[Aarch64] Vector Function Application Binary Interface Specification for OpenMP

[Steve Ellcey]

On Wed, 2018-05-16 at 17:30 +0100, Richard Earnshaw (lists) wrote:
> On 16/05/18 17:21, Steve Ellcey wrote:
> >Â 
> > It doesn't look like GCC has any existing mechanism for having different
> > sets of caller saved/callee saved registers depending on the function
> > attributes of the calling or called function.
> > 
> > Changing what registers a callee function saves and restores shouldn't
> > be too difficult since that can be done when generating the prologue
> > and epilogue code but changing what registers a caller saves/restores
> > when doing the call seems trickier.  The macro
> > TARGET_HARD_REGNO_CALL_PART_CLOBBERED doesn't know anything about the
> > function being called.  It returns true/false depending on just the
> > register number and mode.
> > 
> > Steve Ellcey
> > sellcey@cavium.com
> > 
> 
> Actually, we can.  See, for example, the attribute((pcs)) for the ARM
> port.  I think we could probably handle this automagically for the SVE
> vector calling convention in AArch64.
> 
> R.

Interesting, it looks like one could use aarch64_emit_call to emit
extra use_reg / clobber_reg instructions but in this case we want to
tell the caller that some registers are not being clobbered by the
callee. Â The ARM port does not
define TARGET_HARD_REGNO_CALL_PART_CLOBBERED and that seemed like one
of the most problamatic issues with Aarch64. Â Maybe we would have to
undefine this for aarch64 and use explicit clobbers to say what
floating point registers / vector registers are clobbered for each
call? Â I wonder how that would affect register allocation.

Steve Ellcey
sellcey@cavium.com