> but an unspec is of course easiest for now. So, at this point, should I proceed with UNSPEC considering the complications that might arise as Richard points out?
On Sat, 17 Aug 2019 at 13:51, Richard Sandiford <richard.sandif...@arm.com> wrote: > > Tejas Joshi <tejasjoshi9...@gmail.com> writes: > > Hi, > > > >> It's just a different name, nothing more, nothing less. Because it is > >> a different name it can not be accidentally generated from actual > >> truncations. > > > > I have introduced float_narrow but I could not find appropriate places > > to generate it for a call to fadd instead it to generate a CALL. I > > used GDB to set breakpoints which hit fold_rtx and cse_insn but I got > > confused with the rtx codes and passes which generate respective RTL. > > It should not be similar to FLOAT_TRUNCATE if we want to avoid it > > generating for actual truncations? > > Please don't do it this way. The whole point of the work is that this > is a single operation that cannot be modelled as a post-processing of > a normal double addition result. It's a single operation at the source > level, a single IFN, a single optab, and a single instruction. Splitting > it apart into two operations for rtl only, and making it look in rtl terms > like a post-processing of a normal addition result, seems like it's going > to come back to bite us. > > In lisp terms we're saying that the operand to the float_narrow is > implicitly quoted: > > (float_narrow:m '(plus:n a b)) > > so that when float_narrow is evaluated, the argument is the unevaluated > rtl expression "(plus a b)" rather than the evaluated result a + b. > float_narrow then does its own evaluation of a and b and performs a > fused addition and narrowing on the result. > > No other rtx rvalue works like this. rtx nappings like simplification > or evaluation are normally depth-first, so that the mapping is applied > to the operands first, and then the root is mapped/simplified/evaluated > with the results. Adding implicit lisp quoting would require special > cases in these routines for float_narrow. > > The only current analogue I can think of for this is the handling > of zero_extend on const_ints. Because const_ints are modeless, we have > to avoid cases in which the recursion produces things like: > > (zero_extend:m (const_int -1)) > > because it's no longer clear what mode the zero_extend is extending from. > But I think that's seen as a wart of having modeless const_ints. I don't > think it's something we should actively embrace by adding float_narrow. > > Using float_narrow would also be inconsistent with the way we handle > saturating arithmetic. There we use US_PLUS and SS_PLUS rtx codes for > unsigned and signed saturating plus respectively, rather than: > > (unsigned_sat '(plus a b)) > (signed_sat '(plus a b)) > > Using dedicated codes might seem clunky. But it's simple, safe, and fits > the existing model without special cases. :-) > > Thanks, > Richard > > > > > Thanks, > > Tejas > > > > > > On Fri, 16 Aug 2019 at 15:53, Richard Sandiford > > <richard.sandif...@arm.com> wrote: > >> > >> Segher Boessenkool <seg...@kernel.crashing.org> writes: > >> > On Thu, Aug 15, 2019 at 01:47:47PM +0100, Richard Sandiford wrote: > >> >> Tejas Joshi <tejasjoshi9...@gmail.com> writes: > >> >> > Hello. > >> >> > I just wanted to make sure that I am looking at the correct code here. > >> >> > Except for rtl.def where I should be introducing something like > >> >> > float_contract (or float_narrow?) and also simplify-rtx.c, breakpoints > >> > > >> > I like that "float_narrow" name :-) > >> > > >> >> > set on functions around expr.c, cfgexpand.c where I grep for > >> >> > float_truncate/FLOAT_TRUNCATE did not hit. > >> >> > Also, in what manner should float_contract/narrow be different from > >> >> > float_truncate as both are trying to do similar things? (truncation > >> >> > from DF to SF) > >> >> > >> >> I think the code should instead be a fused addition and truncation, > >> >> a bit like FMA is a fused addition and multiplication. Describing it as > >> >> a DFmode addition followed by some conversion to SF would still involve > >> >> double rounding. > >> > > >> > How so? It would *mean* there is only single rounding, even! That's > >> > the whole point of it. > >> > >> But a PLUS should behave as a PLUS in any context. Making its > >> behaviour dependent on the containing rtxes (if any) would be a > >> can of worms. > >> > >> Richard
