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

[Segher Boessenkool <segher@kernel.crashing.org>]

On Thu, Aug 15, 2019 at 01:47:47PM +0100, Richard Sandiford wrote:
> Tejas Joshi <tejasjoshi9673@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.

> simplify-rtx.c is probably the most important place to handle it.
> It would be easiest to test using the selftests at the end of the file.


Segher