This is question 3.58 (pg 311) from Book: Computer Systems: A Programmers Perspective (3rd Edition):

For a function with prototype

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

GCC generates the following assembly code:

1 decode2:

2 subq %rdx, %rsi

3 imulq %rsi, %rdi

4 movq %rsi, %rax

5 salq $63, %rax

6 sarq $63, %rax

7 xorq %rdi, %rax

8 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.