good point, the gcc optimizer is smart enough to omit the preamble. it's still hacked though. inline assembly is one thing, messing with compiler-owned registers is another especially without a clobber list. btw why do you prefix your x84_64 start code with "mov %rsp,%rbp"? "xor %ebp, %ebp" is more idiomatic and efficient.

according to the abi (http://www.x86-64.org/documentation/abi.pdf), the stack frame is set up like this:

    argc = [RSP+0]
    argv = RSP+8
    envp = RSP+8+8*argc+8

the same for x86 but replace 8 with 4. your code mirrors this but retrieves argv by taking the address of the second function argument (because you pass [RSP+8] to __init(), which is actually argv[0]). C provides no guarantees on the stability of addresses of passed argument values between caller and callee, so this makes your code subject to non-standard behavior. i can see this working for x86 since values as passed through the stack but not for x86_64 where values are passed through registers.

if you're seeing bugs between gcc -O3 and -Os it's likely due to that, or it's due to improper use of inline assembly/clobbering registers.