3.58 Decode

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Problem:

For a function with prototype:

long decode2(long x, long y, long z);

GCC generates the following assembly code:

decode2:
    subq    %rdx, %rsi
    imulq    %rsi, %rdi
    movq    %rsi, %rax
    salq    $63, %rax
    sarq    $63, %rax
    xorq    %rdi, %rax
    ret

Parameters x, y, and z are passed in registers %rdi, %rsi, and %rdx. The code stores the return value in register %rax.

Write C code for decode2 that will have an effect equivalent to the assembly code shown.

Code:

long decode2(long x, long y, long z) {
    long tmp = y - z;
    return (tmp * x) ^ (tmp << 63 >> 63);
}