Re: [Aarch64] Vector Function Application Binary Interface Specification for OpenMP

[Steve Ellcey <sellcey@cavium.com>]

On Tue, 2018-05-15 at 18:29 +0000, Francesco Petrogalli wrote:

> Hi Steve,
> 
> I am happy to let you know that the Vector Function ABI for AArch64
> is now public and available via the link at [1].
> 
> Don’t hesitate to contact me in case you have any questions.
> 
> Kind regards,
> 
> Francesco
> 
> [1] https://developer.arm.com/products/software-development-tools/hpc
> /arm-compiler-for-hpc/vector-function-abi
> 
> > 
> > Steve Ellcey
> > sellcey@cavium.com

Thanks for publishing this Francesco, it looks like the main issue for
GCC is that the Vector Function ABI has different caller saved / callee
saved register conventions than the standard ARM calling convention.

If I understand things correctly, in the standard calling convention
the callee will only save the bottom 64 bits of V8-V15 and so the
caller needs to save those registers if it is using the top half.  In
the Vector calling convention the callee will save all 128 bits of
these registers (and possibly more registers) so the caller does not
have to save these registers at all, even if it is using all 128 bits
of them.

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