Hi, 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. Jose. ----- Mensaje original ----- De: Ian Lance Taylor <iant@xxxxxxxxxx> Para: jose gomez valcarcel <jcgv33@xxxxxxxx> CC: "gcc-help@xxxxxxxxxxx" <gcc-help@xxxxxxxxxxx>; "jcgv@xxxxxxxxxx" <jcgv@xxxxxxxxxx> Enviado: lunes 29 de agosto de 2011 23:29 Asunto: Re: gcc return struct code generation jose gomez valcarcel <jcgv33@xxxxxxxx> writes: > I just read x86_64 ABI code generation, and the result is ok. The > struct is returned in registers RAX, RDX. The only disappointing thing > is the allocation of 40 stack bytes, whic apparently is not used. This > effect cause my kernel code not to mix well with hand-assembler code. That is a good question. I think it is a bug. Although I'm curious as to why allocating extra unused stack space is a problem for your kernel code. Some stack space will always be used, because the x86_64 ABI requires the stack to be aligned on a 16-byte boundary before executing a call instruction. Ian
