two mains

[Vladimir Panteleev]

On Saturday, 26 January 2013 at 20:42:27 UTC, Tyro[17] wrote:
> So why do we need to mov RBP, RSP in [2] but not in [1]? I'm 
> thinking this is because RBP contains the address of args but 
> not sure.

The x64 calling convention passes the first few arguments via 
registers. I think it's most likely that the function prolog is 
allocating stack space to save the value of whatever registers 
(RDI/RDX?) which contain the string[] parameter, so that it could 
reuse those registers in the code of the function - but the 
assignment seems to have been optimized out, yet the stack 
allocation wasn't.

FWIW, on Windows x64, DMD generates slightly different code, 
presumably because it's using the Microsoft x64 calling 
convention instead of the System V one. There is no stack 
allocation when compiled with -O, however without -O, DMD adds a 
"mov [RBP+10h], RCX" instruction. I assume it makes use of the 
32-byte "shadow space" to "spill" ECX: 
http://en.wikipedia.org/wiki/X86_calling_conventions#x86-64_calling_conventions