gcc return struct code generation

[Ian Lance Taylor]

jose gomez valcarcel <jcgv33@yahoo.es> writes:

> I also think is a bug, but i'm not sure. My toy kernel has no assembly (.S) files, but of course it has c functions with
> inline assembly. My context switch function it's a c funtion (yes i know there are better ways of doing this) with some
> inline assembly code first to save context, some c code, and then some assembly code to restore context and do 'iret'.
> It works only if the C code doesn't touch the rsp, because I 'intercept' the end of the function.
>
> I known that is 'questionable' code, but the real question is that i expect that this simple C code not to touch RSP with optimizations
> enabled in x86_64. I got surprised. Note that you point of stack aligned on 16 byte boundary is irrelevant, because it has to be aligned
> on 'fn' entry call.

I see.  The stack alignment is not irrelevant if your function callls
any other functions, because the function will have to adjust for the 8
byte return address pushed on the stack by its caller.  If that is an
issue you can use the -mpreferred-stack-boundary option to compensate.
Of course you need to make sure that the stack pointer is correctly
aligned before returning to any regular C code.

Ian