Re: [PATCH newlib v1 0/4] Add FreeBSD long double functions

[Corinna Vinschen <vinschen@redhat.com>]

On Aug 26 10:45, Joel Sherrill wrote:
> On Fri, Aug 26, 2022 at 10:05 AM Corinna Vinschen <vinschen@redhat.com>
> wrote:
> > No, I wrote, create a *single* _fpmath.h file with the massive amount
> > of definitions (*all* seven) based on LDBL_MANT_DIG.
> >
> > There are very few special targets, like x86/x86_64 which need a tweak
> > in the macros, most of the time the macros should be the same.
> >
> > Instead of having 61 files, only have one.  In theory there should be
> > only two definitions for targets with LDBL_MANT_DIG == DBL_MANT_DIG
> > to support little and big endian, more shouldn't be required.
> >
> > For all others, we already have *ieee*.h files with matching definitions
> > which can be used as role models for the various (but few) definitions of
> > union IEEEl2bits.  See, for instance, newlib/libc/include/machine/ieee.h.
> >
> > > This approach works and doesn't abandon the targets which the
> > > ldbl==dbl method works for.
> >
> > I never said to abandon these targets, but fwiw, I think you're
> > overestimating the complexity to generate a single _fpmath.h
> > file with matching definitions for these targes as well.  The
> > information is basically already present in newlib.
> >
> So don't preserve the FreeBSD files as close to a direct copy
> as possible?

We're only talking about the _fpmath.h files, not all the other
files.

If you like, copy over the handful of target-dependent _fpmath.h
files, but *also* add a generic _fpmath.h file for all other
targets, IMHO.

> Maybe I am overestimating it. I just don't see the single _fpmath.h
> to rule them all with confidence like you do. I am not seeing the
> set of parameters in my head which makes this generic with
> some macros.
> 
> I'd appreciate help on this.

newlib/libc/include/machine/ieee.h does not help?

Kind of like this:

#if LDBL_MANT_DIG == 24
  union IEEEl2bits {
	  long double     e;
#  ifdef __IEEE_LITTLE_ENDIAN
	  struct {
		  __uint32_t   manl    :16;
		  __uint32_t   manh    :7;
		  __uint32_t   exp     :8;
		  __uint32_t   sign    :1;
	  } bits;
	  struct {
		  __uint32_t   manl    :16;
		  __uint32_t   manh    :7;
		  __uint32_t   expsign :9;
	  } xbits;
#  else
	  struct {
		  __uint32_t   sign    :1;
		  __uint32_t   exp     :8;
		  __uint32_t   manh    :7;
		  __uint32_t   manl    :16;
	  } bits;
	  struct {
		  __uint32_t   expsign :9;
		  __uint32_t   manh    :7;
		  __uint32_t   manl    :16;
	  } xbits;
#  endif
  };
#  define LDBL_NBIT       0
#  define LDBL_IMPLICIT_NBIT
#  define mask_nbit_l(u)  ((void)0)

#  define LDBL_MANH_SIZE  7
#  define LDBL_MANL_SIZE  16

#  define LDBL_TO_ARRAY32(u, a) do {                              \
	  (a)[0] = (uint32_t)(u).bits.manl                        \
                   | ((uint32_t)(u).bits.manh << LDBL_MANL_SIZE)  \
	  (a)[1] = 0;                                             \
  } while(0)
#elif LDBL_MANT_DIG == 53
  union IEEEl2bits {
          long double     e;
#  ifdef __IEEE_LITTLE_ENDIAN
          struct {
                  __uint64_t   manl    :32;
                  __uint64_t   manh    :20;
                  __uint64_t   exp     :11;
                  __uint64_t   sign    :1;
          } bits;
          struct {
                  __uint64_t   manl    :32;
                  __uint64_t   manh    :20;
                  __uint64_t   expsign :12;
          } xbits;
#  else 
          struct {
                  __uint64_t   sign    :1;
                  __uint64_t   exp     :11;
                  __uint64_t   manh    :20;
                  __uint64_t   manl    :32;
          } bits;
          struct {
                  __uint64_t   expsign :12;
                  __uint64_t   manh    :20;
                  __uint64_t   manl    :32;
          } xbits;
#  endif
  };
#  define LDBL_NBIT       0
#  define LDBL_IMPLICIT_NBIT
#  define mask_nbit_l(u)  ((void)0)

#  define LDBL_MANH_SIZE  20
#  define LDBL_MANL_SIZE  32

#  define LDBL_TO_ARRAY32(u, a) do {                              \
          (a)[0] = (uint32_t)(u).bits.manl;                       \
          (a)[1] = (uint32_t)(u).bits.manh;                       \
  } while(0)
#elif LDBL_MANT_DIG == 64
  union IEEEl2bits {
          long double     e;
#  ifdef __IEEE_LITTLE_ENDIAN
          struct {
                  __uint64_t   manl    :32;
                  __uint64_t   manh    :32;
                  __uint64_t   exp     :15;
                  __uint64_t   sign    :1;
                  __uint64_t   junk    :16
          } bits;
          struct {
                  __uint64_t   man     :64;
                  __uint64_t   expsign :16;
                  __uint64_t   junk    :16
          } xbits;
#  else 
          struct {
                  __uint64_t   junk    :16
                  __uint64_t   sign    :1;
                  __uint64_t   exp     :15;
                  __uint64_t   manh    :32;
                  __uint64_t   manl    :32;
          } bits;
          struct {
                  __uint64_t   junk    :16
                  __uint64_t   expsign :16;
                  __uint64_t   man     :64;
          } xbits;
#  endif
  };

#  define LDBL_NBIT       0x80000000
#  define mask_nbit_l(u)  ((u).bits.manh &= ~LDBL_NBIT)

#  define LDBL_MANH_SIZE  32
#  define LDBL_MANL_SIZE  32

#  define LDBL_TO_ARRAY32(u, a) do {                              \
          (a)[0] = (uint32_t)(u).bits.manl;                       \
          (a)[1] = (uint32_t)(u).bits.manh;                       \
  } while(0)
#elif LDBL_MANT_DIG == 113
  union IEEEl2bits {
          long double     e;
#  ifdef __IEEE_LITTLE_ENDIAN
          struct {
                  __uint64_t   manl    :64;
                  __uint64_t   manh    :48;
                  __uint64_t   exp     :15;
                  __uint64_t   sign    :1;
          } bits;
          struct {
                  __uint64_t   manl    :64;
                  __uint64_t   manh    :48;
                  __uint64_t   expsign :16;
          } xbits;
#  else 
          struct {
                  __uint64_t   sign    :1;
                  __uint64_t   exp     :15;
                  __uint64_t   manh    :48;
                  __uint64_t   manl    :64;
          } bits;
          struct {
                  __uint64_t   expsign :16;
                  __uint64_t   manh    :48;
                  __uint64_t   manl    :64;
          } xbits;
#  endif
  };

#  define LDBL_NBIT       0
#  define LDBL_IMPLICIT_NBIT
#  define mask_nbit_l(u)  ((void)0)

#  define LDBL_MANH_SIZE  48
#  define LDBL_MANL_SIZE  64

#  define LDBL_TO_ARRAY32(u, a) do {                      \
	  (a)[0] = (uint32_t)(u).bits.manl;               \
	  (a)[1] = (uint32_t)((u).bits.manl >> 32);       \
	  (a)[2] = (uint32_t)(u).bits.manh;               \
	  (a)[3] = (uint32_t)((u).bits.manh >> 32);       \
  } while(0)
#endif

There's also a ___IEEE_BYTES_LITTLE_ENDIAN definition which I ignored
for now.  Also, the LDBL_MANT_DIG == 64 is defined for x86-only and the
manl/manh fields for LDBL_MANT_DIG == 23 are faked on a 16 bit border
just to have a manl and a manh definition.

I don't claim that it works OOTB with the above, but apart from
the LDBL_NBIT/mask_nbit_l values which differ for x86/x86_64 CPUs,
it should be a good start, I think.

> > Jeff, ping?  Your POV would be much appreciated here.
> >
> 
> +1
> 
> I also have come across there needs to be a way to pick between ld80
> and ld128 headers and implementation files. FreeBSD has them in
> separate directories. Apparently their build system builds common
> plus an architecture specific _fpmath.h and a choice of ld80/ld128
> which adds at least two other header files

But then again, FreeBSD supports ARM and PowerPC, both of which come
with 64 bit long double.  So, besides ld80/ld128, there's apparently a
generic implementation, too, to support non-ld80 and non-ld128 long
double, right?


Corinna