gcc feature request / RFC: extra clobbered regs

[Andy Lutomirski <luto at kernel dot org>]

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.)

Thanks,
Andy