Add a panic, pull out constant to keep in sync

Author: jakebailey

compiler/interface.go

@@ -18,6 +18,12 @@ import (
 	"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
+
 // Type kinds for basic types.
 // They must match the constants for the Kind type in src/reflect/type.go.
 var basicTypes = [...]uint8{
@@ -321,8 +327,11 @@ func (c *compilerContext) getTypeCode(typ types.Type) llvm.Value {
 			}
 			pkgPathPtr := c.pkgPathPtr(pkgpath)
 			namedNumMethods := uint64(numMethods)
+			if namedNumMethods&numMethodHasMethodSet != 0 {
+				panic("numMethods overflow: too many exported methods on named type " + name)
+			}
 			if len(methods) > 0 {
-				namedNumMethods |= 0x8000 // numMethodHasMethodSet flag
+				namedNumMethods |= numMethodHasMethodSet
 			}
 			typeFields = []llvm.Value{
 				llvm.ConstInt(c.ctx.Int16Type(), namedNumMethods, false), // numMethods
@@ -359,8 +368,11 @@ func (c *compilerContext) getTypeCode(typ types.Type) llvm.Value {
 			}
 		case *types.Pointer:
 			ptrNumMethods := uint64(numMethods)
+			if ptrNumMethods&numMethodHasMethodSet != 0 {
+				panic("numMethods overflow: too many exported methods on pointer type")
+			}
 			if len(methods) > 0 {
-				ptrNumMethods |= 0x8000 // numMethodHasMethodSet flag
+				ptrNumMethods |= numMethodHasMethodSet
 			}
 			typeFields = []llvm.Value{
 				llvm.ConstInt(c.ctx.Int16Type(), ptrNumMethods, false), // numMethods
@@ -396,8 +408,11 @@ func (c *compilerContext) getTypeCode(typ types.Type) llvm.Value {
 			llvmStructType := c.getLLVMType(typ)
 			size := c.targetData.TypeStoreSize(llvmStructType)
 			structNumMethods := uint64(numMethods)
+			if structNumMethods&numMethodHasMethodSet != 0 {
+				panic("numMethods overflow: too many exported methods on struct type")
+			}
 			if len(methods) > 0 {
-				structNumMethods |= 0x8000 // numMethodHasMethodSet flag
+				structNumMethods |= numMethodHasMethodSet
 			}
 			typeFields = []llvm.Value{
 				llvm.ConstInt(c.ctx.Int16Type(), structNumMethods, false), // numMethods

transform/interface-lowering.go

@@ -36,6 +36,12 @@ import (
 	"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 {
@@ -379,8 +385,8 @@ func (p *lowerInterfacesPass) run() error {
 
 			// 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, bit 15 (0x8000) indicates that an
-			// inline method set is present.
+			// Pointer, and Struct types, the numMethodHasMethodSet flag
+			// indicates that an inline method set is present.
 			var numMethodsConst uint64
 			var numMethodsIsI16 bool
 			if numFields > 2 {
@@ -399,10 +405,10 @@ func (p *lowerInterfacesPass) run() error {
 				// 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&0x8000 != 0 && p.isMethodSetType(field.Type()) {
+				if numMethodsIsI16 && numMethodsConst&numMethodHasMethodSet != 0 && p.isMethodSetType(field.Type()) {
 					elems := field.Type().StructElementTypes()
 					if elems[1].ArrayLength() == 0 {
-						clearedNumMethods := numMethodsConst & ^uint64(0x8000)
+						clearedNumMethods := numMethodsConst & ^uint64(numMethodHasMethodSet)
 						newInitializerFields[1] = llvm.ConstInt(p.ctx.Int16Type(), clearedNumMethods, false)
 						continue
 					}