Re: Expansion of narrowing math built-ins into power instructions

[Tejas Joshi <tejasjoshi9673@gmail.com>]

Hello.

> with it, but first get DP->SP (fadd) working?

Can you please review what have I have been trying and facing the
issues on patch :
<https://gcc.gnu.org/ml/gcc/2019-08/msg00078.html>

Thanks,
Tejas


On Sun, 11 Aug 2019 at 12:50, Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> Hi Tejas,
>
> On Sun, Aug 11, 2019 at 10:34:26AM +0530, Tejas Joshi wrote:
> > > As far as I understand that flag should set the behaviour of the fadd
> > > function, not the __builtin_fadd one.  So I don't know.
> >
> > According to ISO/IEC TS 18661, I am supposed to implement the fadd
> > variants for folding and expand them inline, that take double and long
> > double as arguments and return
> > addition in appropriate narrower type, float and double. As far as I
> > know, we use __builtin_ to call the internal functions? I do not know
> > which the only fadd function is.
>
> See the manual, section "Other Built-in Functions Provided by GCC":
>
>   @opindex fno-builtin
>   GCC includes built-in versions of many of the functions in the standard
>   C library.  These functions come in two forms: one whose names start with
>   the @code{__builtin_} prefix, and the other without.  Both forms have the
>   same type (including prototype), the same address (when their address is
>   taken), and the same meaning as the C library functions even if you specify
>   the @option{-fno-builtin} option @pxref{C Dialect Options}).  Many of these
>   functions are only optimized in certain cases; if they are not optimized in
>   a particular case, a call to the library function is emitted.
>
> > > double precision one.  But instead you want to add two double precision
> > > numbers, producing a single precision one?  The fadds instruction fits
> >
> > Yes.
> >
> > > well to that, but you'll have to check exactly how the fadd() function
> > > should behave with respect to rounding and exceptions and the like.
>
> I read 18661-1 now...  and yup, "fadds" will work fine, and there are
> no complications like this as far as I see.
>
> For QP to either DP or SP, you can do round-to-odd followed by one of the
> conversion instructions.  The ISA manual describes this; I can help you
> with it, but first get DP->SP (fadd) working?
>
> For the non-QP long doubles we have...  There is the option of using DP
> for it, which isn't standard-compliant, many other archs have it too,
> and it is simple anyway, because you have all code for operations
> already.  You can mostly just ignore this option.
>
> For double-double...  Well firstly, double-double is on the way out, so
> adding new features to it is pretty useless?  Just ignore it unless you
> have time left, I'd say.
>
> > In Joseph's initial mail that describes what should be carried out in
> > the course of project, about rounding and exceptions. I have strictly
> > followed this description for my folding patch :
> >
> > * The narrowing functions, e.g. fadd, faddl, daddl, are a bit different
> > from most other built-in math.h functions because the return type is
> > different from the argument types.  You could start by adding them to
> > builtins.def similarly to roundeven (with new macros to handle adding such
> > functions for relevant pairs of _FloatN, _FloatNx types).  These functions
> > could be folded for constant arguments only if the result is exact, or if
> > -fno-rounding-math -fno-trapping-math (and -fno-math-errno if the result
> > involves overflow / underflow).
>
> For Power, all five basic operations (add, sub, mul, div, fma) work fine
> wrt rounding mode if using the fadds etc. insns, for DP->SP.  All
> exceptions work as expected, except maybe underflow and overflow, but
> 18661 doesn't require much at all for those anyway :-)
>
>
> Segher