Re: gcc feature request / RFC: extra clobbered regs

[Vladimir Makarov <vmakarov at redhat dot com>]

On 07/01/2015 11:27 AM, Andy Lutomirski wrote:
> On Wed, Jul 1, 2015 at 8:23 AM, Vladimir Makarov <vmakarov@redhat.com> wrote:
> > On 06/30/2015 05:37 PM, Jakub Jelinek wrote:
> > > 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.
> > One consequence of frequent changing calling convention per function or
> > register usage could be GCC slowdown.  RA calculates too many data and it
> > requires a lot of time to recalculate them after something in the register
> > usage convention is changed.
> Do you mean that RA precalculates things based on the calling
> convention and saves it across functions?
RA calculates a lot info (register classes, class x class relations etc) 
based on register usage convention (fixed regs, call used registers 
etc).  If register usage convention is not changed from previous 
function compilation, RA reuses the info.  Otherwise, RA recalculates it.
>    Hmm.  I don't think this
> would be a big problem in my intended use case -- there would only be
> a handful of functions using this extension, and they'd have very few
> non-asm callers.
Good.  I guess it will be rarely used and people will tolerate some 
extra compilation time.
> > Another consequence would be that RA fails generate the code in some cases
> > and even worse the failure might depend on version of GCC (I already saw PRs
> > where RA worked for an asm in one GCC version because a pseudo was changed
> > by equivalent constant and failed in another GCC version where it did not
> > happen).
> Would this be a problem generating code for a function with extra
> "used" regs or just a problem generating code to call such a function.
> I imagine that, in the former case, RA's job would be easier, not
> harder, since there would be more registers to work with.
Sorry, I meant that the problem will be mostly when the attributes 
describe more fixed regs.  If you describe more clobbered regs, they 
still can be used for allocator which can spill/restore them (around 
calls) when they can not be used. Still i think there will be some rare 
and complicated cases where even describing only clobbered regs can make 
RA fails in a function calling the function with additional clobbered regs.
>    In
> practice, though, I think it would just end up changing the prologue
> and epilogue.