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

[Tejas Joshi <tejasjoshi9673 at gmail dot com>]

Hello.
>
> Similarly addtfsf3 that multiplies TFmode and produces an SFmode result, and so on.

I want to extend this patch for FADDL and DADDL. What operand
constraints should I use for TFmode alongside "f"?

> In cases where long double and double have the same mode,
>the daddl function should use the existing adddf3 pattern.

So, should I use adddf3 for DADDL directly? How would I map the
add<mode>3 optab with DADDL?

Thanks,
Tejas


On Sat, 24 Aug 2019 at 15:23, Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Martin Jambor <mjambor@suse.cz> writes:
> > Hello,
> >
> > On Thu, Aug 22 2019, Segher Boessenkool wrote:
> >>> > Hi Tejas,
> >>> >
> >>> > [ Please do not top-post. ]
> >>
> >> On Thu, Aug 22, 2019 at 01:27:06PM +0530, Tejas Joshi wrote:
> >>> > What happens then?  "It does not work" is very very vague.  At least it
> >>> > seems the compiler does build now?
> >>>
> >>> Oh, compiler builds but instruction is still "bl fadd". It should be
> >>> "fadds" right?
> >>
> >> Yes, but that means the problem is earlier, before it hits RTL perhaps.
> >>
> >> Compile with -dap, look at the expand dump (the lowest numbered one, 234
> >> or so), and see what it looked like in the final Gimple, and then in the
> >> RTL generated from that.  And then drill down.
> >>
> >
> > Tejas sent me his patch and I looked at why it did not work.  I found
> > two reasons:
> >
> > 1. associated_internal_fn (in builtins.c) does not handle
> >    DEF_INTERNAL_OPTAB_FN kind of internal functions, and Tejas
> >    (sensibly, I'd say) used that macro to define the internal function.
> >    But when I worked around that by manually adding a case for it in the
> >    switch statement, I ran into an assert because...
> >
> > 2. direct_internal_fn_supported_p on which replacement_internal_fn
> >    depends to expand built-ins as internal functions cannot handle
> >    conversion optabs... and narrowing is a kind of conversion and the
> >    optab is added as such with OPTAB_CD.
> >
> > Actually, the second statement is not entirely true because somehow it
> > can handle optab while_ult which is a conversion optab but a) the way it
> > is handled, if I can understand it at all, seems to be a big hack and
> > would be even worse if we decided to copy that for all narrowing math
> > functions
>
> Think "big hack" is a bit unfair.  The way that the internal function
> maps argument types to the optab modes, and the way it expands calls
> into rtl, depends on the "optab type" argument (the final argument to
> DEF_INTERNAL_OPTAB_FN).  This is relatively flexible in that it can use
> a single-mode "direct" optab or a dual-mode "conversion" optab, with the
> modes coming from whichever arguments are appropriate.  New optab types
> can be added as needed.
>
> FWIW, several other DEF_INTERNAL_OPTAB_FNs are conversion optabs too
> (e.g. IFN_LOAD_LANES, IFN_STORE_LANES, IFN_MASK_LOAD, etc.).
>
> But...
>
> > and b) it gets both modes from argument types whereas we need one from
> > the result type and so we would have to rewrite
> > replacement_internal_fn anyway.
>
> ...yeah, I agree this breaks the current model.  The reason IFN_WHILE_ULT
> doesn't rely on the return type is that if you have:
>
>   _2 = .WHILE_ULT (_0, _1) // returning a vector of 4 booleans
>   _3 = .WHILE_ULT (_0, _1) // returning a vector of 8 booleans
>
> then the calls look equivalent.  So instead we pass an extra argument
> indicating the required boolean vector "shape".
>
> The same "problem" could in principle apply to FADD if we ever needed
> to support double+double->_Float16 for example.
>
> > Therefore, at least for now (GSoC deadline is kind of looming), I
> > decided that the best way forward would be to not rely on internal
> > functions but plug into expand_builtin() and I wrote the following,
> > lightly tested patch - which of course misses testcases and stuff - but
> > I'd be curious about any feedback now anyway.  When I proposed a very
> > similar approach for the roundeven x86_64 expansion, Uros actually then
> > opted for a solution based on internal functions, so I am curious
> > whether there are simple alternatives I do not see.
> >
> > Tejas, of course cases for other fadd variants should at least be added
> > to expand_builtin.
> >
> > Thanks,
> >
> > Martin
> >
> >
> > 2019-08-23  Tejas Joshi  <tejasjoshi9673@gmail.com>
> >           Martin Jambor  <mjambor@suse.cz>
> >
> >       * builtins.c (expand_builtin_binary_conversion): New function.
> >         (expand_builtin): Call it.
> >       * config/rs6000/rs6000.md (unspec): Add UNSPEC_ADD_NARROWING.
> >       (add_truncdfsf3): New define_insn.
> >       * optabs.def (fadd_optab): New.
> >
> > [...]
> > diff --git a/gcc/internal-fn.def b/gcc/internal-fn.def
> > index 9461693bcd1..3f56880c23f 100644
> > --- a/gcc/internal-fn.def
> > +++ b/gcc/internal-fn.def
> > @@ -140,6 +140,8 @@ DEF_INTERNAL_OPTAB_FN (WHILE_ULT, ECF_CONST | ECF_NOTHROW, while_ult, while)
> >  DEF_INTERNAL_OPTAB_FN (VEC_SHL_INSERT, ECF_CONST | ECF_NOTHROW,
> >                      vec_shl_insert, binary)
> >
> > +DEF_INTERNAL_OPTAB_FN (FADD, ECF_CONST, fadd, binary)
> > +
> >  DEF_INTERNAL_OPTAB_FN (FMS, ECF_CONST, fms, ternary)
> >  DEF_INTERNAL_OPTAB_FN (FNMA, ECF_CONST, fnma, ternary)
> >  DEF_INTERNAL_OPTAB_FN (FNMS, ECF_CONST, fnms, ternary)
>
> Should be dropped now.
>
> OK with that change and the ones Segher asked for.
>
> Thanks,
> Richard