Hi, Thank you for your time, sir. I think the better option is to rearrange my code and forget this issue. For curiosity, today i test this code with the intel compiler (12.0.5.220). It generates this code for fn: icc -O2 -fomit-frame-pointer -c z.c 0000000000000000 <fn>: 0:56 push %rsi 1:e8 00 00 00 00 callq 6 <fn+0x6> 6:48 03 c2 add %rdx,%rax 9:59 pop %rcx a:c3 retq Yo can see the code changes the values of registers RSI and RCX with apparently no reason ! (appart of computing correctly the c code of fn). Also this code uses stack with apparently no reason, or i can not see it. Regards. Jose. ----- Mensaje original ----- De: Ian Lance Taylor <iant@xxxxxxxxxx> Para: jose gomez valcarcel <jcgv33@xxxxxxxx> CC: "gcc-help@xxxxxxxxxxx" <gcc-help@xxxxxxxxxxx> Enviado: martes 30 de agosto de 2011 15:12 Asunto: Re: gcc return struct code generation jose gomez valcarcel <jcgv33@xxxxxxxx> 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
