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;
> + }
> +}
