Re: gcc feature request / RFC: extra clobbered regs

H. Peter Anvin
2015-06-30T21:32:59Z

On 06/30/2015 02:22 PM, Andy Lutomirski wrote:
> Hi all-
>
> I'm working on a massive set of cleanups to Linux's syscall handling.
> We currently have a nasty optimization in which we don't save rbx,
> rbp, r12, r13, r14, and r15 on x86_64 before calling C functions.
> This works, but it makes the code a huge mess. I'd rather save all
> regs in asm and then call C code.
>
> Unfortunately, this will add five cycles (on SNB) to one of the
> hottest paths in the kernel. To counteract it, I have a gcc feature
> request that might not be all that crazy. When writing C functions
> intended to be called from asm, what if we could do:
>
> __attribute__((extra_clobber("rbx", "rbp", "r12", "r13", "r14",
> "r15"))) void func(void);
>
> This will save enough pushes and pops that it could easily give us our
> five cycles back and then some. It's also easy to be compatible with
> old GCC versions -- we could just omit the attribute, since preserving
> a register is always safe.
>
> Thoughts? Is this totally crazy? Is it easy to implement?
>
> (I'm not necessarily suggesting that we do this for the syscall bodies
> themselves. I want to do it for the entry and exit helpers, so we'd
> still lose the five cycles in the full fast-path case, but we'd do
> better in the slower paths, and the slower paths are becoming
> increasingly important in real workloads.)
>

Some gcc targets have done this in the past. There are command-line
options to do that, but using attributes you have to handle cross-ABI
compilation.

However, I don't see this being done in the upstream gcc.

Keep in mind the runway that we'll need, though.

-hpa