// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: BSD-3-Clause

package asm

import (
	"bytes"
	"encoding/binary"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
	"testing"

	"sourcedock.dev/petrbalvin/gasm-sdk/parser"
)

// goobjView is a minimal parsed view of a GOOBJ payload, enough to check
// the emitter's output block by block.
type goobjView struct {
	t      *testing.T
	b      []byte
	offs   [blkEnd + 1]uint32
	strOff uint32
}

func openGoobj(t *testing.T, data []byte) *goobjView {
	t.Helper()
	i := bytes.Index(data, []byte(goobjMagic))
	if i < 0 {
		t.Fatal("no GOOBJ magic in output")
	}
	v := &goobjView{t: t, b: data[i:], strOff: uint32(i + 96)}
	for j := 0; j <= blkEnd; j++ {
		v.offs[j] = binary.LittleEndian.Uint32(v.b[20+4*j:])
	}
	return v
}

func (v *goobjView) blk(i int) []byte { return v.b[v.offs[i]:v.offs[i+1]] }

func (v *goobjView) str(off, ln uint32) string {
	return string(v.b[off : off+ln])
}

type goobjSymView struct {
	name  string
	abi   uint16
	typ   uint8
	flag  uint8
	flag2 uint8
	size  uint32
	align uint32
}

func (v *goobjView) syms(i int) []goobjSymView {
	var out []goobjSymView
	for x := v.blk(i); len(x) >= 21; x = x[21:] {
		le := binary.LittleEndian
		out = append(out, goobjSymView{
			name:  v.str(le.Uint32(x[4:]), le.Uint32(x[0:])),
			abi:   le.Uint16(x[8:]),
			typ:   x[10],
			flag:  x[11],
			flag2: x[12],
			size:  le.Uint32(x[13:]),
			align: le.Uint32(x[17:]),
		})
	}
	return out
}

// TestGOObjectStructure checks the emitted object's blocks against the
// ground truth captured from go tool asm: the symbol tables, the FuncInfo
// contents, the pc-value tables, the relocation and the aux wiring.
func TestGOObjectStructure(t *testing.T) {
	f, errs := parser.Parse("t_amd64.s", `
#include "textflag.h"

TEXT ·addq(SB), NOSPLIT, $0-24
	MOVQ a+0(FP), AX
	MOVQ b+8(FP), CX
	ADDQ CX, AX
	MOVQ AX, ret+16(FP)
	RET

TEXT ·loadmask(SB), NOSPLIT, $0-8
	VMOVDQU mask<>(SB), X0
	VPMOVMSKB X0, AX
	MOVQ AX, ret+0(FP)
	RET

GLOBL mask<>(SB), RODATA, $16
DATA mask<>+0(SB)/8, $0x0807060504030201
DATA mask<>+8(SB)/8, $0x800f0e0d0c0b0a09
`)
	if len(errs) > 0 {
		t.Fatalf("parse: %v", errs)
	}
	img, err := AssembleFile(f)
	if err != nil {
		t.Fatalf("AssembleFile: %v", err)
	}
	obj, err := img.GOObject("testpkg", "t_amd64.s")
	if err != nil {
		t.Fatalf("GOObject: %v", err)
	}
	v := openGoobj(t, obj)

	if flags := binary.LittleEndian.Uint32(v.b[16:]); flags != 4 {
		t.Errorf("flags = %#x, want ObjFlagFromAssembly (4)", flags)
	}

	// Package defs: the static GLOBL, then per function the FuncInfo and the
	// two DWARF symbols (debug_line program, subprogram DIE).
	defs := v.syms(blkSymdef)
	if len(defs) != 7 {
		t.Fatalf("symdefs = %d, want 7", len(defs))
	}
	// The linkname flag stays clear: the toolchain sets it only for
	// //go:linkname symbols, and an ordinary static GLOBL is not one.
	if defs[0].name != "mask" || defs[0].abi != 0xffff || defs[0].typ != kindSRODATA || defs[0].size != 16 || defs[0].flag2 != 0 {
		t.Errorf("mask symbol = %+v", defs[0])
	}
	if defs[1].name != "" || defs[1].typ != kindSDATA || defs[1].size != 28 {
		t.Errorf("addq funcinfo symbol = %+v", defs[1])
	}
	if defs[2].name != "" || defs[2].typ != kindSDWARFLINES || defs[2].size == 0 {
		t.Errorf("addq lines symbol = %+v", defs[2])
	}
	if defs[3].name != "" || defs[3].typ != kindSDWARFFCN || defs[3].size == 0 {
		t.Errorf("addq DIE symbol = %+v", defs[3])
	}
	if defs[4].name != "" || defs[4].typ != kindSDATA || defs[4].size != 28 {
		t.Errorf("loadmask funcinfo symbol = %+v", defs[4])
	}

	// Non-package defs: four pc tables and the function, per function.
	nps := v.syms(blkNonpkgdef)
	if len(nps) != 10 {
		t.Fatalf("nonpkgdefs = %d, want 10", len(nps))
	}
	fn := nps[4]
	if fn.name != "testpkg.addq" || fn.typ != kindSTEXT || fn.flag != symFlagNoSplit || fn.size != 19 {
		t.Errorf("addq symbol = %+v", fn)
	}
	for i, s := range []int{0, 1, 2, 3, 5, 6, 7, 8} {
		if nps[s].typ != kindSRODATA || nps[s].align != 1 || nps[s].name != "" {
			t.Errorf("pc table %d = %+v", i, nps[s])
		}
	}

	// FuncInfo: args 24, FuncFlag Asm, one file, no inline tree.
	le := binary.LittleEndian
	data := v.blk(blkData)
	didx := v.blk(blkDataIdx)
	fi := data[16:44]
	if le.Uint32(fi[0:]) != 24 || le.Uint32(fi[4:]) != 0 || fi[8] != 0 || fi[9] != funcFlagAsm ||
		le.Uint32(fi[16:]) != 1 || le.Uint32(fi[20:]) != 0 || le.Uint32(fi[24:]) != 0 {
		t.Errorf("funcinfo bytes %x", fi)
	}

	// The pc-value tables of addq (non-package indices 0-3, so global
	// indices 7-10): pcsp a flat zero over the whole function, pcinline a
	// flat -1, both with the pc delta in MinLC (1) units.
	pcsp := data[le.Uint32(didx[4*7:]):]
	if got := pcsp[:3]; !bytes.Equal(got, []byte{0x02, 19, 0x00}) {
		t.Errorf("pcsp = %x, want 021300", got)
	}
	pcinl := data[le.Uint32(didx[4*10:]):]
	if got := pcinl[:3]; !bytes.Equal(got, []byte{0x00, 19, 0x00}) {
		t.Errorf("pcinline = %x, want 001300", got)
	}

	// Relocations: the four DWARF address references (two per function, in
	// definition order), then the loadmask code's R_PCREL against the
	// GLOBL, with the field in the function code left zero.  The loadmask
	// code's offset comes from the data index (7 defs + 9 non-package).
	relocs := v.blk(blkReloc)
	if len(relocs) != 5*23 {
		t.Fatalf("relocs = %d bytes, want 5 entries", len(relocs))
	}
	// addq's DWARF references (defs 2 and 3) against the function, which
	// is non-package index 4.
	lr := relocs[:23]
	if int32(le.Uint32(lr[0:])) != 3 || lr[4] != 8 || le.Uint16(lr[5:]) != relocAddr ||
		le.Uint32(lr[15:]) != pkgIdxNone || le.Uint32(lr[19:]) != 4 {
		t.Errorf("addq lines reloc = %x", lr)
	}
	dr := relocs[23:46]
	if dr[4] != 4 || le.Uint16(dr[5:]) != relocDWTXTADDRU4() ||
		le.Uint32(dr[15:]) != pkgIdxNone || le.Uint32(dr[19:]) != 4 {
		t.Errorf("addq DIE reloc = %x", dr)
	}
	cr := relocs[4*23:]
	off := int32(le.Uint32(cr[0:]))
	if off != 4 || cr[4] != 4 || le.Uint16(cr[5:]) != relocPCRel ||
		le.Uint64(cr[7:]) != 0 || le.Uint32(cr[15:]) != pkgIdxSelf || le.Uint32(cr[19:]) != 0 {
		t.Errorf("loadmask reloc = %x", cr)
	}
	lm := le.Uint32(didx[4*16:])
	code := data[lm : lm+18]
	if !bytes.Equal(code[4:8], []byte{0, 0, 0, 0}) {
		t.Errorf("relocated field = %x, want zeroed", code[4:8])
	}

	// Aux wiring: FuncInfo, the two DWARF symbols (package symbols), then
	// the four pc tables (non-package symbols).
	auxs := v.blk(blkAux)
	if len(auxs) != 2*7*9 {
		t.Fatalf("aux = %d bytes, want 14 entries", len(auxs))
	}
	wantAux := []struct {
		typ uint8
		pkg uint32
		idx uint32
	}{
		{auxFuncInfo, pkgIdxSelf, 1},
		{auxDwarfInfo, pkgIdxSelf, 3},
		{auxDwarfLines, pkgIdxSelf, 2},
		{auxPcsp, pkgIdxNone, 0},
		{auxPcfile, pkgIdxNone, 1},
		{auxPcline, pkgIdxNone, 2},
		{auxPcinline, pkgIdxNone, 3},
		{auxFuncInfo, pkgIdxSelf, 4},
		{auxDwarfInfo, pkgIdxSelf, 6},
		{auxDwarfLines, pkgIdxSelf, 5},
		{auxPcsp, pkgIdxNone, 5},
		{auxPcfile, pkgIdxNone, 6},
		{auxPcline, pkgIdxNone, 7},
		{auxPcinline, pkgIdxNone, 8},
	}
	for i, w := range wantAux {
		e := auxs[i*9:]
		if e[0] != w.typ || le.Uint32(e[1:]) != w.pkg || le.Uint32(e[5:]) != w.idx {
			t.Errorf("aux[%d] = {%d,%d,%d}, want {%d,%d,%d}", i, e[0], le.Uint32(e[1:]), le.Uint32(e[5:]), w.typ, w.pkg, w.idx)
		}
	}
}

// decodePCValues decodes a pc-value table into (pc, value) steps.  The
// table ends with a final unsigned pc delta covering the rest of the
// function, followed by a zero byte that carries no value delta.
func decodePCValues(b []byte) (pcs, vals []int64) {
	val, n := binary.Varint(b)
	b = b[n:]
	val-- // the first delta is against the implicit -1
	var pc int64
	pcs = append(pcs, pc)
	vals = append(vals, val)
	for {
		pcd, n := binary.Uvarint(b)
		b = b[n:]
		if pcd == 0 { // zero pc delta terminates the table
			break
		}
		pc += int64(pcd)
		if len(b) == 1 && b[0] == 0 { // final coverage, no value change
			break
		}
		vd, n := binary.Varint(b)
		b = b[n:]
		val += vd
		pcs = append(pcs, pc)
		vals = append(vals, val)
	}
	return pcs, vals
}

// TestGOObjectPcspFrame checks the pcsp table of a frame-pointer function:
// the prologue raises the stack delta to 8+frame, the RET's epilogue
// restores it to zero.
func TestGOObjectPcspFrame(t *testing.T) {
	f, errs := parser.Parse("frame_amd64.s", `
#include "textflag.h"
TEXT ·framed(SB), NOSPLIT, $8-0
	MOVQ BP, AX
	RET
`)
	if len(errs) > 0 {
		t.Fatalf("parse: %v", errs)
	}
	img, err := AssembleFile(f)
	if err != nil {
		t.Fatalf("AssembleFile: %v", err)
	}
	fn := img.Funcs[0]
	pcs, vals := decodePCValues(pcspTable(fn, 1))
	// Prologue: PUSHQ BP (1 byte, +8), MOVQ SP, BP (3 bytes, no change),
	// SUBQ $8, SP (4 bytes, +16 in total); the RET's epilogue unwinds
	// ADDQ $8, SP (+8) then POPQ BP (0).
	wantPCs := []int64{0, 1, 8}
	wantVals := []int64{0, 8, 16}
	if len(pcs) < len(wantPCs) {
		t.Fatalf("pcsp pcs = %v vals = %v", pcs, vals)
	}
	for i := range wantPCs {
		if pcs[i] != wantPCs[i] || vals[i] != wantVals[i] {
			t.Errorf("pcsp[%d] = (%d,%d), want (%d,%d); all: %v %v", i, pcs[i], vals[i], wantPCs[i], wantVals[i], pcs, vals)
		}
	}
	// The last two steps unwind the epilogue to zero.
	n := len(pcs)
	if vals[n-1] != 0 || vals[n-2] != 8 {
		t.Errorf("epilogue steps = %v %v, want …8, 0", pcs, vals)
	}
	// The table covers the whole function.
	if last := pcs[n-1]; last >= int64(fn.Size) {
		t.Errorf("last pc %d beyond function size %d", last, fn.Size)
	}
}

// TestGOObjectExternalRejected checks that a reference to a symbol no GLOBL
// defines is reported: GOOBJ emission resolves only file-local symbols so
// far.
func TestGOObjectExternalRejected(t *testing.T) {
	f, errs := parser.Parse("ext_amd64.s", `
#include "textflag.h"
TEXT ·useext(SB), NOSPLIT, $0-8
	MOVQ elsewhere(SB), AX
	MOVQ AX, ret+0(FP)
	RET
`)
	if len(errs) > 0 {
		t.Fatalf("parse: %v", errs)
	}
	img, err := AssembleFile(f)
	if err != nil {
		t.Fatalf("AssembleFile: %v", err)
	}
	if _, err := img.GOObject("p", "ext_amd64.s"); err == nil || !strings.Contains(err.Error(), "external") {
		t.Errorf("error = %v, want an external-symbol error", err)
	}
}

// TestGOObjectLinkAndRun is the end-to-end check: assemble the test
// functions to a GOOBJ, swap it into a go build in place of the toolchain's
// assembly object, link, and run; the output must match the baseline
// binary the Go assembler produced.  Skipped when no Go toolchain is
// available.
func TestGOObjectLinkAndRun(t *testing.T) {
	goBin, err := exec.LookPath("go")
	if err != nil {
		t.Skip("no Go toolchain available")
	}
	dir := t.TempDir()

	const asmSrc = `
#include "textflag.h"

TEXT ·addq(SB), NOSPLIT, $0-24
	MOVQ a+0(FP), AX
	MOVQ b+8(FP), CX
	ADDQ CX, AX
	MOVQ AX, ret+16(FP)
	RET

TEXT ·loadmask(SB), NOSPLIT, $0-8
	VMOVDQU mask<>(SB), X0
	VPMOVMSKB X0, AX
	MOVQ AX, ret+0(FP)
	RET

GLOBL mask<>(SB), RODATA, $16
DATA mask<>+0(SB)/8, $0x0807060504030201
DATA mask<>+8(SB)/8, $0x800f0e0d0c0b0a09
`
	const mainSrc = `package main

func addq(a, b int64) int64
func loadmask() int64

func main() {
	println(addq(41, 1))
	println(loadmask())
}
`
	if err := os.WriteFile(filepath.Join(dir, "main_amd64.s"), []byte(asmSrc), 0o644); err != nil {
		t.Fatal(err)
	}
	if err := os.WriteFile(filepath.Join(dir, "main.go"), []byte(mainSrc), 0o644); err != nil {
		t.Fatal(err)
	}
	if err := os.WriteFile(filepath.Join(dir, "go.mod"), []byte("module goobjtest\n\ngo 1.27\n"), 0o644); err != nil {
		t.Fatal(err)
	}

	// Baseline build with the toolchain's assembler; keep the work
	// directory and the commands the build used.
	cmd := exec.Command(goBin, "build", "-x", "-work", "-o", "app", ".")
	cmd.Dir = dir
	buildLog, err := cmd.CombinedOutput()
	if err != nil {
		t.Fatalf("baseline build: %v\n%s", err, buildLog)
	}
	var work string
	var asmObj, pkgArch, linkLine string
	for line := range strings.SplitSeq(string(buildLog), "\n") {
		switch {
		case strings.HasPrefix(line, "WORK="):
			work = strings.TrimPrefix(line, "WORK=")
		case strings.Contains(line, "/asm ") && strings.Contains(line, "-o ") && strings.Contains(line, "main_amd64.s") && !strings.Contains(line, "-gensymabis"):
			asmObj = fieldAfter(line, "-o")
		case strings.Contains(line, "pack r") && strings.Contains(line, "_pkg_.a"):
			pkgArch = strings.TrimSpace(strings.SplitN(line, "pack r", 2)[1])
			pkgArch = strings.Fields(strings.SplitN(pkgArch, "#", 2)[0])[0]
		case strings.Contains(line, "/link ") && strings.Contains(line, "-importcfg"):
			linkLine = line
		}
	}
	if work == "" || asmObj == "" || pkgArch == "" || linkLine == "" {
		t.Fatalf("could not locate the build steps:\n%s", buildLog)
	}
	asmObj = strings.ReplaceAll(asmObj, "$WORK", work)
	pkgArch = strings.ReplaceAll(pkgArch, "$WORK", work)

	// The baseline's answer.
	baseOut, err := exec.Command(filepath.Join(dir, "app")).CombinedOutput()
	if err != nil {
		t.Fatalf("run baseline: %v\n%s", err, baseOut)
	}

	// Assemble the same source with gasm and swap the object in.
	pf, perrs := parser.Parse(filepath.Join(dir, "main_amd64.s"), asmSrc)
	if len(perrs) > 0 {
		t.Fatalf("parse: %v", perrs)
	}
	img, err := AssembleFile(pf)
	if err != nil {
		t.Fatalf("AssembleFile: %v", err)
	}
	obj, err := img.GOObject("main", filepath.Join(dir, "main_amd64.s"))
	if err != nil {
		t.Fatalf("GOObject: %v", err)
	}

	// Rebuild the package archive with our object in place of the
	// toolchain's (go tool pack has no replace-in-place that dedupes, so
	// extract, substitute and repack).  The archive member holding the
	// assembler's output is named after the asm object file, e.g.
	// main_amd64.o.
	extract := exec.Command(goBin, "tool", "pack", "x", pkgArch)
	membersDir := filepath.Join(dir, "members")
	if err := os.MkdirAll(membersDir, 0o755); err != nil {
		t.Fatal(err)
	}
	extract.Dir = membersDir
	if out, err := extract.CombinedOutput(); err != nil {
		t.Fatalf("pack x: %v\n%s", err, out)
	}
	member := filepath.Join(membersDir, filepath.Base(asmObj))
	if err := os.Chmod(member, 0o644); err != nil {
		t.Fatal(err)
	}
	if err := os.WriteFile(member, obj, 0o644); err != nil {
		t.Fatal(err)
	}
	listCmd := exec.Command(goBin, "tool", "pack", "t", pkgArch)
	listOut, err := listCmd.CombinedOutput()
	if err != nil {
		t.Fatalf("pack t: %v\n%s", err, listOut)
	}
	newArch := filepath.Join(dir, "pkg.a")
	args := []string{"tool", "pack", "c", newArch}
	seen := map[string]bool{}
	for m := range strings.FieldsSeq(string(listOut)) {
		if seen[m] {
			continue
		}
		seen[m] = true
		if err := os.Chmod(filepath.Join(membersDir, m), 0o644); err != nil {
			t.Fatal(err)
		}
		args = append(args, filepath.Join(membersDir, m))
	}
	pack := exec.Command(goBin, args...)
	pack.Dir = membersDir
	if out, err := pack.CombinedOutput(); err != nil {
		t.Fatalf("pack c: %v\n%s", err, out)
	}

	// Link with our archive.  The link line carries a GOROOT assignment
	// and $WORK placeholders; run it through the shell with the
	// GOEXPERIMENT the toolchain expects (the linker compares the object
	// header against its own, experiments included).
	goExp, _ := exec.Command(goBin, "env", "GOEXPERIMENT").Output()
	linkLine = strings.ReplaceAll(linkLine, "$WORK", work)
	linkLine = strings.ReplaceAll(linkLine, filepath.Join(work, "b001", "_pkg_.a"), newArch)
	linkLine = strings.ReplaceAll(linkLine, filepath.Join(work, "b001", "exe", "a.out"), filepath.Join(dir, "app2"))
	link := exec.Command("sh", "-c", linkLine)
	link.Dir = dir
	link.Env = append(os.Environ(), "GOEXPERIMENT="+strings.TrimSpace(string(goExp)))
	if out, err := link.CombinedOutput(); err != nil {
		t.Fatalf("link with gasm object: %v\n%s", err, out)
	}
	got, err := exec.Command(filepath.Join(dir, "app2")).CombinedOutput()
	if err != nil {
		t.Fatalf("run gasm-linked binary: %v\n%s", err, got)
	}
	if !bytes.Equal(got, baseOut) {
		t.Errorf("gasm-linked output %q, want baseline %q", got, baseOut)
	}
}

// fieldAfter returns the whitespace-delimited field following the first
// occurrence of flag in line.
func fieldAfter(line, flag string) string {
	fields := strings.Fields(line)
	for i, f := range fields {
		if f == flag && i+1 < len(fields) {
			return fields[i+1]
		}
	}
	return ""
}

// TestGOObjectExternalPackageLink is the cross-package end-to-end check: a
// GOOBJ whose code references a real external package symbol (runtime's
// morestack, a plain reference rather than the builtin noctxt form) must
// carry a package index that points past the blkPkgIdx table's dummy entry
// 0, and the object must link against the real runtime.  Pre-fix, the
// relocations carried block index 0, which the loader never fills, so the
// reference resolved against whatever object was loaded first and the link
// failed.  The binary is not run: morestack returns to the call site's
// stack check, which a hand-written caller has none of.
func TestGOObjectExternalPackageLink(t *testing.T) {
	goBin, err := exec.LookPath("go")
	if err != nil {
		t.Skip("no Go toolchain available")
	}
	dir := t.TempDir()

	const asmSrc = `
#include "textflag.h"

TEXT ·fn(SB), NOSPLIT, $0-0
	CALL ·helper(SB)
	RET

TEXT ·helper(SB), NOSPLIT, $0-0
	RET
`
	const mainSrc = `package main

func fn()
func helper()

func main() {
	fn()
	helper()
}
`
	if err := os.WriteFile(filepath.Join(dir, "main_amd64.s"), []byte(asmSrc), 0o644); err != nil {
		t.Fatal(err)
	}
	if err := os.WriteFile(filepath.Join(dir, "main.go"), []byte(mainSrc), 0o644); err != nil {
		t.Fatal(err)
	}
	if err := os.WriteFile(filepath.Join(dir, "go.mod"), []byte("module extlink\n\ngo 1.27\n"), 0o644); err != nil {
		t.Fatal(err)
	}

	// Capture the build the toolchain performs and re-run only its link
	// step with our object swapped into the package archive, mirroring
	// TestGOObjectLinkAndRun.
	build := exec.Command(goBin, "build", "-x", "-work", "-o", filepath.Join(dir, "prog"), ".")
	build.Dir = dir
	buildLog, err := build.CombinedOutput()
	if err != nil {
		t.Fatalf("baseline build: %v\n%s", err, buildLog)
	}
	var work, linkLine, asmObj, pkgArch string
	for line := range strings.SplitSeq(string(buildLog), "\n") {
		switch {
		case strings.HasPrefix(line, "WORK="):
			work = strings.TrimPrefix(line, "WORK=")
		case strings.Contains(line, "/asm ") && strings.Contains(line, "main_amd64.s") && !strings.Contains(line, "-gensymabis"):
			asmObj = fieldAfter(line, "-o")
		case strings.Contains(line, "pack r") && strings.Contains(line, "_pkg_.a"):
			pkgArch = strings.TrimSpace(strings.SplitN(line, "pack r", 2)[1])
			pkgArch = strings.Fields(strings.SplitN(pkgArch, "#", 2)[0])[0]
		case strings.Contains(line, "/link ") && strings.Contains(line, "-importcfg"):
			linkLine = line
		}
	}
	if work == "" || asmObj == "" || pkgArch == "" || linkLine == "" {
		t.Skipf("could not parse build log (work=%q asmObj=%q)", work, asmObj)
	}
	defer os.RemoveAll(work)
	asmObj = strings.ReplaceAll(asmObj, "$WORK", work)
	pkgArch = strings.ReplaceAll(pkgArch, "$WORK", work)

	// Assemble the source with gasm, then retarget fn's internal call at
	// a real external package symbol: the reloc's qualified name drives
	// the export-data resolution the way a source-level runtime·sym(SB)
	// reference would.
	f, errs := parser.Parse("main_amd64.s", asmSrc)
	if len(errs) > 0 {
		t.Fatalf("parse: %v", errs)
	}
	img, err := AssembleFile(f)
	if err != nil {
		t.Fatalf("AssembleFile: %v", err)
	}
	fn := &img.Funcs[0]
	for i := range fn.Relocs {
		fn.Relocs[i].Name = "runtime\u00b7morestack"
		fn.Relocs[i].External = true
	}
	img.Externals = []string{"runtime\u00b7morestack"}
	obj, err := img.GOObject("main", "main_amd64.s")
	if err != nil {
		t.Fatalf("GOObject: %v", err)
	}

	// Structural check: the blkPkgIdx block reserves entry 0 for the
	// dummy invalid package and places runtime at entry 1, and fn's call
	// relocation carries PkgIdx 1.
	v := openGoobj(t, obj)
	pkgBlk := v.blk(blkPkgIdx)
	if len(pkgBlk) != 2*8 {
		t.Fatalf("blkPkgIdx = %d bytes, want two entries", len(pkgBlk))
	}
	le := binary.LittleEndian
	strEntry := func(i int) string {
		e := pkgBlk[i*8 : (i+1)*8]
		return v.str(le.Uint32(e[4:]), le.Uint32(e[0:]))
	}
	if s := strEntry(0); s != "" {
		t.Errorf("blkPkgIdx[0] = %q, want the dummy empty package", s)
	}
	if s := strEntry(1); s != "runtime" {
		t.Errorf("blkPkgIdx[1] = %q, want runtime", s)
	}
	relocs := v.blk(blkReloc)
	// fn is the last non-package symbol (two functions, four pc tables
	// each); its one reloc is the final record.
	fnRec := relocs[len(relocs)-23:]
	if pIdx := le.Uint32(fnRec[15:]); pIdx != 1 {
		t.Errorf("external reloc PkgIdx = %d, want 1 (runtime)", pIdx)
	}

	// Swap the object into the package archive and link with cmd/link;
	// the link line consumes the archive, not the loose object file.
	membersDir := filepath.Join(dir, "members")
	if err := os.MkdirAll(membersDir, 0o755); err != nil {
		t.Fatal(err)
	}
	extract := exec.Command(goBin, "tool", "pack", "x", pkgArch)
	extract.Dir = membersDir
	if out, err := extract.CombinedOutput(); err != nil {
		t.Fatalf("pack x: %v\n%s", err, out)
	}
	member := filepath.Join(membersDir, filepath.Base(asmObj))
	if err := os.Chmod(member, 0o644); err != nil {
		t.Fatal(err)
	}
	if err := os.WriteFile(member, obj, 0o644); err != nil {
		t.Fatal(err)
	}
	listCmd := exec.Command(goBin, "tool", "pack", "t", pkgArch)
	listOut, err := listCmd.CombinedOutput()
	if err != nil {
		t.Fatalf("pack t: %v\n%s", err, listOut)
	}
	newArch := filepath.Join(dir, "pkg.a")
	args := []string{"tool", "pack", "c", newArch}
	seen := map[string]bool{}
	for m := range strings.FieldsSeq(string(listOut)) {
		if seen[m] {
			continue
		}
		seen[m] = true
		if err := os.Chmod(filepath.Join(membersDir, m), 0o644); err != nil {
			t.Fatal(err)
		}
		args = append(args, filepath.Join(membersDir, m))
	}
	pack := exec.Command(goBin, args...)
	pack.Dir = membersDir
	if out, err := pack.CombinedOutput(); err != nil {
		t.Fatalf("pack c: %v\n%s", err, out)
	}
	linkLine = strings.ReplaceAll(linkLine, "$WORK", work)
	linkLine = strings.ReplaceAll(linkLine, pkgArch, newArch)
	linkLine = strings.ReplaceAll(linkLine, filepath.Join(work, "b001", "exe", "a.out"), filepath.Join(dir, "prog2"))
	linkCmd := exec.Command("sh", "-c", "cd "+dir+" && "+linkLine)
	if out, err := linkCmd.CombinedOutput(); err != nil {
		t.Fatalf("re-link with gasm object: %v\n%s", err, out)
	}

	// The call must have resolved to the real runtime symbol.
	dump, err := exec.Command(goBin, "tool", "objdump", "-s", "main.fn", filepath.Join(dir, "prog2")).CombinedOutput()
	if err != nil {
		t.Fatalf("objdump main.fn: %v\n%s", err, dump)
	}
	if !bytes.Contains(dump, []byte("runtime.morestack")) {
		t.Errorf("main.fn does not call runtime.morestack:\n%s", dump)
	}
}