For a function with prototype

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

gcc generates the following assembly code:

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

Parameters x, y, and z are passed in registers %rdi, %rsi, and %rdx. The code stores the return value in register %rax. Reverse-engineer decode2. (In other words, write C code for decode2 that will have an effect equivalent to the assembly code shown.)