Function emit_fetch_byte_dec

src/codec.cpp:111–238

fetch one byte from [ip], decrypt with current cipher state, update state, advance ip, leave result in low 8 of dst

    // fetch one byte from [ip], decrypt with current cipher state, update state,
    // advance ip, leave result in low 8 of dst 
    void emit_fetch_byte_dec(X64Emitter& e, const VMConfig& vm, std::uint8_t dst) {
        const auto& r = vm.dispatcher_regs();
        const std::int32_t add_off  = static_cast<std::int32_t>(vm.state_layout().cipher_extra) + kCipherAddOff;
        const std::int32_t mult_off = static_cast<std::int32_t>(vm.state_layout().cipher_extra) + kCipherMultOff;
        const std::int32_t sbox_off = static_cast<std::int32_t>(vm.state_layout().cipher_extra) + kSboxInvOff;

        // movzx dst, byte [ip]
        e.emit_rex(
            false,
            dst >= 8,
            false,
            r.ip >= 8
        );
        e.u8(0x0F); e.u8(0xB6);
        e.emit_modrm_mem(
            dst & 7,
            r.ip,
            rx::none,
            0,
            0
        );

        // inc ip
        e.inc_reg(r.ip, true);

        // mix byte with cipher_state low, then advance state
        auto add_cs_mem = [&](std::int32_t disp) {
            e.emit_rex(
                true,
                r.cipher_state >= 8,
                false,
                r.state_ptr >= 8
            );
            e.u8(0x03); // ADD r64, r/m64

            e.emit_modrm_mem(
                r.cipher_state & 7,
                r.state_ptr,
                rx::none,
                0,
                disp
            );
        };

        auto imul_cs_mem = [&](std::int32_t disp) {
            e.emit_rex(
                true,
                r.cipher_state >= 8,
                false,
                r.state_ptr >= 8
            );
            e.u8(0x0F); e.u8(0xAF); // IMUL r64, r/m64

            e.emit_modrm_mem(
                r.cipher_state & 7,
                r.state_ptr,
                rx::none,
                0,