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Re: gcc feature request / RFC: extra clobbered regs

On Tue, Jun 30, 2015 at 02:22:33PM -0700, Andy Lutomirski wrote:
> 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.)

GCC already supports -ffixed-REG, -fcall-used-REG and -fcall-saved-REG
options, which allow to tweak the calling conventions; but it is per
translation unit right now.  It isn't clear which of these options
you mean with the extra_clobber.
I assume you are looking for a possibility to change this to be
per-function, with caller with a different calling convention having to
adjust for different ABI callee.  To some extent, recent GCC versions
do that automatically with -fipa-ra already - if some call used registers
are not clobbered by some call and the caller can analyze that callee,
it can stick values in such registers across the call.
I'd say the most natural API for this would be to allow
f{fixed,call-{used,saved}}-REG in target attribute.


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