From: Richard Sandiford <richard.sandiford@arm.com>
To: Richard Biener <richard.guenther@gmail.com>
Cc: David Sherwood <david.sherwood@arm.com>,
GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PING][Patch] Add support for IEEE-conformant versions of scalar fmin* and fmax*
Date: Tue, 18 Aug 2015 14:20:00 -0000 [thread overview]
Message-ID: <87fv3gbs36.fsf@e105548-lin.cambridge.arm.com> (raw)
In-Reply-To: <CAFiYyc3CLF8beK5GaB86Ad7623gWc9yhc8nTom-ByoaHTEMyOg@mail.gmail.com> (Richard Biener's message of "Tue, 18 Aug 2015 15:30:12 +0200")
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
next prev parent reply other threads:[~2015-08-18 14:15 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-08-13 10:13 David Sherwood
2015-08-13 11:12 ` Richard Biener
2015-08-17 9:41 ` David Sherwood
2015-08-17 14:02 ` Richard Biener
2015-08-18 11:10 ` David Sherwood
2015-08-18 13:31 ` Richard Biener
2015-08-18 14:20 ` Richard Sandiford [this message]
2015-08-19 9:48 ` Richard Biener
2015-08-19 10:04 ` Richard Sandiford
2015-08-19 10:31 ` Richard Biener
2015-08-19 12:23 ` Richard Sandiford
2015-08-19 12:35 ` Richard Biener
2015-08-19 13:16 ` Richard Sandiford
2015-08-19 13:41 ` Richard Biener
2015-09-14 10:47 ` David Sherwood
2015-09-14 13:42 ` Richard Biener
2015-09-14 20:38 ` Joseph Myers
2015-08-19 15:32 ` Joseph Myers
2015-11-23 9:21 ` David Sherwood
2015-11-25 12:39 ` Richard Biener
2015-08-19 15:07 ` Michael Matz
2015-08-19 15:25 ` Richard Biener
2015-08-19 15:39 ` Richard Sandiford
-- strict thread matches above, loose matches on Subject: below --
2015-08-06 9:39 David Sherwood
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