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[RFC PATCH 00/15] softfloat: alternate conversion of float128_addsub

From: Richard Henderson
Subject: [RFC PATCH 00/15] softfloat: alternate conversion of float128_addsub
Date: Tue, 20 Oct 2020 21:51:34 -0700

Hi Alex,

Here's my first adjustment to your conversion for 128-bit floats.

The Idea is to use a set of macros and an include file so that we
can re-use the same large chunk of code that performs the basic
operations on various fraction lengths.  It's ugly, but without
proper language support it seems to be less ugly than most.

While I've just gone and added lots of stuff to int128... I have
had another idea, half-baked because I'm tired and it's late:

typedef struct {
    FloatClass cls;
    int exp;
    bool sign;
    uint64_t frac[];
} FloatPartsBase;

typedef struct {
    FloatPartsBase base;
    uint64_t frac;
} FloatParts64;

typedef struct {
    FloatPartsBase base;
    uint64_t frac_hi, frac_lo;
} FloatParts128;

typedef struct {
    FloatPartsBase base;
    uint64_t frac[4]; /* big endian word ordering */
} FloatParts256;

This layout, with the big-endian ordering, means that storage
can be shared between them, just by ignoring the least significant
words of the fraction as needed.  Which may make muladd more


static void muladd_floats64(FloatParts128 *r, FloatParts64 *a,
                            FloatParts64 *b, FloatParts128 *c, ...)
    // handle nans
    // produce 128-bit product into r
    // handle p vs c special cases.
    // zero-extend c to 128-bits
    c->frac[1] = 0;
    // perform 128-bit fractional addition
    addsub_floats128(r, c, ...);
    // fold 128-bit fraction to 64-bit sticky bit.
    r->frac[0] |= r->frac[1] != 0;

float64 float64_muladd(float64 a, float64 b, float64 c, ...)
    FloatParts64 pa, pb;
    FloatParts128 pc, pr;

    float64_unpack_canonical(&pa.base, a, status);
    float64_unpack_canonical(&pb.base, b, status);
    float64_unpack_canonical(&pc.base, c, status);
    muladd_floats64(&pr, &pa, &pb, &pc, flags, status);

    return float64_round_pack_canonical(&pr.base, status);

Similarly, muladd_floats128 would use addsub_floats256.

However, the big-endian word ordering means that Int128
cannot be used directly; so a set of wrappers are needed.
If added the Int128 routine just for use here, then it's
probably easier to bypass Int128 and just code it here.



Richard Henderson (15):
  qemu/int128: Add int128_or
  qemu/int128: Add int128_clz, int128_ctz
  qemu/int128: Rename int128_rshift, int128_lshift
  qemu/int128: Add int128_shr
  qemu/int128: Add int128_geu
  softfloat: Use mulu64 for mul64To128
  softfloat: Use int128.h for some operations
  softfloat: Tidy a * b + inf return
  softfloat: Add float_cmask and constants
  softfloat: Inline float_raise
  Test split to softfloat-parts.c.inc
  softfloat: Streamline FloatFmt
  Test float128_addsub
  softfloat: Use float_cmask for addsub_floats
  softfloat: Improve subtraction of equal exponent

 include/fpu/softfloat-macros.h |  89 ++--
 include/fpu/softfloat.h        |   5 +-
 include/qemu/int128.h          |  61 ++-
 fpu/softfloat.c                | 802 ++++++++++-----------------------
 softmmu/physmem.c              |   4 +-
 target/ppc/int_helper.c        |   4 +-
 tests/test-int128.c            |  44 +-
 fpu/softfloat-parts.c.inc      | 339 ++++++++++++++
 fpu/softfloat-specialize.c.inc |  45 +-
 9 files changed, 716 insertions(+), 677 deletions(-)
 create mode 100644 fpu/softfloat-parts.c.inc


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