How can I perform arithmetic right shift in C in a portable way?

Question

We are writing an emulator where we need sign propagating right shift. The emulated system uses 2\'s complement numbers.

I read that the >> operat

Answer 1

Arithmetic right shift is equivalent to division by 2-to-the-number-you're- shifting-by, rounding towards zero if the divisor was nonnegative or towards infinity otherwise.

Hence a generic portable arithmetic right-shift macro in GNU C could be:

#if /*$auto*{{{*/ \
    __GNUC__>=7
    #define $let(L,R) __auto_type L = R
#else
    #define $let(L,R) __typeof(R) L = R
#endif //}}}
#define $sar(X,By) /*{{{*/ \
    (__extension__({ \
       $let($sar_X,X); \
       $let($sar_divisor, ($sar_X*0+1)<<(By) ); \
       $sar__($sar_X, $sar_divisor);  \
    }))
    #define $sar__(X,Divisor) ((X)/(Divisor)+((X)%(Divisor)>=0?0:-1))
    /*}}}*/

signedX >> shift often does the equivalent with less machine code (whether it does is implementation defined), and so you could have:

#if (-2>>1==-1 && -2l>>1==-1 && -2ll>>1==-1) 
    //does signedX>>Shift do an arithmetic right shift?

    #define $sar(X,By) ((X)>>(By))
#else
   //...the previous snippet...
#endif