Compilers: Understanding assembly code generated from small programs

Question

I\'m self-studying how compilers works. I\'m learning by reading the disassembly of GCC generated code from small 64-bit Linux programs.

I wrote this C

Answer 1

1. Yes, the nop is for alignment. Compilers use different instructions for different lengths of padding needed, knowing that modern CPU will be pre-fetching and decoding several instructions ahead.

2. As others have said, the C99 standard returns 0 from main() by default if there's no explicit return statement (see 5.1.2.2.3 in C99 TC3), so no warning is raised.

3. The 64-bit System V Linux ABI reserves a 128-byte "red zone" below the current stack pointer that leaf functions (functions that do not call any other functions - and your main() is one such) can use for local variables and other scratch values without having to sub rsp / add rsp. And so rbp == rsp.

And for the PS: when you call a function in the for() loop (or anywhere in your main()), main() is no longer a leaf function, so the compiler can no longer use the red zone. That's why the it allocates space on the stack with sub rsp, 0x10. However, it knows the relationship between rsp and rbp, so it can use either when accessing data.