feat(amd64): LOCK and REP prefixes, literal data pseudo-ops and ADJSP

Assisted-by: GLM 5.3 Flash

asm/assemble.go

@@ -67,6 +67,9 @@ type spadjStep struct {
 // patch sites (for the file-level layout to resolve), the label table and the
 // stack-adjustment boundaries.
 func assemble(t *ast.Text, link *linkInfo) ([]byte, []sbPatch, map[string]int, []spadjStep, []LineEntry, error) {
+	if err := checkAdjspBalance(t); err != nil {
+		return nil, nil, nil, nil, nil, err
+	}
 	fi := computeFrame(t)
 	chain := jumpChain(t)
 	resolve := func(name string) string {
@@ -203,6 +206,14 @@ func assemble(t *ast.Text, link *linkInfo) ([]byte, []sbPatch, map[string]int, [
 			spadjStep{guardLen + len(fi.prologue), 8 + fi.size},
 		)
 	}
+	// frameBase is the SP delta the prologue leaves: 8 for the saved base
+	// pointer plus the frame, 0 frameless.  bodyDelta tracks the ADJSP
+	// statements' straight-line sum, so a mid-body step's value is the
+	// frame base plus what the body has opened so far.
+	frameBase, bodyDelta := 0, 0
+	if fi.useFP {
+		frameBase = 8 + fi.size
+	}
 	pos := guardLen + len(fi.prologue)
 	for i, stmt := range t.Body {
 		s, ok := stmt.(*ast.Instr)
@@ -230,6 +241,16 @@ func assemble(t *ast.Text, link *linkInfo) ([]byte, []sbPatch, map[string]int, [
 				ps[k].kind = RelCall
 			}
 		}
+		if strings.ToUpper(s.Mnemonic.Text) == "ADJSP" && len(s.Operands) == 1 && s.Operands[0].Imm.HasVal {
+			// The statement shifted SP mid-body: record the new running
+			// delta as the value in effect from just past the instruction.
+			v := s.Operands[0].Imm.Val
+			if s.Operands[0].Imm.Neg {
+				v = -v
+			}
+			bodyDelta += int(v)
+			steps = append(steps, spadjStep{pos + len(code), frameBase + bodyDelta})
+		}
 		patches = append(patches, ps...)
 		lines = append(lines, LineEntry{Offset: pos, Line: s.Pos().Line})
 		out = append(out, code...)
@@ -412,6 +433,40 @@ func hasCall(t *ast.Text) bool {
 	return false
 }
 
+// checkAdjspBalance mirrors the toolchain's push/pop walk: every ADJSP
+// shifts SP away from the entry state and every RET must see the shifts
+// closed.  The assembler's own prologue and epilogue contribute matching
+// deltas on both sides, so the statements' straight-line sum must be zero
+// at each RET; branches do not reset the walk, which runs over the program
+// list in source order.  go tool asm reports an offender as "unbalanced
+// PUSH/POP" (verified against ADJSP $16 before a RET, accepted as a
+// $16/$-16 pair, per-RET rather than per-function).
+func checkAdjspBalance(t *ast.Text) error {
+	delta := 0
+	for _, stmt := range t.Body {
+		in, ok := stmt.(*ast.Instr)
+		if !ok {
+			continue
+		}
+		switch strings.ToUpper(in.Mnemonic.Text) {
+		case "ADJSP":
+			if len(in.Operands) != 1 || !in.Operands[0].Imm.HasVal {
+				continue // reported during emission
+			}
+			v := in.Operands[0].Imm.Val
+			if in.Operands[0].Imm.Neg {
+				v = -v
+			}
+			delta += int(v)
+		case "RET":
+			if delta != 0 {
+				return fmt.Errorf("unbalanced PUSH/POP")
+			}
+		}
+	}
+	return nil
+}
+
 // guardLen returns the byte length of the stack-split guard prefix.  The
 // final conditional branch (JBE, and JB in the big class) is 2 bytes in the
 // short form and 6 in the long form.

asm/assemble_test.go

@@ -439,3 +439,132 @@ func TestSubSPEncodings(t *testing.T) {
 		}
 	}
 }
+
+// TestAssemblePseudoStatements runs LOCK/REP, BYTE/WORD and END through the
+// full statement pipeline, pinned against go tool asm (Go 1.27, amd64).  It
+// asserts the three behaviours the toolchain shows: each prefix statement is
+// a standalone byte with a PC of its own (so a label placed on the LOCK
+// points at the F0), the data pseudo-ops write their literal bytes inline,
+// and END terminates nothing (the statements after it still belong to the
+// function and carry no trace of it).
+func TestAssemblePseudoStatements(t *testing.T) {
+	fn := firstText(t, `
+#include "textflag.h"
+TEXT ·pseudo(SB), NOSPLIT, $0-0
+pfx:
+	LOCK
+	CMPXCHGQ AX, (BX)
+	REP
+	MOVSQ
+	BYTE $0x0f
+	BYTE $0x1f
+	WORD $0x1234
+	END
+	BYTE $0x02
+	RET
+`)
+	code, labels, err := Assemble(fn)
+	if err != nil {
+		t.Fatalf("Assemble: %v", err)
+	}
+	// go tool asm: f0 480fb103 f3 48a5 0f 1f 3412 02 c3
+	want := []byte{
+		0xf0,
+		0x48, 0x0f, 0xb1, 0x03,
+		0xf3, 0x48, 0xa5,
+		0x0f, 0x1f, 0x34, 0x12,
+		0x02, 0xc3,
+	}
+	if hexBytes(code) != hexBytes(want) {
+		t.Errorf("pseudo statements:\n got:  %s\n want: %s", hexBytes(code), hexBytes(want))
+	}
+	// The label sits on the LOCK byte, exactly where the toolchain's PC
+	// listing puts it.
+	if off := labels["pfx"]; off != 0 {
+		t.Errorf("label pfx = %d, want 0 (the LOCK's own byte)", off)
+	}
+	// The trailing BYTE lands where the layout says: after the 8 bytes of
+	// LOCK, CMPXCHGQ, REP and MOVSQ plus the 4 data bytes, END contributing
+	// none.
+	if code[12] != 0x02 {
+		t.Errorf("byte at 12 = %02x, want 02 (the BYTE after END)", code[12])
+	}
+}
+
+// TestAssembleAdjspBalance pins the toolchain's push/pop balance rule over
+// ADJSP: the straight-line sum of the adjustments must be zero at each
+// RET, branches in between counting for nothing (verified against go tool
+// asm: ADJSP $16 before a RET is reported as "unbalanced PUSH/POP", a
+// $16/$-16 pair with a JMP in between assembles).
+func TestAssembleAdjspBalance(t *testing.T) {
+	// Balanced pair with a branch in between, bytes pinned from go tool asm.
+	fn := firstText(t, `
+#include "textflag.h"
+TEXT ·adjsp(SB), NOSPLIT, $0-0
+	ADJSP $16
+	JMP body
+body:
+	ADJSP $-16
+	RET
+`)
+	code, _, err := Assemble(fn)
+	if err != nil {
+		t.Fatalf("Assemble: %v", err)
+	}
+	want := []byte{0x48, 0x83, 0xEC, 0x10, 0xEB, 0x00, 0x48, 0x83, 0xC4, 0x10, 0xC3}
+	if hexBytes(code) != hexBytes(want) {
+		t.Errorf("adjsp pair:\n got:  %s\n want: %s", hexBytes(code), hexBytes(want))
+	}
+
+	// Unbalanced at the RET: the toolchain diagnoses, so must we.
+	_, _, err = Assemble(firstText(t, `
+#include "textflag.h"
+TEXT ·unbalanced(SB), NOSPLIT, $0-0
+	ADJSP $16
+	RET
+`))
+	if err == nil || !strings.Contains(err.Error(), "unbalanced PUSH/POP") {
+		t.Errorf("unbalanced ADJSP: err = %v, want unbalanced PUSH/POP", err)
+	}
+
+	// The check runs per RET: a closed pair before the first RET does not
+	// excuse an open adjustment before the second.
+	_, _, err = Assemble(firstText(t, `
+#include "textflag.h"
+TEXT ·tworet(SB), NOSPLIT, $0-0
+	ADJSP $8
+	ADJSP $-8
+	RET
+mid:
+	ADJSP $8
+	RET
+`))
+	if err == nil || !strings.Contains(err.Error(), "unbalanced PUSH/POP") {
+		t.Errorf("second RET with open ADJSP: err = %v, want unbalanced PUSH/POP", err)
+	}
+
+	// A framed function: the assembler's own prologue and epilogue
+	// contribute matching deltas, so the pair in the body still balances,
+	// and the bytes match go tool asm end to end.
+	fn = firstText(t, `
+#include "textflag.h"
+TEXT ·framed(SB), $16-8
+	ADJSP $8
+	ADJSP $-8
+	RET
+`)
+	code, _, err = Assemble(fn)
+	if err != nil {
+		t.Fatalf("Assemble framed: %v", err)
+	}
+	want = []byte{
+		0x55, 0x48, 0x89, 0xE5, 0x48, 0x83, 0xEC, 0x10, // prologue
+		0x48, 0x83, 0xEC, 0x08, // ADJSP $8
+		0x48, 0x83, 0xC4, 0x08, // ADJSP $-8
+		0x48, 0x83, 0xC4, 0x10, 0x5D, // epilogue
+		0xC3,
+	}
+	if hexBytes(code) != hexBytes(want) {
+		t.Errorf("framed adjsp:\n got:  %s\n want: %s", hexBytes(code), hexBytes(want))
+	}
+}

asm/encodable.go

@@ -19,7 +19,10 @@ func Encodable(mnemonic string) bool {
 	// Fixed-name instructions (no size suffix).
 	switch upper {
 	case "RET", "NOP", "CALL", "JMP",
-		"POPFQ", "PUSHFQ", "INT", "LDMXCSR", "STMXCSR", "CMPSD", "SHA256RNDS2":
+		"POPFQ", "PUSHFQ", "INT", "LDMXCSR", "STMXCSR", "CMPSD", "SHA256RNDS2",
+		// The literal-data pseudo-ops, the accepted-and-ignored END and the
+		// SP adjust.
+		"BYTE", "WORD", "LONG", "QUAD", "END", "ADJSP":
 		return true
 	}
 	if _, ok := noOperandTable[upper]; ok {

asm/encode.go

@@ -92,6 +92,16 @@ func (e *enc) encode(mnem string, ops []Operand) error {
 	// SHA256RNDS2 carries the round constant in a literal X0 first operand.
 	case "SHA256RNDS2":
 		return e.encodeSha256rnds2(ops)
+	// BYTE, WORD, LONG and QUAD write the immediate into the text stream
+	// itself: 1, 2, 4 or 8 literal bytes, little-endian.  END is accepted
+	// and ignored.  ADJSP adjusts SP by the immediate, sign-chosen between
+	// the SUBQ and ADDQ forms.
+	case "BYTE", "WORD", "LONG", "QUAD":
+		return e.encodeData(upper, ops)
+	case "END":
+		return e.encodeEnd(ops)
+	case "ADJSP":
+		return e.encodeAdjsp(ops)
 	}
 
 	// VEX (AVX/AVX2) and EVEX (AVX-512) instructions: the trailing
@@ -241,6 +251,73 @@ var prefetchVariant = map[string]int{
 	"PREFETCHT2":  3,
 }
 
+// dataWidth is the literal byte count of each data-emission pseudo-op.
+var dataWidth = map[string]int{
+	"BYTE": 1,
+	"WORD": 2,
+	"LONG": 4,
+	"QUAD": 8,
+}
+
+// encodeData emits the literal-data pseudo-ops: BYTE, WORD, LONG and QUAD
+// write the immediate into the text stream as 1, 2, 4 or 8 bytes,
+// little-endian, with no opcode lookup.  The value is truncated to the
+// width rather than range-checked, exactly as go tool asm behaves (BYTE
+// $0x1FF emits FF, WORD $0x12345 emits 45 23, both without an error), and
+// exactly one immediate is accepted: the toolchain rejects a list such as
+// BYTE $1, $2, $3.
+func (e *enc) encodeData(mnem string, ops []Operand) error {
+	if len(ops) != 1 {
+		return fmt.Errorf("%s expects 1 immediate operand, got %d", mnem, len(ops))
+	}
+	imm, ok := ops[0].(Imm)
+	if !ok {
+		return fmt.Errorf("%s requires an integer immediate", mnem)
+	}
+	width := dataWidth[mnem]
+	out := make([]byte, width)
+	u := uint64(imm)
+	for i := range width {
+		out[i] = byte(u >> (8 * i))
+	}
+	e.out = append(e.out, out...)
+	return nil
+}
+
+// encodeEnd accepts-and-ignores END.  go tool asm drops the statement
+// entirely: the AEND Prog is skipped when the program list is flushed, so
+// the statements after an END still belong to the same function and the
+// encoded body carries no trace of it, whatever operands follow the name
+// (the toolchain takes END $0 and END AX alike).  Zero bytes, no effect.
+func (e *enc) encodeEnd(ops []Operand) error {
+	return nil
+}
+
+// encodeAdjsp emits ADJSP $imm: a positive value is SUBQ $imm, SP, a
+// negative one ADDQ $-imm, SP, in the imm8 or imm32 form the magnitude
+// picks (the same selection subSP and addSP make for the frame).  go tool
+// asm refuses ADJSP $0 outright, so a zero value is an error here too; the
+// statement's effect on the SP balance is checked by the function-level
+// assembly (checkAdjspBalance), as the toolchain's push/pop walk does.
+func (e *enc) encodeAdjsp(ops []Operand) error {
+	if len(ops) != 1 {
+		return fmt.Errorf("ADJSP expects 1 immediate operand, got %d", len(ops))
+	}
+	imm, ok := ops[0].(Imm)
+	if !ok {
+		return fmt.Errorf("ADJSP requires an integer immediate")
+	}
+	switch v := int(imm); {
+	case v > 0:
+		e.out = append(e.out, subSP(v)...)
+	case v < 0:
+		e.out = append(e.out, addSP(-v)...)
+	default:
+		return fmt.Errorf("ADJSP $0 has no encoding")
+	}
+	return nil
+}
+
 // splitSize separates a trailing B/W/L/Q size suffix from the mnemonic.
 func splitSize(upper string) (base string, size int) {
 	if upper == "" {

asm/encode_test.go

@@ -925,3 +925,142 @@ func TestMOVQXMMGroundTruth(t *testing.T) {
 		}
 	}
 }
+
+// TestPrefixStatements pins LOCK, REP and REPN.  go tool asm encodes each as
+// a standalone one-byte instruction with a PC of its own (F0, F3, F2), not a
+// prefix field merged into the following instruction, and it validates
+// nothing about the pairing (LOCK before NOP assembles).  The prefixed
+// atomic and string shapes are the bytes the runtime's own kernels need.
+func TestPrefixStatements(t *testing.T) {
+	cases := []struct {
+		name string
+		mnem string
+		ops  []Operand
+		want string
+	}{
+		{"LOCK", "LOCK", nil, "f0"},
+		{"REP", "REP", nil, "f3"},
+		{"REPN", "REPN", nil, "f2"},
+		// LOCK; CMPXCHGQ AX, (BX)
+		{"LOCK CMPXCHGQ", "CMPXCHGQ", []Operand{AX, Ptr(BX, 0, 8)}, "480fb103"},
+		// REP; MOVSQ
+		{"REP MOVSQ", "MOVSQ", nil, "48a5"},
+		// REPN; MOVSB
+		{"REPN MOVSB", "MOVSB", nil, "a4"},
+	}
+	for _, c := range cases {
+		code, err := Encode(c.mnem, c.ops...)
+		if err != nil {
+			t.Errorf("%s: %v", c.name, err)
+			continue
+		}
+		if got := fmt.Sprintf("%x", code); got != c.want {
+			t.Errorf("%s = %s, want %s", c.name, got, c.want)
+		}
+	}
+	// The prefix statements take no operands, as the toolchain reports for
+	// LOCK AX.
+	if _, err := Encode("LOCK", AX); err == nil {
+		t.Error("LOCK AX assembled, want an error")
+	}
+	if _, err := Encode("REP", Imm(1)); err == nil {
+		t.Error("REP $1 assembled, want an error")
+	}
+}
+
+// TestDataEmission pins BYTE, WORD, LONG and QUAD: the immediate lands in
+// the text stream as 1, 2, 4 or 8 little-endian bytes with no opcode
+// lookup, truncated to the width rather than range-checked (go tool asm
+// emits FF for BYTE $0x1FF and 45 23 for WORD $0x12345, both silently).
+func TestDataEmission(t *testing.T) {
+	cases := []struct {
+		name string
+		mnem string
+		imm  Imm
+		want string
+	}{
+		{"BYTE", "BYTE", 0x0f, "0f"},
+		{"BYTE negative", "BYTE", -1, "ff"},
+		{"BYTE truncated", "BYTE", 0x1ff, "ff"},
+		{"WORD", "WORD", 0x1234, "3412"},
+		{"WORD negative", "WORD", -1, "ffff"},
+		{"WORD truncated", "WORD", 0x12345, "4523"},
+		{"LONG", "LONG", 0x11223344, "44332211"},
+		{"LONG negative", "LONG", -1, "ffffffff"},
+		{"QUAD", "QUAD", 0x1122334455667788, "8877665544332211"},
+		{"QUAD negative", "QUAD", -2, "feffffffffffffff"},
+	}
+	for _, c := range cases {
+		code, err := Encode(c.mnem, c.imm)
+		if err != nil {
+			t.Errorf("%s: %v", c.name, err)
+			continue
+		}
+		if got := fmt.Sprintf("%x", code); got != c.want {
+			t.Errorf("%s = %s, want %s", c.name, got, c.want)
+		}
+	}
+	// Exactly one immediate: the toolchain rejects BYTE $1, $2, $3, and a
+	// register or a missing operand is no immediate at all.
+	if _, err := Encode("BYTE"); err == nil {
+		t.Error("BYTE with no operand assembled, want an error")
+	}
+	if _, err := Encode("BYTE", Imm(1), Imm(2)); err == nil {
+		t.Error("BYTE $1, $2 assembled, want an error")
+	}
+	if _, err := Encode("WORD", AX); err == nil {
+		t.Error("WORD AX assembled, want an error")
+	}
+}
+
+// TestEndIgnored pins END: go tool asm drops the statement entirely, so it
+// encodes to zero bytes and takes any operands without complaint (the
+// toolchain accepts END $0 and END AX alike).
+func TestEndIgnored(t *testing.T) {
+	for _, ops := range [][]Operand{nil, {Imm(0)}, {AX}} {
+		code, err := Encode("END", ops...)
+		if err != nil {
+			t.Errorf("END: %v", err)
+			continue
+		}
+		if len(code) != 0 {
+			t.Errorf("END = %x, want no bytes", code)
+		}
+	}
+}
+
+// TestAdjsp pins ADJSP: a positive immediate is SUBQ $imm, SP, a negative
+// one ADDQ $-imm, SP, in the imm8 or imm32 form the magnitude picks; $0
+// has no encoding (go tool asm refuses ADJSP $0 outright).
+func TestAdjsp(t *testing.T) {
+	cases := []struct {
+		name string
+		imm  Imm
+		want string
+	}{
+		{"imm8", 112, "4883ec70"},
+		{"imm8 negative", -112, "4883c470"},
+		{"imm32", 200, "4881ecc8000000"},
+		{"imm32 negative", -200, "4881c4c8000000"},
+		{"small", 8, "4883ec08"},
+	}
+	for _, c := range cases {
+		code, err := Encode("ADJSP", c.imm)
+		if err != nil {
+			t.Errorf("%s: %v", c.name, err)
+			continue
+		}
+		if got := fmt.Sprintf("%x", code); got != c.want {
+			t.Errorf("ADJSP %d = %s, want %s", int64(c.imm), got, c.want)
+		}
+	}
+	if _, err := Encode("ADJSP", Imm(0)); err == nil {
+		t.Error("ADJSP $0 assembled, want an error")
+	}
+	if _, err := Encode("ADJSP"); err == nil {
+		t.Error("ADJSP with no operand assembled, want an error")
+	}
+	if _, err := Encode("ADJSP", AX); err == nil {
+		t.Error("ADJSP AX assembled, want an error")
+	}
+}

asm/instrs.go

@@ -65,6 +65,15 @@ var bitTestOp = map[string]int{
 // noOperandTable maps a fixed no-operand mnemonic to its opcode bytes.  The
 // fence names carry their opcode inside the 0F AE /digit group spelled out in
 // full (E8/F0/F8), and PAUSE is F3 90.
+//
+// LOCK, REP and REPN are the prefix statements.  go tool asm encodes each as
+// a standalone one-byte instruction with a PC of its own (F0, F3 and F2
+// respectively), not as a prefix field merged into the next instruction: the
+// statement that follows is encoded unaware of it, and nothing validates
+// that the pairing is a legal one (LOCK before NOP assembles without
+// complaint, each byte pinned against the toolchain).  Because the bytes
+// land in the stream before the following statement anyway, a LOCKed
+// CMPXCHGQ encodes identically to a prefixed form.
 var noOperandTable = map[string][]byte{
 	"CPUID":   {0x0F, 0xA2},
 	"RDTSC":   {0x0F, 0x31},
@@ -78,6 +87,9 @@ var noOperandTable = map[string][]byte{
 	"MFENCE":  {0x0F, 0xAE, 0xF0},
 	"SFENCE":  {0x0F, 0xAE, 0xF8},
 	"UNDEF":   {0x0F, 0x0B},
+	"LOCK":    {0xF0},
+	"REP":     {0xF3},
+	"REPN":    {0xF2},
 }
 
 // --- MOV --------------------------------------------------------------------

testdata/verify/pfx_amd64.s

@@ -0,0 +1,103 @@
+// Instruction prefixes: LOCK, REP and REPN.  go tool asm encodes each
+// statement as a standalone one-byte instruction with a PC of its own (F0,
+// F3 and F2 respectively); the statement that follows is encoded unaware of
+// it, and nothing validates the pairing.  The shapes are the runtime's
+// atomic read-modify-write family and the string moves, every result folded
+// back.
+
+#include "textflag.h"
+
+// func cas64(ptr *uint64, old, new uint64) bool
+TEXT ·cas64(SB), NOSPLIT, $0-25
+	MOVQ     ptr+0(FP), BX
+	MOVQ     old+8(FP), AX
+	MOVQ     new+16(FP), CX
+	LOCK
+	CMPXCHGQ CX, 0(BX)
+	SETEQ    ret+24(FP)
+	RET
+
+// func casloop(addr *uint64, v uint64) uint64
+// The runtime's Or64 shape: a LOCK inside a branch loop, the backward jump
+// measuring over the prefix statement's own byte.
+TEXT ·casloop(SB), NOSPLIT, $0-24
+	MOVQ     addr+0(FP), BX
+	MOVQ     v+8(FP), CX
+
+loop:
+	MOVQ     CX, DX
+	MOVQ     (BX), AX
+	ORQ      AX, DX
+	LOCK
+	CMPXCHGQ DX, (BX)
+	JNZ      loop
+	MOVQ     AX, ret+16(FP)
+	RET
+
+// func xadd64(p *uint64, v uint64) uint64
+TEXT ·xadd64(SB), NOSPLIT, $0-24
+	MOVQ  p+0(FP), AX
+	MOVQ  v+8(FP), BX
+	LOCK
+	XADDQ BX, (AX)
+	MOVQ  AX, ret+16(FP)
+	RET
+
+// func xaddw(p *uint16, v uint16) uint16
+TEXT ·xaddw(SB), NOSPLIT, $0-12
+	MOVQ  p+0(FP), AX
+	MOVW  v+8(FP), BX
+	LOCK
+	XADDW BX, (AX)
+	MOVW  AX, ret+8(FP)
+	RET
+
+// func lockarith(p *uint64)
+TEXT ·lockarith(SB), NOSPLIT, $0-8
+	MOVQ p+0(FP), AX
+	LOCK
+	ORQ  CX, (AX)
+	LOCK
+	ANDL CX, (AX)
+	LOCK
+	INCQ (AX)
+	LOCK
+	DECQ (AX)
+	LOCK
+	ORB  BX, (AX)
+	RET
+
+// func repstring(dst, src *byte, n int)
+// The memmove shapes: forward copy by quadwords, backward tails.
+TEXT ·repstring(SB), NOSPLIT, $0-24
+	MOVQ  dst+0(FP), DI
+	MOVQ  src+8(FP), SI
+	REP
+	MOVSQ
+	REP
+	MOVSB
+	REPN
+	MOVSB
+	REP
+	STOSQ
+	REP
+	STOSB
+	RET
+
+// func pfxlabel()
+// Labels pinned on prefix statements' own bytes: pfx: sits on the LOCK,
+// mid: on the REPN.
+TEXT ·pfxlabel(SB), NOSPLIT, $0-0
+pfx:
+	LOCK
+	XCHGL BX, (AX)
+	JMP   done
+
+mid:
+	REPN
+	MOVSB
+
+done:
+	REP
+	STOSB
+	RET

testdata/verify/rawdata_amd64.s

@@ -0,0 +1,62 @@
+// Literal data emission: BYTE, WORD, LONG and QUAD write the immediate
+// into the text stream as 1, 2, 4 or 8 little-endian bytes with no opcode
+// lookup, truncated to the width rather than range-checked; END is
+// accepted and ignored, contributing no bytes and ending nothing.  The
+// shapes mirror the runtime's hand-laid markers
+// (crypto/internal/boring/sig/sig_amd64.s) and its syscall stubs
+// (runtime/sys_linux_amd64.s).
+
+#include "textflag.h"
+
+// func marker()
+// A boring/crypto-style marker: a hand-laid forward branch whose skip
+// distance is patched at runtime.  One BYTE per statement, as the
+// runtime's own file spells it: the semicolon-separated one-liner the
+// sys_linux_amd64.s stub uses does not survive gasm fmt, which drops the
+// statement separators.
+TEXT ·marker(SB), NOSPLIT, $0-0
+	BYTE $0xEB
+	BYTE $0x1D
+	BYTE $0xF4
+	BYTE $0x48
+	BYTE $0xF4
+	BYTE $0x4B
+	BYTE $0xC3
+	RET
+
+// func stub()
+// The sys_linux_amd64.s stub bytes: the sign-extended
+// "48 c7 c0 0f 00 00 00" form of MOVQ $rt_sigreturn, AX.
+TEXT ·stub(SB), NOSPLIT, $0-0
+	BYTE $0x48
+	BYTE $0xc7
+	BYTE $0xc0
+	BYTE $0x0f
+	BYTE $0x00
+	BYTE $0x00
+	BYTE $0x00
+	RET
+
+// func words()
+// The wider literals, and an END that ends nothing: the WORD after it
+// still lands in this function.
+TEXT ·words(SB), NOSPLIT, $0-0
+	WORD $0x1234
+	WORD $-1
+	LONG $0x11223344
+	LONG $-1
+	QUAD $0x1122334455667788
+	QUAD $-2
+	END
+	WORD $0xBEEF
+	RET
+
+// func trunc()
+// Truncation, not a range check: each literal keeps its low bytes, exactly
+// as go tool asm emits them.
+TEXT ·trunc(SB), NOSPLIT, $0-0
+	BYTE $0x1FF
+	WORD $0x12345
+	LONG $0x123456789
+	QUAD $-2
+	RET