Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 30 Jun 2015 14:22:33 -0700 | Subject | gcc feature request / RFC: extra clobbered regs |
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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
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