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

[Segher Boessenkool <segher at kernel dot crashing dot org>]

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