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Re: Expansion of narrowing math built-ins into power instructions
> 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?
On Sat, 24 Aug 2019 at 15:23, Richard Sandiford
> Martin Jambor <firstname.lastname@example.org> 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.).
> > 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 <email@example.com>
> > Martin Jambor <firstname.lastname@example.org>
> > * 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.