After entering _start, is rsp aligned?

Question

When programs enter the _start routine at the program start, is the stack pointer aligned to a 16 byte boundary, or should it be manually aligned? I mean, is it aligned even before the prologue (push rbp; mov rbp, rsp) in _start?

I know that on x86-64 at the start of the program RSP is aligned to 8 bytes, but I do now know if it's aligned to 16 bytes. For some tasks I might need that alignment to properly execute SSE instructions which require alignment on a 16 byte boundary.

Answer 1

The x86-64 ABI explicitly says (3.4.1 Initial Stack and Register State) :

%rsp The stack pointer holds the address of the byte with lowest address which is part of the stack. It is guaranteed to be 16-byte aligned at process entry.

Since _start is the first symbol that's called when a process is entered, you can be entirely sure that it is 16-byte aligned when the OS calls _start in your executable.