Re: [PATCH v3 0/6] Static calls

[Josh Poimboeuf]

On Thu, Jan 10, 2019 at 07:45:26PM +0000, Nadav Amit wrote:
> >> Iâm not GCC expert either and writing this code was not making me full of
> >> joy, etc.. Iâll be happy that my code would be reviewed, but it does work. I
> >> donât think an early pass is needed, as long as hardware registers were not
> >> allocated.
> >> 
> >>> Would it work with more than 5 arguments, where args get passed on the
> >>> stack?
> >> 
> >> It does.
> >> 
> >>> At the very least, it would (at least partially) defeat the point of the
> >>> callee-saved paravirt ops.
> >> 
> >> Actually, I think you can even deal with callee-saved functions and remove
> >> all the (terrible) macros. You would need to tell the extension not to
> >> clobber the registers through a new attribute.
> > 
> > Ok, it does sound interesting then.  I assume you'll be sharing the
> > code?
> 
> Of course. If this what is going to convince, Iâll make a small version for
> PV callee-saved first.

It wasn't *only* the PV callee-saved part which interested me, so if you
already have something which implements the other parts, I'd still like
to see it.

> >>> What if we just used a plugin in a simpler fashion -- to do call site
> >>> alignment, if necessary, to ensure the instruction doesn't cross
> >>> cacheline boundaries.  This could be done in a later pass, with no side
> >>> effects other than code layout.  And it would allow us to avoid
> >>> breakpoints altogether -- again, assuming somebody can verify that
> >>> intra-cacheline call destination writes are atomic with respect to
> >>> instruction decoder reads.
> >> 
> >> The plugin should not be able to do so. Layout of the bytecode is done by
> >> the assembler, so I donât think a plugin would help you with this one.
> > 
> > Actually I think we could use .bundle_align_mode for this purpose:
> > 
> >  https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fsourceware.org%2Fbinutils%2Fdocs-2.31%2Fas%2FBundle-directives.html&data=02%7C01%7Cnamit%40vmware.com%7Cfa29fb8be208498d039008d67727fe30%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636827411004664549&sdata=elDuAVOsSlidG7pZSZfjbhrgnMOHeX6AWKs0hJM4cCE%3D&reserved=0
> 
> Hmâ I donât understand what you have in mind (i.e., when would this
> assembly directives would be emitted).

For example, it could replace

  callq ____static_call_tramp_my_key

with

  .bundle_align_mode 6
  callq ____static_call_tramp_my_key
  .bundle_align_mode 0

which ensures the instruction is within a cache line, aligning it with
NOPs if necessary.  That would allow my current implementation to
upgrade out-of-line calls to inline calls 100% of the time, instead of
95% of the time.

-- 
Josh