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

[Tom Musta <tommusta@gmail.com>]

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
       include/libdecnumber and the code is placed under libdecnumber.  A minimalist approach
       is taken -- only the source required to support PPC DFP instruction models is imported.

    2) The code is modified to account for this structure (see patch 02/12)

    3) The libdecnumber configuration script is jettisoned.  Instead, the configuration header
       that is normally generated is modified to integrate properly with QEMU's configuration.
       See patch 03/12.

    4) Some trivial modifications to source are made to address compiler warnings. See
       patches 04/12 - 06/12

    5) Compilation is enabled for PowerPC emulation models (see patch 07/12).


Tom Musta (12):
  target-ppc: Introduce libdecnumber Code
  target-ppc: Prepare libdecnumber for QEMU include structure
  target-ppc: Modify dconfig.h to Integrate with QEMU
  target-ppc: Change gstdint.h to stdint.h
  target-ppc: Eliminate redundant declarations
  target-ppc: Eliminate Unused Variable in decSetSubnormal
  target-ppc: Enable Building of libdecnumber
  target-ppc: Define FPR Pointer Type for Helpers
  target-ppc: Introduce Translation Macros for DFP Arithmetic Forms
  target-ppc: Introduce DFP Helper Utilities
  target-ppc: Introduce DFP Post Processor Utilities
  target-ppc: Introduce DFP Add

 Makefile.target                              |    5 +
 default-configs/ppc-linux-user.mak           |    1 +
 default-configs/ppc-softmmu.mak              |    1 +
 default-configs/ppc64-linux-user.mak         |    1 +
 default-configs/ppc64-softmmu.mak            |    1 +
 include/libdecnumber/dconfig.h               |   35 +
 include/libdecnumber/decCommonSymbols.h      |   14 +
 include/libdecnumber/decContext.h            |  253 +
 include/libdecnumber/decContextSymbols.h     |   44 +
 include/libdecnumber/decDPD.h                | 1211 ++++
 include/libdecnumber/decDPDSymbols.h         |   26 +
 include/libdecnumber/decDouble.h             |  161 +
 include/libdecnumber/decDoubleSymbols.h      |  165 +
 include/libdecnumber/decNumber.h             |  195 +
 include/libdecnumber/decNumberLocal.h        |  662 +++
 include/libdecnumber/decNumberSymbols.h      |  143 +
 include/libdecnumber/decQuad.h               |  183 +
 include/libdecnumber/decQuadSymbols.h        |  160 +
 include/libdecnumber/dpd/decimal128.h        |   96 +
 include/libdecnumber/dpd/decimal128Local.h   |   42 +
 include/libdecnumber/dpd/decimal128Symbols.h |   48 +
 include/libdecnumber/dpd/decimal64.h         |   96 +
 include/libdecnumber/dpd/decimal64Symbols.h  |   48 +
 libdecnumber/decContext.c                    |  427 ++
 libdecnumber/decNumber.c                     | 8115 ++++++++++++++++++++++++++
 libdecnumber/dpd/decimal128.c                |  559 ++
 libdecnumber/dpd/decimal128Local.h           |   42 +
 libdecnumber/dpd/decimal64.c                 |  845 +++
 target-ppc/Makefile.objs                     |    1 +
 target-ppc/dfp_helper.c                      |  264 +
 target-ppc/helper.h                          |    7 +
 target-ppc/translate.c                       |   65 +
 32 files changed, 13916 insertions(+), 0 deletions(-)
 create mode 100644 include/libdecnumber/dconfig.h
 create mode 100644 include/libdecnumber/decCommonSymbols.h
 create mode 100644 include/libdecnumber/decContext.h
 create mode 100644 include/libdecnumber/decContextSymbols.h
 create mode 100644 include/libdecnumber/decDPD.h
 create mode 100644 include/libdecnumber/decDPDSymbols.h
 create mode 100644 include/libdecnumber/decDouble.h
 create mode 100644 include/libdecnumber/decDoubleSymbols.h
 create mode 100644 include/libdecnumber/decNumber.h
 create mode 100644 include/libdecnumber/decNumberLocal.h
 create mode 100644 include/libdecnumber/decNumberSymbols.h
 create mode 100644 include/libdecnumber/decQuad.h
 create mode 100644 include/libdecnumber/decQuadSymbols.h
 create mode 100644 include/libdecnumber/dpd/decimal128.h
 create mode 100644 include/libdecnumber/dpd/decimal128Local.h
 create mode 100644 include/libdecnumber/dpd/decimal128Symbols.h
 create mode 100644 include/libdecnumber/dpd/decimal64.h
 create mode 100644 include/libdecnumber/dpd/decimal64Symbols.h
 create mode 100644 libdecnumber/decContext.c
 create mode 100644 libdecnumber/decNumber.c
 create mode 100644 libdecnumber/dpd/decimal128.c
 create mode 100644 libdecnumber/dpd/decimal128Local.h
 create mode 100644 libdecnumber/dpd/decimal64.c
 create mode 100644 target-ppc/dfp_helper.c