tinygo/transform/interface-lowering.go

package transform

// This file provides function to lower interface intrinsics to their final LLVM
// form, optimizing them in the process.
//
// During SSA construction, the following pseudo-call is created (see
// src/runtime/interface.go):
//     runtime.typeAssert(typecode, assertedType)
// Additionally, interface type asserts and interface invoke functions are
// declared but not defined, so the optimizer will leave them alone.
//
// This pass lowers these functions to their final form:
//
// typeAssert:
//     Replaced with an icmp instruction so it can be directly used in a type
//     switch.
//
// interface type assert:
//     These functions are defined by creating a big type switch over all the
//     concrete types implementing this interface.
//
// interface invoke:
//     These functions are defined with a similar type switch, but instead of
//     checking for the appropriate type, these functions will call the
//     underlying method instead.
//
// 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/tinygo-org/tinygo/compileopts"
	"tinygo.org/x/go-llvm"
)

// numMethodHasMethodSet is a flag in bit 15 of the numMethod field (uint16) in
// Named, Pointer, and Struct type descriptors. When set, an inline method set
// is present in the type descriptor. Must match the constant in
// src/internal/reflectlite/type.go.
const numMethodHasMethodSet = 0x8000

// 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
}

// 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
	typecodeGEP llvm.Value
	methodSet   llvm.Value
	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")
}

// interfaceInfo keeps information about a Go interface type, including all
// methods it has.
type interfaceInfo struct {
	name       string                    // "tinygo-methods" attribute
	signatures map[string]*signatureInfo // method set
	types      []*typeInfo               // types this interface implements
}

// 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 {
	mod         llvm.Module
	config      *compileopts.Config
	builder     llvm.Builder
	dibuilder   *llvm.DIBuilder
	difiles     map[string]llvm.Metadata
	ctx         llvm.Context
	uintptrType llvm.Type
	targetData  llvm.TargetData
	ptrType     llvm.Type
	types       map[string]*typeInfo
	signatures  map[string]*signatureInfo
	interfaces  map[string]*interfaceInfo
}

// LowerInterfaces lowers all intermediate interface calls and globals that are
// emitted by the compiler as higher-level intrinsics. They 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 LowerInterfaces(mod llvm.Module, config *compileopts.Config) error {
	ctx := mod.Context()
	targetData := llvm.NewTargetData(mod.DataLayout())
	defer targetData.Dispose()
	p := &lowerInterfacesPass{
		mod:         mod,
		config:      config,
		builder:     ctx.NewBuilder(),
		ctx:         ctx,
		targetData:  targetData,
		uintptrType: mod.Context().IntType(targetData.PointerSize() * 8),
		ptrType:     llvm.PointerType(ctx.Int8Type(), 0),
		types:       make(map[string]*typeInfo),
		signatures:  make(map[string]*signatureInfo),
		interfaces:  make(map[string]*interfaceInfo),
	}
	defer p.builder.Dispose()

	if config.Debug() {
		p.dibuilder = llvm.NewDIBuilder(mod)
		defer p.dibuilder.Destroy()
		defer p.dibuilder.Finalize()
		p.difiles = make(map[string]llvm.Metadata)
	}

	return p.run()
}

// run runs the pass itself.
func (p *lowerInterfacesPass) run() error {
	if p.dibuilder != nil {
		p.dibuilder.CreateCompileUnit(llvm.DICompileUnit{
			Language:  0xb, // DW_LANG_C99 (0xc, off-by-one?)
			File:      "<unknown>",
			Dir:       "",
			Producer:  "TinyGo",
			Optimized: true,
		})
	}

	// Collect all type codes.
	for global := p.mod.FirstGlobal(); !global.IsNil(); global = llvm.NextGlobal(global) {
		if strings.HasPrefix(global.Name(), "reflect/types.type:") {
			// Retrieve Go type information based on an opaque global variable.
			// Only the name of the global is relevant, the object itself is
			// discarded afterwards.
			name := strings.TrimPrefix(global.Name(), "reflect/types.type:")
			if _, ok := p.types[name]; !ok {
				t := &typeInfo{
					name:     name,
					typecode: global,
				}
				p.types[name] = t
				initializer := global.Initializer()
				firstField := p.builder.CreateExtractValue(initializer, 0, "")
				if firstField.Type() != p.ctx.Int8Type() {
					// This type has a method set at index 0. Change the GEP to
					// point to index 1 (the meta byte).
					t.typecodeGEP = llvm.ConstGEP(global.GlobalValueType(), global, []llvm.Value{
						llvm.ConstInt(p.ctx.Int32Type(), 0, false),
						llvm.ConstInt(p.ctx.Int32Type(), 1, false),
					})
					methodSet := stripPointerCasts(firstField)
					if !strings.HasSuffix(methodSet.Name(), "$methodset") {
						panic("expected method set")
					}
					p.addTypeMethods(t, methodSet)
				} else {
					// This type has no method set.
					t.typecodeGEP = llvm.ConstGEP(global.GlobalValueType(), global, []llvm.Value{
						llvm.ConstInt(p.ctx.Int32Type(), 0, false),
						llvm.ConstInt(p.ctx.Int32Type(), 0, false),
					})
				}
			}
		}
	}

	// Find all interface type asserts and interface method thunks.
	var interfaceAssertFunctions []llvm.Value
	var interfaceInvokeFunctions []llvm.Value
	for fn := p.mod.FirstFunction(); !fn.IsNil(); fn = llvm.NextFunction(fn) {
		methodsAttr := fn.GetStringAttributeAtIndex(-1, "tinygo-methods")
		if methodsAttr.IsNil() {
			continue
		}
		if !hasUses(fn) {
			// Don't bother defining this function.
			continue
		}
		p.addInterface(methodsAttr.GetStringValue())
		invokeAttr := fn.GetStringAttributeAtIndex(-1, "tinygo-invoke")
		if invokeAttr.IsNil() {
			// Type assert.
			interfaceAssertFunctions = append(interfaceAssertFunctions, fn)
		} else {
			// Interface invoke.
			interfaceInvokeFunctions = append(interfaceInvokeFunctions, fn)
		}
	}

	// 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.
	for _, itf := range p.interfaces {
		sort.Slice(itf.types, func(i, j int) bool {
			return itf.types[i].name > itf.types[j].name
		})
	}

	// Define all interface invoke thunks.
	for _, fn := range interfaceInvokeFunctions {
		methodsAttr := fn.GetStringAttributeAtIndex(-1, "tinygo-methods")
		invokeAttr := fn.GetStringAttributeAtIndex(-1, "tinygo-invoke")
		itf := p.interfaces[methodsAttr.GetStringValue()]
		signature := itf.signatures[invokeAttr.GetStringValue()]
		p.defineInterfaceMethodFunc(fn, itf, signature)
	}

	// Define all interface type assert functions.
	for _, fn := range interfaceAssertFunctions {
		methodsAttr := fn.GetStringAttributeAtIndex(-1, "tinygo-methods")
		itf := p.interfaces[methodsAttr.GetStringValue()]
		p.defineInterfaceAssertFunc(fn, itf)
	}

	// Replace each type assert with an actual type comparison or (if the type
	// assert is impossible) the constant false.
	llvmFalse := llvm.ConstInt(p.ctx.Int1Type(), 0, false)
	for _, use := range getUses(p.mod.NamedFunction("runtime.typeAssert")) {
		actualType := use.Operand(0)
		name := strings.TrimPrefix(use.Operand(1).Name(), "reflect/types.typeid:")
		gepOffset := uint64(0)
		for strings.HasPrefix(name, "pointer:pointer:") {
			// This is a type like **int, which has the name pointer:pointer:int
			// but is encoded using pointer tagging.
			// Calculate the pointer tag, which is emitted as a GEP instruction.
			name = name[len("pointer:"):]
			gepOffset++
		}

		if t, ok := p.types[name]; ok {
			// The type exists in the program, so lower to a regular pointer
			// comparison.
			p.builder.SetInsertPointBefore(use)
			typecodeGEP := t.typecodeGEP
			if gepOffset != 0 {
				// This is a tagged pointer.
				typecodeGEP = llvm.ConstInBoundsGEP(p.ctx.Int8Type(), typecodeGEP, []llvm.Value{
					llvm.ConstInt(p.ctx.Int64Type(), gepOffset, false),
				})
			}
			commaOk := p.builder.CreateICmp(llvm.IntEQ, typecodeGEP, actualType, "typeassert.ok")
			use.ReplaceAllUsesWith(commaOk)
		} else {
			// The type does not exist in the program, so lower to a constant
			// false. This is trivially further optimized.
			// TODO: eventually it'll be necessary to handle reflect.PtrTo and
			// reflect.New calls which create new types not present in the
			// original program.
			use.ReplaceAllUsesWith(llvmFalse)
		}
		use.EraseFromParentAsInstruction()
	}

	// Create a sorted list of type names, for predictable iteration.
	var typeNames []string
	for name := range p.types {
		typeNames = append(typeNames, name)
	}
	sort.Strings(typeNames)

	// Check whether runtime.typeImplementsMethodSet still has uses. Now that
	// interface type assertions have been lowered to type-ID comparison
	// chains, the only remaining callers would be from reflect
	// (AssignableTo/Implements). If none remain, we can strip the inline
	// method-set data from type descriptors to save binary size.
	stripMethodSets := false
	typeImplementsFn := p.mod.NamedFunction("runtime.typeImplementsMethodSet")
	if !typeImplementsFn.IsNil() && !hasUses(typeImplementsFn) {
		stripMethodSets = true
	}

	// Collect all method signatures that appear in any interface type
	// descriptor. When reflect is imported and method sets are kept,
	// concrete type method sets are pruned: individual methods not in any
	// interface are removed, and types that can't fully satisfy at least
	// one interface have their method sets emptied entirely.
	//
	// When method sets are stripped entirely (reflect not imported),
	// methodFilter is nil and filterMethodSet replaces with empty.
	var methodFilter map[string]struct{}
	var ifaceMethodSets []map[string]struct{}
	if !stripMethodSets {
		methodFilter = make(map[string]struct{})
		for _, name := range typeNames {
			if !strings.HasPrefix(name, "interface:") {
				continue
			}
			t := p.types[name]
			initializer := t.typecode.Initializer()
			ifaceSet := make(map[string]struct{})
			for i := 0; i < initializer.Type().StructElementTypesCount(); i++ {
				field := p.builder.CreateExtractValue(initializer, i, "")
				for _, sig := range p.extractMethodSigs(field) {
					methodFilter[sig] = struct{}{}
					ifaceSet[sig] = struct{}{}
				}
			}
			if len(ifaceSet) > 0 {
				ifaceMethodSets = append(ifaceMethodSets, ifaceSet)
			}
		}
	}

	// Remove all method sets, which are now unnecessary and inhibit later
	// optimizations if they are left in place.
	zero := llvm.ConstInt(p.ctx.Int32Type(), 0, false)
	for _, name := range typeNames {
		t := p.types[name]
		if !t.methodSet.IsNil() {
			initializer := t.typecode.Initializer()
			numFields := initializer.Type().StructElementTypesCount()

			// Read numMethods from the original type descriptor (index 2:
			// after prefix pointer at 0 and kind byte at 1). For Named,
			// Pointer, and Struct types, the numMethodHasMethodSet flag
			// indicates that an inline method set is present.
			var numMethodsConst uint64
			var numMethodsIsI16 bool
			if numFields > 2 {
				nmField := p.builder.CreateExtractValue(initializer, 2, "")
				if nmField.Type() == p.ctx.Int16Type() {
					numMethodsConst = nmField.ZExtValue()
					numMethodsIsI16 = true
				}
			}

			var newInitializerFields []llvm.Value
			for i := 1; i < numFields; i++ {
				field := p.builder.CreateExtractValue(initializer, i, "")
				field = p.filterMethodSet(field, methodFilter, ifaceMethodSets)
				// Strip empty inline method sets for Named, Pointer, and
				// Struct types. When the method set is pruned to empty, we
				// remove it and clear the numMethodHasMethodSet flag (bit 15
				// of numMethod) so the runtime skips reading it.
				if numMethodsIsI16 && numMethodsConst&numMethodHasMethodSet != 0 && p.isMethodSetType(field.Type()) {
					elems := field.Type().StructElementTypes()
					if elems[1].ArrayLength() == 0 {
						clearedNumMethods := numMethodsConst & ^uint64(numMethodHasMethodSet)
						newInitializerFields[1] = llvm.ConstInt(p.ctx.Int16Type(), clearedNumMethods, false)
						continue
					}
				}
				newInitializerFields = append(newInitializerFields, field)
			}
			newInitializer := p.ctx.ConstStruct(newInitializerFields, false)
			typecodeName := t.typecode.Name()
			newGlobal := llvm.AddGlobal(p.mod, newInitializer.Type(), typecodeName+".tmp")
			newGlobal.SetInitializer(newInitializer)
			newGlobal.SetLinkage(t.typecode.Linkage())
			newGlobal.SetGlobalConstant(true)
			newGlobal.SetAlignment(t.typecode.Alignment())
			for _, use := range getUses(t.typecode) {
				if !use.IsAConstantExpr().IsNil() {
					opcode := use.Opcode()
					if opcode == llvm.GetElementPtr && use.OperandsCount() == 3 {
						if use.Operand(1).ZExtValue() == 0 && use.Operand(2).ZExtValue() == 1 {
							gep := p.builder.CreateInBoundsGEP(newGlobal.GlobalValueType(), newGlobal, []llvm.Value{zero, zero}, "")
							use.ReplaceAllUsesWith(gep)
						}
					}
				}
			}
			// Fallback.
			if hasUses(t.typecode) {
				negativeOffset := -int64(p.targetData.TypeAllocSize(p.ptrType))
				gep := p.builder.CreateInBoundsGEP(p.ctx.Int8Type(), newGlobal, []llvm.Value{llvm.ConstInt(p.ctx.Int32Type(), uint64(negativeOffset), true)}, "")
				t.typecode.ReplaceAllUsesWith(gep)
			}
			t.typecode.EraseFromParentAsGlobal()
			newGlobal.SetName(typecodeName)
			t.typecode = newGlobal
		}
	}

	return nil
}

// 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() {
		// no methods or methods already read
		return
	}

	// This type has methods, collect all methods of this type.
	t.methodSet = methodSet
	set := methodSet.Initializer() // get value from global
	signatures := p.builder.CreateExtractValue(set, 1, "")
	wrappers := p.builder.CreateExtractValue(set, 2, "")
	numMethods := signatures.Type().ArrayLength()
	for i := 0; i < numMethods; i++ {
		signatureGlobal := p.builder.CreateExtractValue(signatures, i, "")
		function := p.builder.CreateExtractValue(wrappers, i, "")
		function = stripPointerCasts(function) // strip bitcasts
		signatureName := signatureGlobal.Name()
		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(methodsString string) {
	if _, ok := p.interfaces[methodsString]; ok {
		return
	}
	t := &interfaceInfo{
		name:       methodsString,
		signatures: make(map[string]*signatureInfo),
	}
	p.interfaces[methodsString] = t
	for _, method := range strings.Split(methodsString, "; ") {
		signature := p.getSignature(method)
		signature.interfaces = append(signature.interfaces, t)
		t.signatures[method] = 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]
}

// defineInterfaceMethodFunc defines this thunk by calling the concrete method
// of the type that implements this interface.
//
// Matching the actual type is implemented using an if/else chain over all
// possible types.  This is later converted to a switch statement by the LLVM
// simplifycfg pass.
func (p *lowerInterfacesPass) defineInterfaceMethodFunc(fn llvm.Value, itf *interfaceInfo, signature *signatureInfo) {
	context := fn.LastParam()
	actualType := llvm.PrevParam(context)
	returnType := fn.GlobalValueType().ReturnType()
	context.SetName("context")
	actualType.SetName("actualType")
	fn.SetLinkage(llvm.InternalLinkage)
	fn.SetUnnamedAddr(true)
	AddStandardAttributes(fn, p.config)

	// 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()-3)
	for i := range params {
		params[i] = fn.Param(i + 1)
	}
	params = append(params,
		llvm.Undef(p.ptrType),
	)

	// Start chain in the entry block.
	entry := p.ctx.AddBasicBlock(fn, "entry")
	p.builder.SetInsertPointAtEnd(entry)

	if p.dibuilder != nil {
		difile := p.getDIFile("<Go interface method>")
		diFuncType := p.dibuilder.CreateSubroutineType(llvm.DISubroutineType{
			File: difile,
		})
		difunc := p.dibuilder.CreateFunction(difile, llvm.DIFunction{
			Name:         "(Go interface method)",
			File:         difile,
			Line:         0,
			Type:         diFuncType,
			LocalToUnit:  true,
			IsDefinition: true,
			ScopeLine:    0,
			Flags:        llvm.FlagPrototyped,
			Optimized:    true,
		})
		fn.SetSubprogram(difunc)
		p.builder.SetCurrentDebugLocation(0, 0, difunc, llvm.Metadata{})
	}

	// Define all possible functions that can be called.
	for _, typ := range itf.types {
		// Create type check (if/else).
		bb := p.ctx.AddBasicBlock(fn, typ.name)
		next := p.ctx.AddBasicBlock(fn, typ.name+".next")
		cmp := p.builder.CreateICmp(llvm.IntEQ, actualType, typ.typecodeGEP, typ.name+".icmp")
		p.builder.CreateCondBr(cmp, bb, next)

		// The function we will redirect to when the interface has this type.
		function := typ.getMethod(signature).function

		p.builder.SetInsertPointAtEnd(bb)
		receiver := fn.FirstParam()

		paramTypes := []llvm.Type{receiver.Type()}
		for _, param := range params {
			paramTypes = append(paramTypes, param.Type())
		}
		functionType := llvm.FunctionType(returnType, paramTypes, false)
		retval := p.builder.CreateCall(functionType, function, append([]llvm.Value{receiver}, params...), "")
		if retval.Type().TypeKind() == llvm.VoidTypeKind {
			p.builder.CreateRetVoid()
		} else {
			p.builder.CreateRet(retval)
		}

		// Start next comparison in the 'next' block (which is jumped to when
		// the type doesn't match).
		p.builder.SetInsertPointAtEnd(next)
	}

	// The builder now points to the last *.then block, after all types have
	// been checked. Call runtime.nilPanic here.
	// The only other possible value remaining is nil for nil interfaces. We
	// could panic with a different message here such as "nil interface" but
	// that would increase code size and "nil panic" is close enough. Most
	// importantly, it avoids undefined behavior when accidentally calling a
	// method on a nil interface.
	nilPanic := p.mod.NamedFunction("runtime.nilPanic")
	p.builder.CreateCall(nilPanic.GlobalValueType(), nilPanic, []llvm.Value{
		llvm.Undef(p.ptrType),
	}, "")
	p.builder.CreateUnreachable()
}

func (p *lowerInterfacesPass) getDIFile(file string) llvm.Metadata {
	difile, ok := p.difiles[file]
	if !ok {
		difile = p.dibuilder.CreateFile(file, "")
		p.difiles[file] = difile
	}
	return difile
}

// defineInterfaceAssertFunc defines a $typeassert function for the given
// interface. The function returns true if the concrete type (passed as a
// type-ID pointer) implements the interface, using a chain of type-ID
// comparisons. This avoids pulling in runtime.typeImplementsMethodSet for
// programs that don't use reflect.
func (p *lowerInterfacesPass) defineInterfaceAssertFunc(fn llvm.Value, itf *interfaceInfo) {
	actualType := fn.FirstParam()
	actualType.SetName("actualType")
	fn.SetLinkage(llvm.InternalLinkage)
	fn.SetUnnamedAddr(true)
	AddStandardAttributes(fn, p.config)

	entry := p.ctx.AddBasicBlock(fn, "entry")
	p.builder.SetInsertPointAtEnd(entry)

	if p.dibuilder != nil {
		difile := p.getDIFile("<Go interface type assert>")
		diFuncType := p.dibuilder.CreateSubroutineType(llvm.DISubroutineType{
			File: difile,
		})
		difunc := p.dibuilder.CreateFunction(difile, llvm.DIFunction{
			Name:         "(Go interface type assert)",
			File:         difile,
			Line:         0,
			Type:         diFuncType,
			LocalToUnit:  true,
			IsDefinition: true,
			ScopeLine:    0,
			Flags:        llvm.FlagPrototyped,
			Optimized:    true,
		})
		fn.SetSubprogram(difunc)
		p.builder.SetCurrentDebugLocation(0, 0, difunc, llvm.Metadata{})
	}

	// Build an OR chain: return (type == T1) || (type == T2) || ...
	llvmFalse := llvm.ConstInt(p.ctx.Int1Type(), 0, false)
	result := llvmFalse
	for _, typ := range itf.types {
		cmp := p.builder.CreateICmp(llvm.IntEQ, actualType, typ.typecodeGEP, typ.name+".icmp")
		result = p.builder.CreateOr(result, cmp, "")
	}
	p.builder.CreateRet(result)
}

// isMethodSetType reports whether ty has the shape of a method-set struct:
// { uintptr, [N x ptr] }.
func (p *lowerInterfacesPass) isMethodSetType(ty llvm.Type) bool {
	if ty.TypeKind() != llvm.StructTypeKind {
		return false
	}
	elems := ty.StructElementTypes()
	if len(elems) != 2 {
		return false
	}
	if elems[0] != p.uintptrType {
		return false
	}
	return elems[1].TypeKind() == llvm.ArrayTypeKind && elems[1].ElementType() == p.ptrType
}

// extractMethodSigs returns the names of method signature globals inside a
// method-set field ({ uintptr, [N x ptr] }). Returns nil if field is not a
// method set.
func (p *lowerInterfacesPass) extractMethodSigs(field llvm.Value) []string {
	if !p.isMethodSetType(field.Type()) {
		return nil
	}
	methodArray := p.builder.CreateExtractValue(field, 1, "")
	n := methodArray.Type().ArrayLength()
	sigs := make([]string, 0, n)
	for j := 0; j < n; j++ {
		sig := p.builder.CreateExtractValue(methodArray, j, "")
		sig = stripPointerCasts(sig)
		sigs = append(sigs, sig.Name())
	}
	return sigs
}

// filterMethodSet processes a type-descriptor field that may be a method set.
// Non-method-set fields are returned unchanged.
//
// If keepSigs is nil, the method set is replaced with an empty one (strip mode,
// used when reflect is not imported). If keepSigs is non-nil, the method set is
// pruned in two stages: first, methods not in keepSigs (the union of all
// interface signatures) are removed; then, if the remaining methods cannot
// fully satisfy at least one interface in ifaceSets, the entire method set is
// emptied.
func (p *lowerInterfacesPass) filterMethodSet(field llvm.Value, keepSigs map[string]struct{}, ifaceSets []map[string]struct{}) llvm.Value {
	if !p.isMethodSetType(field.Type()) {
		return field
	}

	methodArray := p.builder.CreateExtractValue(field, 1, "")
	numMethods := methodArray.Type().ArrayLength()

	// Strip mode: replace with empty method set.
	if keepSigs == nil {
		return p.ctx.ConstStruct([]llvm.Value{
			llvm.ConstInt(p.uintptrType, 0, false),
			llvm.ConstArray(p.ptrType, nil),
		}, false)
	}

	if numMethods == 0 {
		return field
	}

	// Extract all methods and their signature names.
	type methodEntry struct {
		value llvm.Value
		name  string
	}
	entries := make([]methodEntry, numMethods)
	nameSet := make(map[string]struct{}, numMethods)
	for j := 0; j < numMethods; j++ {
		sig := p.builder.CreateExtractValue(methodArray, j, "")
		stripped := stripPointerCasts(sig)
		name := stripped.Name()
		entries[j] = methodEntry{sig, name}
		nameSet[name] = struct{}{}
	}

	// Check whether this type can fully implement at least one interface.
	// If not, its method set can never produce a true result from
	// typeImplementsMethodSet, so we can empty it entirely.
	implementsAny := false
	for _, ifaceSet := range ifaceSets {
		if isSubsetOf(ifaceSet, nameSet) {
			implementsAny = true
			break
		}
	}
	if !implementsAny {
		return p.ctx.ConstStruct([]llvm.Value{
			llvm.ConstInt(p.uintptrType, 0, false),
			llvm.ConstArray(p.ptrType, nil),
		}, false)
	}

	// Prune: keep only methods whose signature appears in keepSigs.
	var kept []llvm.Value
	for _, e := range entries {
		if _, ok := keepSigs[e.name]; ok {
			kept = append(kept, e.value)
		}
	}

	if len(kept) == numMethods {
		return field
	}

	return p.ctx.ConstStruct([]llvm.Value{
		llvm.ConstInt(p.uintptrType, uint64(len(kept)), false),
		llvm.ConstArray(p.ptrType, kept),
	}, false)
}

// isSubsetOf reports whether every key in sub is also in super.
func isSubsetOf(sub, super map[string]struct{}) bool {
	for k := range sub {
		if _, ok := super[k]; !ok {
			return false
		}
	}
	return true
}