refactor(arch): extract the displacement tail and SIB builders

Assisted-by: GLM 5.3 Flash

arch/amd64_ext.go

@@ -183,34 +183,48 @@ func (in ExtInstr) amd64Memory(op ExtOperand, pos int) (base int, disp int64, er
 	return op.Reg, op.Imm, nil
 }
 
+// amd64DispTail lays the displacement bytes and their ModR/M mod field: no
+// bytes at displacement zero, a disp8 when the value fits a signed byte and
+// a disp32 otherwise.  The force flag makes a zero displacement encode
+// anyway, the mod-01 shape the RBP and R13 bases demand, whose bare r/m 101
+// would otherwise address RIP-relative.
+func amd64DispTail(disp int64, force bool) (mod byte, tail []byte) {
+	if !force && disp == 0 {
+		return 0, nil
+	}
+	if disp >= -128 && disp <= 127 {
+		return 1, []byte{byte(disp)}
+	}
+	return 2, []byte{byte(disp), byte(disp >> 8), byte(disp >> 16), byte(disp >> 24)}
+}
+
+// amd64Sib builds the SIB byte the scaled forms carry: the scale field over
+// the index and the base, whose number rides the r/m field.
+func amd64Sib(base, index, scale int) byte {
+	sib := byte(base & 7)
+	sib |= byte(index&7) << 3
+	switch scale {
+	case 2:
+		sib |= 1 << 6
+	case 4:
+		sib |= 2 << 6
+	case 8:
+		sib |= 3 << 6
+	}
+	return sib
+}
+
 // amd64EncodeMemory returns the register-form template with a base-relative
 // memory operand filled in: dest and vvvv keep their register meanings, the
 // ModR/M r/m field carries the base, and the high base bit rides EVEX.B as
 // amd64Encode lays it.  The ModR/M mod bits and the trailing SIB and
 // displacement bytes follow the canonical choices the GNU assembler makes
-// for the plain, unscaled SDM displacements: no displacement bytes at
-// displacement zero, a disp8 when the value fits a signed byte and a disp32
-// otherwise, the SIB byte 0x24 when the base is RSP or R12, whose r/m
-// encoding 100 demands it, and a forced displacement on RBP and R13, whose
-// mod-00 r/m encoding 101 means RIP-relative.  The operand must have passed
-// amd64Memory first.
+// for the plain, unscaled SDM displacements, and the operand must have
+// passed amd64Memory first.
 func amd64EncodeMemory(b []byte, dest, vvvv, base int, disp int64) []byte {
 	out := amd64Encode(b, dest, vvvv, base)
 	rm := base & 7
-	var tail []byte
-	mod := byte(0)
-	switch {
-	case rm == 5 || disp != 0:
-		// RBP and R13 cannot drop the displacement: mod 00 with r/m 101
-		// addresses RIP-relative, not through the base.
-		if disp >= -128 && disp <= 127 {
-			mod = 1
-			tail = []byte{byte(disp)}
-		} else {
-			mod = 2
-			tail = []byte{byte(disp), byte(disp >> 8), byte(disp >> 16), byte(disp >> 24)}
-		}
-	}
+	mod, tail := amd64DispTail(disp, rm == 5)
 	if rm == 4 {
 		// RSP and R12 need the SIB byte: no index, base 100.
 		tail = append([]byte{0x24}, tail...)
@@ -247,29 +261,8 @@ func amd64EncodeScaledMemory(b []byte, dest, vvvv, base, index, scale int, disp
 		out[1] &^= 0x40
 	}
 	rm := base & 7
-	var tail []byte
-	mod := byte(0)
-	switch {
-	case rm == 5 || disp != 0:
-		if disp >= -128 && disp <= 127 {
-			mod = 1
-			tail = []byte{byte(disp)}
-		} else {
-			mod = 2
-			tail = []byte{byte(disp), byte(disp >> 8), byte(disp >> 16), byte(disp >> 24)}
-		}
-	}
-	sib := byte(rm)
-	sib |= byte(index&7) << 3
-	switch scale {
-	case 2:
-		sib |= 1 << 6
-	case 4:
-		sib |= 2 << 6
-	case 8:
-		sib |= 3 << 6
-	}
-	tail = append([]byte{sib}, tail...)
+	mod, tail := amd64DispTail(disp, rm == 5)
+	tail = append([]byte{amd64Sib(rm, index, scale)}, tail...)
 	out[5] = out[5]&0x38 | mod<<6 | 4
 	return append(out, tail...)
 }