Re: [PING][Patch] Add support for IEEE-conformant versions of scalar fmin* and fmax*

[Richard Sandiford <richard dot sandiford at arm dot com>]

Richard Biener <richard.guenther@gmail.com> writes:
> On Tue, Aug 18, 2015 at 1:07 PM, David Sherwood <david.sherwood@arm.com> wrote:
>>> On Mon, Aug 17, 2015 at 11:29 AM, David Sherwood
>>> <david.sherwood@arm.com> wrote:
>>> > Hi Richard,
>>> >
>>> > Thanks for the reply. I'd chosen to add new expressions as this seemed more
>>> > consistent with the existing MAX_EXPR and MIN_EXPR tree codes. In
>>> > addition it
>>> > would seem to provide more opportunities for optimisation than a
>>> > target-specific
>>> > builtin implementation would. I accept that optimisation opportunities will
>>> > be more limited for strict math compilation, but that it was still
>>> > worth having
>>> > them. Also, if we did map it to builtins then the scalar version would go
>>> > through the optabs and the vector version would go through the
>>> > target's builtin
>>> > expansion, which doesn't seem very consistent.
>>>
>>> On another note ISTR you can't associate STRICT_MIN/MAX_EXPR and thus
>>> you can't vectorize anyway?  (strict IEEE behavior is about NaNs, correct?)
>> I thought for this particular case associativity wasn't an issue?
>> We're not doing any
>> reductions here, just simply performing max/min operations on each
>> pair of elements
>> in the vectors. I thought for IEEE-compliant behaviour we just need to
>> ensure that for
>> each pair of elements if one element is a NaN we return the other one.
>
> Hmm, true.  Ok, my comment still stands - I don't see that using a
> tree code is the best thing to do here.  You can add fmin/max optabs
> and special expansion of BUILT_IN_FMIN/MAX and you can use a target
> builtin for the vectorized variant.
>
> The reason I am pushing against a new tree code is that we'd have an
> awful lot of similar codes when pushing other flag related IL
> specialities to actual IL constructs.  And we still need to find a
> consistent way to do that.

In this case though the new code is really the "native" min/max operation
for fp, rather than some weird flag-dependent behaviour.  Maybe it's
a bit unfortunate that the non-strict min/max fp operation got mapped
to the generic MIN_EXPR and MAX_EXPR when the non-strict version is really
the flag-related modification.  The STRICT_* prefix is forced by that and
might make it seem like more of a special case than it really is.

If you're still not convinced, how about an internal function instead
of a built-in function, so that we can continue to use optabs for all
cases?  I'd really like to avoid forcing such a generic concept down to
target-specific builtins with target-specific expansion code, especially
when the same concept is exposed by target-independent code for scalars.

TBH though I'm not sure why an internal_fn value (or a target-specific
builtin enum value) is worse than a tree-code value, unless the limit
of the tree_code bitfield is in sight (maybe it is).

Thanks,
Richard