Re: [Qemu-devel] [Consult] tilegx: About floating point instructions

[Richard Henderson <rth@twiddle.net>]

On 08/01/2015 02:47 AM, Chen Gang wrote:
> I am just adding floating point instructions (e.g. fsingle_add1), but
> for me, I can not find any details about them (the ISA documents only
> give a summary description, but not details), e.g.

The tilegx splits the four/six cycle arithmetic into multiple black-box
instructions.  You need only really implement one of the four, with the
rest of them being implemented as nops or moves.

Looking at what gcc produces gives the hints:

	fdouble_unpack_min	min, srca, srcb
	fdouble_unpack_max	max, srca, srcb
	fdouble_add_flags	flg, srca, srcb
	fdouble_addsub		max, min, flg
	fdouble_pack1		dst, max, flg
	fdouble_pack2		dst, max, zero

The unpack, addsub, and pack2 insns can be ignored, the add_flags insn can
perform the whole operation, the pack1 insn performs a move from "flg" to "dst".

Similarly for the single-precision:

	fsingle_add1		tmp, srca, srcb
	fsingle_addsub2		tmp, srca, srcb
	fsingle_pack1		flg, tmp
	fsingle_pack2		dst, tmp, flg

The add1 insn performs the whole operation, the addsub2 and pack1 insns are
ignored, and the pack2 insn is a move from tmp to dst.


r~