[PATCH/AARCH64] Decimal floating point support for AARCH64

[Peter Bergner]

On 7/13/17 7:12 PM, Andrew Pinski wrote:
>   This patch adds Decimal floating point support to aarch64.  It is
> the base support in that since there is no hardware support for DFP,
> it just defines the ABI.  The ABI I chose is that _Decimal32 is
> treated like float, _Decimal64 is treated like double and _Decimal128
> is treated like long double.  In that they are passed via the floating
> registers (sN, dN, qN).
> Is this ok an ABI?

It depends on whether AARCH ever plans on implementing HW DFP.
On POWER, we handle things similarly to what you mention above,
except for one extra constraint for _Decimal128 and that is that
they must live in even/odd register pairs.  This was due to how
the instructions were implemented in the HW, they required even/odd
reg pairs.

If there's zero chance AARCH ever implements HW DFP, then you're
probably fine with the above, but if you go with the above and HW DFP
is eventually added, then the HW would need handle even/odd and
odd/even register pairs in it's instructions...or you'd need to
add potential prologue/epilogue code to move formal args into
even/odd regs if the HW demands it.  If there is a non-zero chance
or you just want to be safe, you could enforce even/odd reg usage
in the ABI upfront.

Peter