CODEWITHSUNDEEP
cgccassembly
Ben
Ben

Reputation: 17

Writing a C program from Assembly language

I'm attempting to learn assembly language and I'm not sure if I'm on the right track. I have a program in assembly language that I need to convert to C.

addq    %rsi, %rdi
addq    %rdi, %rdx
movq    %rdx, %rax
ret

Trying to work through it myself I come up with something along the lines of

long p1(long x, long y, long z)
{
 x = x + y;
 z = z + x;
 long a = z;
 return a;
}

No matter which way I look at it I feel like I'm close but when I objdump I get nothing of the sort. Looking for some guidance.. thanks!

Upvotes: 0

Views: 156

Answers (1)

Nate Eldredge
Nate Eldredge

Reputation: 58673

Your C code looks fine, and as far as I can tell, it does the same thing as your assembly code.

You shouldn't necessarily expect that compiling your C code will give exactly the same assembly as the example. There are many different ways to convert C into assembly.

For instance, if I compile your C code with gcc -O0 (no optimization), I get much longer assembly, which stores all the function arguments and variables onto the stack. This is unnecessary (and slower) but makes things easier for a debugger. If I compile with gcc -O2, I get 

    leaq    (%rdi,%rsi), %rax
    addq    %rdx, %rax
    ret

Here the compiler has made clever use of the lea instruction to do an addition and a move in one instruction.

Upvotes: 3

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