGlobal(c.mod, funcArray.Type(), "runtime.methodSetFunctions.tmp")
-	funcArrayNewGlobal.SetInitializer(funcArray)
-	funcArrayNewGlobal.SetLinkage(llvm.InternalLinkage)
-	funcArrayOldGlobal := c.mod.NamedGlobal("runtime.methodSetFunctions")
-	funcArrayOldGlobal.ReplaceAllUsesWith(llvm.ConstBitCast(funcArrayNewGlobal, funcArrayOldGlobal.Type()))
-	funcArrayOldGlobal.EraseFromParentAsGlobal()
-	funcArrayNewGlobal.SetName("runtime.methodSetFunctions")
-	signatureArray := llvm.ConstArray(c.ctx.Int16Type(), signatures)
-	signatureArrayNewGlobal := llvm.AddGlobal(c.mod, signatureArray.Type(), "runtime.methodSetSignatures.tmp")
-	signatureArrayNewGlobal.SetInitializer(signatureArray)
-	signatureArrayNewGlobal.SetLinkage(llvm.InternalLinkage)
-	signatureArrayOldGlobal := c.mod.NamedGlobal("runtime.methodSetSignatures")
-	signatureArrayOldGlobal.ReplaceAllUsesWith(llvm.ConstBitCast(signatureArrayNewGlobal, signatureArrayOldGlobal.Type()))
-	signatureArrayOldGlobal.EraseFromParentAsGlobal()
-	signatureArrayNewGlobal.SetName("runtime.methodSetSignatures")
-	interfaceIndexArray := llvm.ConstArray(c.ctx.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(c.ctx.Int8Type(), interfaceLengths)
-	interfaceLengthsArrayNewGlobal := llvm.AddGlobal(c.mod, interfaceLengthsArray.Type(), "runtime.interfaceLengths.tmp")
-	interfaceLengthsArrayNewGlobal.SetInitializer(interfaceLengthsArray)
-	interfaceLengthsArrayNewGlobal.SetLinkage(llvm.InternalLinkage)
-	interfaceLengthsArrayOldGlobal := c.mod.NamedGlobal("runtime.interfaceLengths")
-	interfaceLengthsArrayOldGlobal.ReplaceAllUsesWith(llvm.ConstBitCast(interfaceLengthsArrayNewGlobal, interfaceLengthsArrayOldGlobal.Type()))
-	interfaceLengthsArrayOldGlobal.EraseFromParentAsGlobal()
-	interfaceLengthsArrayNewGlobal.SetName("runtime.interfaceLengths")
-	interfaceMethodsArray := llvm.ConstArray(c.ctx.Int16Type(), interfaceMethods)
-	interfaceMethodsArrayNewGlobal := llvm.AddGlobal(c.mod, interfaceMethodsArray.Type(), "runtime.interfaceMethods.tmp")
-	interfaceMethodsArrayNewGlobal.SetInitializer(interfaceMethodsArray)
-	interfaceMethodsArrayNewGlobal.SetLinkage(llvm.InternalLinkage)
-	interfaceMethodsArrayOldGlobal := c.mod.NamedGlobal("runtime.interfaceMethods")
-	interfaceMethodsArrayOldGlobal.ReplaceAllUsesWith(llvm.ConstBitCast(interfaceMethodsArrayNewGlobal, interfaceMethodsArrayOldGlobal.Type()))
-	interfaceMethodsArrayOldGlobal.EraseFromParentAsGlobal()
-	interfaceMethodsArrayNewGlobal.SetName("runtime.interfaceMethods")
-
-	c.mod.NamedGlobal("runtime.firstTypeWithMethods").SetInitializer(llvm.ConstInt(c.ctx.Int16Type(), uint64(c.ir.FirstDynamicType()), false))
-
-	return nil
+// interfaceInvokeWrapper keeps some state between getInterfaceInvokeWrapper and
+// createInterfaceInvokeWrapper. The former is called during IR construction
+// itself and the latter is called when finishing up the IR.
+type interfaceInvokeWrapper struct {
+	fn           *ir.Function
+	wrapper      llvm.Value
+	receiverType llvm.Type
 }
 
 // Wrap an interface method function pointer. The wrapper takes in a pointer to
 // the underlying value, dereferences it, and calls the real method. This
 // wrapper is only needed when the interface value actually doesn't fit in a
 // pointer and a pointer to the value must be created.
-func (c *Compiler) wrapInterfaceInvoke(f *ir.Function) (llvm.Value, error) {
+func (c *Compiler) getInterfaceInvokeWrapper(f *ir.Function) (llvm.Value, error) {
+	wrapperName := f.LinkName() + "$invoke"
+	wrapper := c.mod.NamedFunction(wrapperName)
+	if !wrapper.IsNil() {
+		// Wrapper already created. Return it directly.
+		return wrapper, nil
+	}
+
+	// Get the expanded receiver type.
 	receiverType, err := c.getLLVMType(f.Params[0].Type())
 	if err != nil {
 		return llvm.Value{}, err
 	}
 	expandedReceiverType := c.expandFormalParamType(receiverType)
 
-	if c.targetData.TypeAllocSize(receiverType) <= c.targetData.TypeAllocSize(c.i8ptrType) && len(expandedReceiverType) == 1 {
-		// nothing to wrap
+	// Does this method even need any wrapping?
+	if len(expandedReceiverType) == 1 && receiverType.TypeKind() == llvm.PointerTypeKind {
+		// Nothing to wrap.
+		// Casting a function signature to a different signature and calling it
+		// with a receiver pointer bitcasted to *i8 (as done in calls on an
+		// interface) is hopefully a safe (defined) operation.
 		return f.LLVMFn, nil
 	}
 
@@ -398,16 +384,30 @@ func (c *Compiler) wrapInterfaceInvoke(f *ir.Function) (llvm.Value, error) {
 	fnType := f.LLVMFn.Type().ElementType()
 	paramTypes := append([]llvm.Type{c.i8ptrType}, fnType.ParamTypes()[len(expandedReceiverType):]...)
 	wrapFnType := llvm.FunctionType(fnType.ReturnType(), paramTypes, false)
-	wrapper := llvm.AddFunction(c.mod, f.LinkName()+"$invoke", wrapFnType)
+	wrapper = llvm.AddFunction(c.mod, wrapperName, wrapFnType)
+	c.interfaceInvokeWrappers = append(c.interfaceInvokeWrappers, interfaceInvokeWrapper{
+		fn:           f,
+		wrapper:      wrapper,
+		receiverType: receiverType,
+	})
+	return wrapper, nil
+}
+
+// createInterfaceInvokeWrapper finishes the work of getInterfaceInvokeWrapper,
+// see that function for details.
+func (c *Compiler) createInterfaceInvokeWrapper(state interfaceInvokeWrapper) error {
+	wrapper := state.wrapper
+	fn := state.fn
+	receiverType := state.receiverType
 	wrapper.SetLinkage(llvm.InternalLinkage)
 	wrapper.SetUnnamedAddr(true)
 
-	// add debug info
+	// add debug info if needed
 	if c.Debug {
-		pos := c.ir.Program.Fset.Position(f.Pos())
-		difunc, err := c.attachDebugInfoRaw(f, wrapper, "$invoke", pos.Filename, pos.Line)
+		pos := c.ir.Program.Fset.Position(fn.Pos())
+		difunc, err := c.attachDebugInfoRaw(fn, wrapper, "$invoke", pos.Filename, pos.Line)
 		if err != nil {
-			return llvm.Value{}, err
+			return err
 		}
 		c.builder.SetCurrentDebugLocation(uint(pos.Line), uint(pos.Column), difunc, llvm.Metadata{})
 	}
@@ -424,7 +424,7 @@ func (c *Compiler) wrapInterfaceInvoke(f *ir.Function) (llvm.Value, error) {
 		// Load the underlying value.
 		receiverPtrType := llvm.PointerType(receiverType, 0)
 		receiverPtr = c.builder.CreateBitCast(wrapper.Param(0), receiverPtrType, "receiver.ptr")
-	} else if len(expandedReceiverType) != 1 {
+	} else {
 		// The value is stored in the interface, but it is of type struct which
 		// is expanded to multiple parameters (e.g. {i8, i8}). So we have to
 		// receive the struct as parameter, expand it, and pass it on to the
@@ -435,19 +435,17 @@ func (c *Compiler) wrapInterfaceInvoke(f *ir.Function) (llvm.Value, error) {
 		alloca := c.builder.CreateAlloca(c.i8ptrType, "receiver.alloca")
 		c.builder.CreateStore(wrapper.Param(0), alloca)
 		receiverPtr = c.builder.CreateBitCast(alloca, llvm.PointerType(receiverType, 0), "receiver.ptr")
-	} else {
-		panic("unreachable")
 	}
 
 	receiverValue := c.builder.CreateLoad(receiverPtr, "receiver")
 	params := append(c.expandFormalParam(receiverValue), wrapper.Params()[1:]...)
-	if fnType.ReturnType().TypeKind() == llvm.VoidTypeKind {
-		c.builder.CreateCall(f.LLVMFn, params, "")
+	if fn.LLVMFn.Type().ElementType().ReturnType().TypeKind() == llvm.VoidTypeKind {
+		c.builder.CreateCall(fn.LLVMFn, params, "")
 		c.builder.CreateRetVoid()
 	} else {
-		ret := c.builder.CreateCall(f.LLVMFn, params, "ret")
+		ret := c.builder.CreateCall(fn.LLVMFn, params, "ret")
 		c.builder.CreateRet(ret)
 	}
 
-	return wrapper, nil
+	return nil
 }

compiler/optimizer.go

@@ -1,12 +1,14 @@
 package compiler
 
 import (
+	"errors"
+
 	"github.com/aykevl/go-llvm"
 )
 
 // Run the LLVM optimizer over the module.
 // The inliner can be disabled (if necessary) by passing 0 to the inlinerThreshold.
-func (c *Compiler) Optimize(optLevel, sizeLevel int, inlinerThreshold uint) {
+func (c *Compiler) Optimize(optLevel, sizeLevel int, inlinerThreshold uint) error {
 	builder := llvm.NewPassManagerBuilder()
 	defer builder.Dispose()
 	builder.SetOptLevel(optLevel)
@@ -40,7 +42,13 @@ func (c *Compiler) Optimize(optLevel, sizeLevel int, inlinerThreshold uint) {
 		c.OptimizeMaps()
 		c.OptimizeStringToBytes()
 		c.OptimizeAllocs()
-		c.Verify()
+		c.LowerInterfaces()
+	} else {
+		// Must be run at any optimization level.
+		c.LowerInterfaces()
+	}
+	if err := c.Verify(); err != nil {
+		return errors.New("optimizations caused a verification failure")
 	}
 
 	// Run module passes.
@@ -48,6 +56,8 @@ func (c *Compiler) Optimize(optLevel, sizeLevel int, inlinerThreshold uint) {
 	defer modPasses.Dispose()
 	builder.Populate(modPasses)
 	modPasses.Run(c.mod)
+
+	return nil
 }
 
 // Eliminate created but not used maps.
@@ -299,6 +309,9 @@ func (c *Compiler) hasFlag(call, param llvm.Value, kind string) bool {
 // Return a list of values (actually, instructions) where this value is used as
 // an operand.
 func getUses(value llvm.Value) []llvm.Value {
+	if value.IsNil() {
+		return nil
+	}
 	var uses []llvm.Value
 	use := value.FirstUse()
 	for !use.IsNil() {