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Re: [PING][Patch] Add support for IEEE-conformant versions of scalar fmin* and fmax*
- From: Richard Sandiford <richard dot sandiford at arm dot com>
- To: Richard Biener <richard dot guenther at gmail dot com>
- Cc: David Sherwood <david dot sherwood at arm dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Tue, 18 Aug 2015 15:15:41 +0100
- Subject: Re: [PING][Patch] Add support for IEEE-conformant versions of scalar fmin* and fmax*
- Authentication-results: sourceware.org; auth=none
- References: <000001d0d5b0$5da4dbb0$18ee9310$ at arm dot com> <CAFiYyc1bgWwdV4PRLBuUv3yC0X-k5gJVuyyV9V7Vrz3Lte+wZw at mail dot gmail dot com> <000001d0d8cf$2fb42770$8f1c7650$ at arm dot com> <CAFiYyc2XT+iqyRNgp+N2gWsaP-=1xVUWsuUEj+bOq_UmE_1eLw at mail dot gmail dot com> <000001d0d9a6$1efdc350$5cf949f0$ at arm dot com> <CAFiYyc3CLF8beK5GaB86Ad7623gWc9yhc8nTom-ByoaHTEMyOg at mail dot gmail dot com>
Richard Biener <email@example.com> writes:
> On Tue, Aug 18, 2015 at 1:07 PM, David Sherwood <firstname.lastname@example.org> wrote:
>>> On Mon, Aug 17, 2015 at 11:29 AM, David Sherwood
>>> <email@example.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).