build,ir,printer: add InternalFunction

Adds support for internal assembly functions. The new InternalFunction()
call outputs a TEXT directive sans the pesky unicode dot used in Go
assembly. This allows Avo to generate an internal assembly function that
is not linked to any symbols in the corresponding package.

Closes mmcloughlin#442

build/context.go

@@ -90,6 +90,12 @@ func (c *Context) Function(name string) {
 	c.file.AddSection(c.function)
 }
 
+// InternalFunction starts building a new internal function (not linked to any package) with the given name.
+func (c *Context) InternalFunction(name string) {
+	c.function = ir.NewInternalFunction(name)
+	c.file.AddSection(c.function)
+}
+
 // Doc sets documentation comment lines for the currently active function.
 func (c *Context) Doc(lines ...string) {
 	c.activefunc().Doc = lines
@@ -112,6 +118,11 @@ func (c *Context) Signature(s *gotypes.Signature) {
 
 // SignatureExpr parses the signature expression and sets it as the active function's signature.
 func (c *Context) SignatureExpr(expr string) {
+	if c.activefunc().IsInternal {
+		e := fmt.Sprintf("cannot link SignatureExpr \"%s\" with InternalFunction \"%s\"", expr, c.activefunc().Name)
+		c.adderror(errors.New(e))
+		return
+	}
 	s, err := gotypes.ParseSignatureInPackage(c.types(), expr)
 	if err != nil {
 		c.adderror(err)

build/global.go

@@ -127,6 +127,9 @@ func Dereference(ptr gotypes.Component) gotypes.Component { return ctx.Dereferen
 // Function starts building a new function with the given name.
 func Function(name string) { ctx.Function(name) }
 
+// InternalFunction starts building a new internal function (not linked to any package) with the given name.
+func InternalFunction(name string) { ctx.InternalFunction(name) }
+
 // Doc sets documentation comment lines for the currently active function.
 func Doc(lines ...string) { ctx.Doc(lines...) }
 

ir/ir.go

@@ -176,6 +176,7 @@ type Function struct {
 	Doc        []string
 	Signature  *gotypes.Signature
 	LocalSize  int
+	IsInternal bool
 
 	Nodes []Node
 
@@ -195,10 +196,20 @@ func (f *Function) section() {}
 func NewFunction(name string) *Function {
 	return &Function{
 		Name:      name,
+		IsInternal: false,
 		Signature: gotypes.NewSignatureVoid(),
 	}
 }
 
+// NewInternalFunction builds an empty internal function (not linked to any package) of the given name.
+func NewInternalFunction(name string) *Function {
+	return &Function{
+		Name:       name,
+		IsInternal: true,
+		Signature:  gotypes.NewSignatureVoid(),
+	}
+}
+
 // AddPragma adds a pragma to this function.
 func (f *Function) AddPragma(directive string, args ...string) {
 	f.Pragmas = append(f.Pragmas, Pragma{

ir/ir_test.go

@@ -24,6 +24,20 @@ func TestFunctionLabels(t *testing.T) {
 	}
 }
 
+func TestIsNotInternal(t *testing.T) {
+	f := NewFunction("isNotInternal")
+	if f.IsInternal {
+		t.Fatalf("expected f.IsInternal to be false, got %t", f.IsInternal)
+	}
+}
+
+func TestIsInternal(t *testing.T) {
+	f := NewInternalFunction("isInternal")
+	if !f.IsInternal {
+		t.Fatalf("expected f.IsInternal to be true, got %t", f.IsInternal)
+	}
+}
+
 func TestInputRegisters(t *testing.T) {
 	cases := []struct {
 		Name   string

printer/goasm.go

@@ -67,12 +67,22 @@ func (p *goasm) includes(paths []string) {
 
 func (p *goasm) function(f *ir.Function) {
 	p.NL()
-	p.Comment(f.Stub())
+	if !f.IsInternal {
+		p.Comment(f.Stub())
+	} else {
+		p.Comment("Internal Function (not linked to any package)")
+	}
 
 	if len(f.ISA) > 0 {
 		p.Comment("Requires: " + strings.Join(f.ISA, ", "))
 	}
 
+	if f.IsInternal {
+		p.Printf("TEXT %s(SB)", f.Name)
+	} else {
+		p.Printf("TEXT %s%s(SB)", dot, f.Name)
+	}
+
 	// Reference: https://github.com/golang/go/blob/b115207baf6c2decc3820ada4574ef4e5ad940ec/src/cmd/internal/obj/util.go#L166-L176
 	//
 	//		if p.As == ATEXT {
@@ -87,7 +97,6 @@ func (p *goasm) function(f *ir.Function) {
 	//			}
 	//		}
 	//
-	p.Printf("TEXT %s%s(SB)", dot, f.Name)
 	if f.Attributes != 0 {
 		p.Printf(", %s", f.Attributes.Asm())
 	}

printer/goasm_test.go

@@ -1,6 +1,8 @@
 package printer_test
 
 import (
+	"errors"
+	"strings"
 	"testing"
 
 	"github.com/mmcloughlin/avo/attr"
@@ -34,6 +36,43 @@ func TestBasic(t *testing.T) {
 	})
 }
 
+func TestInternal(t *testing.T) {
+	ctx := build.NewContext()
+	ctx.InternalFunction("internal")
+	ctx.AllocLocal(16)
+	ctx.RET()
+
+	AssertPrintsLines(t, ctx, printer.NewGoAsm, []string{
+		"// Code generated by avo. DO NOT EDIT.",
+		"",
+		"// Internal Function (not linked to any package)",
+		"TEXT internal(SB), $16",
+		"\tRET",
+		"",
+	})
+}
+
+func TestInternalSigExp(t *testing.T) {
+	ctx := build.NewContext()
+	ctx.InternalFunction("internal")
+	ctx.SignatureExpr("invalidSig(b bool)")
+	ctx.ADDQ(reg.EAX, reg.ECX)
+
+	_, err := ctx.Result()
+	expect := "cannot link SignatureExpr \"invalidSig(b bool)\" with InternalFunction \"internal\""
+
+	var list build.ErrorList
+	if errors.As(err, &list) {
+		for _, e := range list {
+			if strings.Contains(e.Error(), expect) {
+				return
+			}
+		}
+	}
+
+	t.Fatal("Calling SignatureExpr() while the current function is an InternalFunction should generate an error")
+}
+
 func TestTextDecl(t *testing.T) {
 	ctx := build.NewContext()