Re: limiting call clobbered registers for library functions

[Yury Gribov <y dot gribov at samsung dot com>]

On 01/30/2015 11:16 AM, Matthew Fortune wrote:
> Yury Gribov <y.gribov@samsung.com> writes:
> > On 01/29/2015 08:32 PM, Richard Henderson wrote:
> > > On 01/29/2015 02:08 AM, Paul Shortis wrote:
> > > > I've ported GCC to a small 16 bit CPU that has single bit shifts. So
> > > > I've handled variable / multi-bit shifts using a mix of inline shifts
> > > > and calls to assembler support functions.
> > > >
> > > > The calls to the asm library functions clobber only one (by const) or
> > > > two
> > > > (variable) registers but of course calling these functions causes all
> > > > of the standard call clobbered registers to be considered clobbered,
> > > > thus wasting lots of candidate registers for use in expressions
> > > > surrounding these shifts and causing unnecessary register saves in
> > the surrounding function prologue/epilogue.
> > > > I've scrutinized and cloned the actions of other ports that do the
> > > > same, however I'm unable to convince the various passes that only r1
> > > > and r2 can be clobbered by these library calls.
> > > >
> > > > Is anyone able to point me in the proper direction for a solution to
> > > > this problem ?
> > >
> > > You wind up writing a pattern that contains a call, but isn't
> > > represented in rtl as a call.
> >
> > Could it be useful to provide a pragma for specifying function register
> > usage? This would allow e.g. library writer to write a hand-optimized
> > assembly version and then inform compiler of it's binary interface.
> >
> > Currently a surrogate of this can be achieved by putting inline asm code
> > in static inline functions in public library headers but this has it's
> > own disadvantages (e.g. code bloat).
>
> This sounds like a good idea in principle. I seem to recall seeing something
> similar to this in other compiler frameworks that allow a number of special
> calling conventions to be defined and enable functions to be attributed to use
> one of them. I.e. not quite so general as specifying an arbitrary clobber list
> but some sensible pre-defined alternative conventions.

FYI a colleague from kernel mentioned that they already achieve this by 
wrapping the actual call with inline asm e.g.

static inline int foo(int x) {
  asm(
    ".global foo_core\n"
    // foo_core accepts single parameter in %rax,
    // returns result in %rax and
    // clobbers %rbx
    "call foo_core\n"
    : "+a"(x)
    :
    : "rbx"
  );
  return x;
}

We still can't mark inline asm with things like __attribute__((pure)), 
etc. though so it's not an ideal solution.

-Y