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

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Aug 18, 2015 at 4:15 PM, Richard Sandiford
<richard.sandiford@arm.com> wrote:
> 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.

In some sense.  But the "strict" version already has a builtin (just no
special expander in builtins.c).  We usually don't add 1:1 tree codes
for existing builtins (why have builtins at all then?).

> 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.

The target builtin is for the vectorized variant - not all targets might have
that and we'd need to query the target about this.  So using a IFN would
mean adding a target hook for that query.

> 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).

I think tree_code is 64bits now.

Richard.

> Thanks,
> Richard
>