build: unify Label function signatures

The Context.Label method and LABEL global function did not agree. Also
breaks the convention I'd like to set that capitalized functions must
agree with existing Go assembly syntax.

To help avoid a conflict with `avo.Label`, attributes were moved to
their own package.

Fixes #35

attr/attr.go

@@ -0,0 +1,97 @@
+// Package attr provides attributes for text and data sections.
+package attr
+
+import (
+	"fmt"
+	"math/bits"
+	"strings"
+)
+
+// Attribute represents TEXT or DATA flags.
+type Attribute uint16
+
+// Reference: https://github.com/golang/go/blob/35f4ec152b44ae5fc83aaf68e2eb3aa1a778e5cd/src/runtime/textflag.h#L11-L34
+//
+//	// Don't profile the marked routine. This flag is deprecated.
+//	#define NOPROF	1
+//	// It is ok for the linker to get multiple of these symbols. It will
+//	// pick one of the duplicates to use.
+//	#define DUPOK	2
+//	// Don't insert stack check preamble.
+//	#define NOSPLIT	4
+//	// Put this data in a read-only section.
+//	#define RODATA	8
+//	// This data contains no pointers.
+//	#define NOPTR	16
+//	// This is a wrapper function and should not count as disabling 'recover'.
+//	#define WRAPPER 32
+//	// This function uses its incoming context register.
+//	#define NEEDCTXT 64
+//	// Allocate a word of thread local storage and store the offset from the
+//	// thread local base to the thread local storage in this variable.
+//	#define TLSBSS	256
+//	// Do not insert instructions to allocate a stack frame for this function.
+//	// Only valid on functions that declare a frame size of 0.
+//	// TODO(mwhudson): only implemented for ppc64x at present.
+//	#define NOFRAME 512
+//	// Function can call reflect.Type.Method or reflect.Type.MethodByName.
+//	#define REFLECTMETHOD 1024
+//
+const (
+	NOPROF Attribute = 1 << iota
+	DUPOK
+	NOSPLIT
+	RODATA
+	NOPTR
+	WRAPPER
+	NEEDCTXT
+	_
+	TLSBSS
+	NOFRAME
+	REFLECTMETHOD
+)
+
+// Asm returns a representation of the attributes in assembly syntax. This may use macros from "textflags.h"; see ContainsTextFlags() to determine if this header is required.
+func (a Attribute) Asm() string {
+	parts, rest := a.split()
+	if len(parts) == 0 || rest != 0 {
+		parts = append(parts, fmt.Sprintf("%d", rest))
+	}
+	return strings.Join(parts, "|")
+}
+
+// ContainsTextFlags returns whether the Asm() representation requires macros in "textflags.h".
+func (a Attribute) ContainsTextFlags() bool {
+	flags, _ := a.split()
+	return len(flags) > 0
+}
+
+// split splits a into known flags and any remaining bits.
+func (a Attribute) split() ([]string, Attribute) {
+	var flags []string
+	var rest Attribute
+	for a != 0 {
+		i := uint(bits.TrailingZeros16(uint16(a)))
+		bit := Attribute(1) << i
+		if flag := attrname[bit]; flag != "" {
+			flags = append(flags, flag)
+		} else {
+			rest |= bit
+		}
+		a ^= bit
+	}
+	return flags, rest
+}
+
+var attrname = map[Attribute]string{
+	NOPROF:   "NOPROF",
+	DUPOK:    "DUPOK",
+	NOSPLIT:  "NOSPLIT",
+	RODATA:   "RODATA",
+	NOPTR:    "NOPTR",
+	WRAPPER:  "WRAPPER",
+	NEEDCTXT: "NEEDCTXT",
+	TLSBSS:   "TLSBSS",
+	NOFRAME:  "NOFRAME",
+	// REFLECTMETHOD excluded due to https://golang.org/issue/29487
+}