Why local variable s32 tmp is getting generated in code when bitwise OR operation is used in stateflow

Question

Niraj Deshmukh 2016-2-12

回答： Walter Roberson 2016-2-12

When I use bitwise OR operation in stateflow of matlab model, signed 32 bit (s32) local variable "tmp" gets generated in the autogenerated code.

tmp = GetU8_CAN_St_Lidf_Ft_Motbk_2010() | lfc_U8_TEST_BIT4;
    if (tmp > 255) {
      tmp = 255;
    }
SetU8_CAN_St_Lidf_Ft_Motbk_2010((T_u8)tmp)

But same is not the case if bitwise AND operation is used.

lfc_DW.u8_Temp_FT_Motbk = (T_u8)(GetU8_CAN_St_Lidf_Ft_Motbk_2010() &
        lfc_U8_TEST_BIT4_OP);

Both "lfc_U8_TEST_BIT4" and "lfc_U8_TEST_BIT4_OP" are defined as Uint8 constant in stateflow.

Why such difference?

Answer 1

Walter Roberson 2016-2-12

bitwise-or could increase the range of a value beyond 255. bitwise-and can never increase the range of a value.

The behaviour is like the "saturation" of uint8 values in MATLAB. If you have

X = uint8(250);
X = X + 10;

then the result in MATLAB is not 260, and it is not 5, it is 255 (just by definition in the language)