feat(asm): add AArch64 arm64 encoder with ground-truth verification

Assisted-by: MiMo V2.5 Pro

asm/goobjarm64.go

@@ -0,0 +1,84 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+package asm
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"sync"
+)
+
+// GOObjectAARCH64 emits a GOOBJ object file for AArch64.  The layout is
+// the shared one in goobj.go — the toolchain preamble, the go120ld header
+// with its block offsets, the string table, the symbol definitions and the
+// reloc/aux/data index arrays — with the arm64 preamble, the MinLC of 4
+// for the pc-value deltas, and R_ADDRARM64 relocation types for the
+// ADRP+ADD/LDR/STR address pairs.
+func (img *Image) GOObjectAARCH64(pkgPath, srcPath string) ([]byte, error) {
+	pre, err := toolchainObjectPreambleAARCH64()
+	if err != nil {
+		return nil, err
+	}
+	return img.emitGOObject(pkgPath, srcPath, pre, 4, func(r Reloc) (uint16, uint8) {
+		return relocArm64Addr, 4
+	})
+}
+
+// arm64 relocation types (cmd/internal/objabi).  R_ADDRARM64 resolves an
+// ADRP+ADD/LDR/STR pair to a symbol's address.
+const (
+	relocArm64Addr = 9 // R_ADDRARM64
+)
+
+// toolchainObjectPreambleAARCH64 returns the "go object ...\n!\n" header
+// the installed go tool asm writes for arm64, captured by assembling a
+// one-instruction probe.
+var (
+	preambleAARCH64Once sync.Once
+	preambleAARCH64     []byte
+	preambleAARCH64Err  error
+)
+
+func toolchainObjectPreambleAARCH64() ([]byte, error) {
+	preambleAARCH64Once.Do(func() {
+		goBin, err := exec.LookPath("go")
+		if err != nil {
+			preambleAARCH64Err = fmt.Errorf("GOOBJ emission needs the Go toolchain: %w", err)
+			return
+		}
+		dir, err := os.MkdirTemp("", "gasm-preamble-arm64")
+		if err != nil {
+			preambleAARCH64Err = err
+			return
+		}
+		defer os.RemoveAll(dir)
+		src := filepath.Join(dir, "probe_arm64.s")
+		if err := os.WriteFile(src, []byte("TEXT \u00b7x(SB), $0-0\n\tRET\n"), 0o644); err != nil {
+			preambleAARCH64Err = err
+			return
+		}
+		obj := filepath.Join(dir, "probe.o")
+		cmd := exec.Command(goBin, "tool", "asm", "-p", "probe", "-o", obj, src)
+		cmd.Env = append(os.Environ(), "GOARCH=arm64")
+		if out, err := cmd.CombinedOutput(); err != nil {
+			preambleAARCH64Err = fmt.Errorf("probing the assembler for the object header: %v\n%s", err, out)
+			return
+		}
+		data, err := os.ReadFile(obj)
+		if err != nil {
+			preambleAARCH64Err = err
+			return
+		}
+		i := bytes.Index(data, []byte("\n!\n"))
+		if i < 0 || !bytes.HasPrefix(data[i+3:], []byte(goobjMagic)) {
+			preambleAARCH64Err = fmt.Errorf("unrecognised assembler object layout")
+			return
+		}
+		preambleAARCH64 = data[:i+3]
+	})
+	return preambleAARCH64, preambleAARCH64Err
+}