[sunfish]

The compiler is using push and pop here just for their side effect of adjusting the stack pointer, to keep the stack pointer 16-byte aligned.

[saagarjha]

Shouldn't the C runtime do that before calling main?

[sunfish]

It does; the stack pointer starts out aligned, but then the function does a call, and the call instruction adjusts the stack pointer by 8 bytes to push the return address, which would cause it to be misaligned. The push pushes an extra 8 bytes so that the stack pointer is aligned in the callee.

[SilasX]

Any reason not to just do some other “increment stack pointer” option?

[glandium]

The instruction is smaller. Push rax is encoded into 1 bytes, while an increment stack pointer would be 4.