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


From: Richard Henderson
Subject: Re: [Qemu-devel] [Consult] tilegx: About floating point instructions
Date: Mon, 3 Aug 2015 09:40:21 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0

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~



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