Re: [Qemu-devel] [Qemu-ppc] [RFC 00/12] target-ppc: Decimal Floating Point

[Alexander Graf <agraf@suse.de>]

On 13.03.14 16:12, Tom Musta wrote:
> This patch series provides a prototype of Decimal Floating Point (DFP) instruction
> support in QEMU.  The topic was briefly discussed here:
> http://lists.nongnu.org/archive/html/qemu-ppc/2014-02/msg00129.html
>
>
> I. Overview
>
> The core of the DFP model is a library named libdecnumber, which is a sub-library
> of GCC's libdfp.  Libdecnumber's model uses an expanded internal representation
> of a decimal floating point number as:
>
>     - exponent (base 10)
>     - the number of digits (aka declets)
>     - flags (sign bit and "special" bits for NaNs and infinities)
>     - an array of units.  Each unit contains DECPUN declets.
>
> Libdecnumber provides routines for various arithmetic operations (e.g. decNumberAdd for
> addition) as well as converters for going to/from binary form.
>
> PowerPC's support of DFP uses Densely Packed Decimal (DPD) format.  The standard floating
> point registers are re-used to hold DFP operands.  There are typically PowerPC instructions
> for both the long (64 bit) and extended (128 bit, aka quad) formats.  The extended format
> instructions use adjacent pairs of FPRs to hold the 128 bit operands.
>
>
> II. A Simple Example
>
> PowerPC DFP instructions are implemented using helpers, which in turn use libdecnumber
> routines to perform the detailed computations.  This is similar to how softfloat is used
> to model binary floating point instructions.
>
> For example, the implementation of the helper for DFP Add Long (dadd) looks like this:
>
>      1) Establish a libdecnumber context for 64-bit computation (decContext)
>      2) Convert the two source FPRs from DPD to decNumbers (the expanded internal representation)
>      3) Invoke the add operation (decNumberAdd)
>      4) Inspect the context's flags for various exception situations (e.g. overflow), mapping
>         them to the corresponding FPSCR status flags.
>      5) Convert the summand to DPD form and store into the target FPR.
>
> The helper for the DPD Add Extended (daddq) is nearly identical.  It differs only in that
> it establishes a 128 bit context and the conversions are from and to 128 bit DPD format.
>
>
> III. Integration of libdecnumber in QEMU
>
> The appoach taken here is direct and simple:
>
>      1) A copy of libdecnumber code is imported into QEMU.  The header files are placed under

I don't think this is possible. Libdecnumber is GPLv3+ code while QEMU 
overall is licensed at GPLv2 only. Gotta love licenses...

Is there an older, GPLv2 licensed version available?


Alex