On Fri, Sep 05, 2025 at 10:24:47AM +0200, Peter Zijlstra wrote:

+bool insn_is_nop(struct insn *insn)
+{
+	u8 rex, rex_b = 0, rex_x = 0, rex_r = 0, rex_w = 0;
+	u8 modrm, modrm_mod, modrm_reg, modrm_rm;
+	u8 sib = 0, sib_scale, sib_index, sib_base;
+
+	if (insn->rex_prefix.nbytes) {
+		rex = insn->rex_prefix.bytes[0];
+		rex_w = !!X86_REX_W(rex);
+		rex_r = !!X86_REX_R(rex);
+		rex_x = !!X86_REX_X(rex);
+		rex_b = !!X86_REX_B(rex);
+	}
+
+	if (insn->modrm.nbytes) {
+		modrm = insn->modrm.bytes[0];
+		modrm_mod = X86_MODRM_MOD(modrm);
+		modrm_reg = X86_MODRM_REG(modrm) + 8*rex_r;
+		modrm_rm  = X86_MODRM_RM(modrm)  + 8*rex_b;
+	}
+
+	if (insn->sib.nbytes) {
+		sib = insn->sib.bytes[0];
+		sib_scale = X86_SIB_SCALE(sib);
+		sib_index = X86_SIB_INDEX(sib) + 8*rex_x;
+		sib_base  = X86_SIB_BASE(sib)  + 8*rex_b;
+
+		modrm_rm = sib_base;
+	}
+
+	switch (insn->opcode.bytes[0]) {
+	case 0x0f: /* 2nd byte */
+		break;
+
+	case 0x89: /* MOV */
+		if (modrm_mod != 3) /* register-direct */
+			return false;
+
+		if (insn->x86_64 && !rex_w) /* native size */
+			return false;
+
+		for (int i = 0; i < insn->prefixes.nbytes; i++) {
+			if (insn->prefixes.bytes[i] == 0x66) /* OSP */
+				return false;
+		}
+
+		return modrm_reg == modrm_rm; /* MOV %reg, %reg */
+
+	case 0x8d: /* LEA */
+		if (modrm_mod == 0 || modrm_mod == 3) /* register-indirect with disp */
+			return false;
+
+		if (insn->x86_64 && !rex_w) /* native size */
+			return false;
+
+		if (insn->displacement.value != 0)
+			return false;
+
+		if (sib & (sib_scale != 0 || sib_index != 4)) /* (%reg, %eiz, 1) */

Argh, that should obviously be: &&

+			return false;
+
+		for (int i = 0; i < insn->prefixes.nbytes; i++) {
+			if (insn->prefixes.bytes[i] != 0x3e) /* DS */
+				return false;
+		}
+
+		return modrm_reg == modrm_rm; /* LEA 0(%reg), %reg */
+
+	case 0x90: /* NOP */
+		for (int i = 0; i < insn->prefixes.nbytes; i++) {
+			if (insn->prefixes.bytes[i] == 0xf3) /* REP */
+				return false; /* REP NOP -- PAUSE */
+		}
+		return true;
+
+	case 0xe9: /* JMP.d32 */
+	case 0xeb: /* JMP.d8 */
+		return insn->immediate.value == 0; /* JMP +0 */
+
+	default:
+		return false;
+	}
+
+	switch (insn->opcode.bytes[1]) {
+	case 0x1f:
+		return modrm_reg == 0; /* 0f 1f /0 -- NOPL */
+
+	default:
+		return false;
+	}
+}