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Re: Expansion of narrowing math built-ins into power instructions
Segher Boessenkool <firstname.lastname@example.org> writes:
> On Thu, Aug 15, 2019 at 01:47:47PM +0100, Richard Sandiford wrote:
>> Tejas Joshi <email@example.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.