Re: [Patch 2/3][Aarch64] Add support for IEEE-conformant versions of scalar fmin* and fmax*

[James Greenhalgh <james dot greenhalgh at arm dot com>]

On Thu, Nov 26, 2015 at 04:20:35PM -0000, David Sherwood wrote:
> Hi,
> 
> Here is the second patch of the fmin/fmax change, which adds the optabs
> to the aarch64 backend.
> 
> Tested:
> 
> x86_64-linux: no regressions
> aarch64-none-elf: no regressions
> 
> Good to go?
> David Sherwood.

Could you also update the comment a few lines above the pattern you add
in aarch64-simd.md? Unless I've misunderstood the point of this patch set,
that looks to be out of date now:

  ;; FP Max/Min
  ;; Max/Min are introduced by idiom recognition by GCC's mid-end.  An
  ;; expression like:
  ;;      a = (b < c) ? b : c;
  ;; is idiom-matched as MIN_EXPR<b,c> only if -ffinite-math-only is enabled
  ;; either explicitly or indirectly via -ffast-math.
  ;;
  ;; MIN_EXPR and MAX_EXPR eventually map to 'smin' and 'smax' in RTL.
  ;; The 'smax' and 'smin' RTL standard pattern names do not specify which
  ;; operand will be returned when both operands are zero (i.e. they may not
  ;; honour signed zeroes), or when either operand is NaN.  Therefore GCC
  ;; only introduces MIN_EXPR/MAX_EXPR in fast math mode or when not honouring
  ;; NaNs.

Either that, or reorder the patterns you add so the existing patterns that
this comment pertains to are kept close to it, and add a new comment for
your new pattern - explaining that it is the auto-vectorized form of the
IEEE-754 fmax/fmin functions.

Thanks,
James



> 
> ChangeLog:
> 
> 2015-11-26  David Sherwood  <david.sherwood@arm.com>
> 
>     gcc/
> 	* config/aarch64/aarch64.md: New pattern.
> 	* config/aarch64/aarch64-simd.md: Likewise.
> 	* config/aarch64/iterators.md: New unspecs, iterators.
>     gcc/testsuite
> 	* gcc.target/aarch64/fmaxmin.c: New test.
> 
> > -----Original Message-----
> > From: Richard Biener [mailto:richard.guenther@gmail.com]
> > Sent: 25 November 2015 12:39
> > To: David Sherwood
> > Cc: GCC Patches; Richard Sandiford
> > Subject: Re: [PING][Patch] Add support for IEEE-conformant versions of scalar fmin* and fmax*
> > 
> > On Mon, Nov 23, 2015 at 10:21 AM, David Sherwood <david.sherwood@arm.com> wrote:
> > > Hi,
> > >
> > > This is part 1 of a reworked version of a patch I originally submitted in
> > > August, rebased after Richard Sandiford's recent work on the internal
> > > functions. This first patch adds the internal function definitions and optabs
> > > that provide support for IEEE fmax()/fmin() functions.
> > >
> > > Later patches will add the appropriate aarch64/aarch32 vector instructions.
> > 
> > Ok.
> > 
> > Thanks,
> > Richard.
> > 
> > > Tested:
> > >
> > > x86_64-linux: no regressions
> > > aarch64-none-elf: no regressions
> > > arm-none-eabi: no regressions
> > >
> > > Regards,
> > > David Sherwood.
> > >
> > > ChangeLog:
> > >
> > > 2015-11-19  David Sherwood  <david.sherwood@arm.com>
> > >
> > >     gcc/
> > >         * optabs.def: Add new optabs fmax_optab/fmin_optab.
> > >         * internal-fn.def: Add new fmax/fmin internal functions.
> > >         * config/aarch64/aarch64.md: New pattern.
> > >         * config/aarch64/aarch64-simd.md: Likewise.
> > >         * config/aarch64/iterators.md: New unspecs, iterators.
> > >         * config/arm/iterators.md: New iterators.
> > >         * config/arm/unspecs.md: New unspecs.
> > >         * config/arm/neon.md: New pattern.
> > >         * config/arm/vfp.md: Likewise.
> > >         * doc/md.texi: Add fmin and fmax patterns.
> > >     gcc/testsuite
> > >         * gcc.target/aarch64/fmaxmin.c: New test.
> > >         * gcc.target/arm/fmaxmin.c: New test.
> > >
> > >
> > >> -----Original Message-----
> > >> From: Richard Biener [mailto:richard.guenther@gmail.com]
> > >> Sent: 19 August 2015 13:35
> > >> To: Richard Biener; David Sherwood; GCC Patches; Richard Sandiford
> > >> Subject: Re: [PING][Patch] Add support for IEEE-conformant versions of scalar fmin* and fmax*
> > >>
> > >> On Wed, Aug 19, 2015 at 2:11 PM, Richard Sandiford
> > >> <richard.sandiford@arm.com> wrote:
> > >> > Richard Biener <richard.guenther@gmail.com> writes:
> > >> >> On Wed, Aug 19, 2015 at 11:54 AM, Richard Sandiford
> > >> >> <richard.sandiford@arm.com> wrote:
> > >> >>> Richard Biener <richard.guenther@gmail.com> writes:
> > >> >>>> 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?).
> > >> >>>
> > >> >>> We still need the builtin to match the C function (and to allow direct
> > >> >>> calls to __builtin_fmin, etc., which are occasionally useful).
> > >> >>>
> > >> >>>>> 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.
> > >> >>>
> > >> >>> No, the idea is that if we have a tree code or an internal function, the
> > >> >>> decision about whether we have target support is based on a query of the
> > >> >>> optabs (just like it is for scalar, and for other vectorisable tree codes).
> > >> >>> No new hooks are needed.
> > >> >>>
> > >> >>> The patch checked for target support that way.
> > >> >>
> > >> >> Fair enough.  Still this means we should have tree codes for all builtins
> > >> >> that eventually are vectorized?  So why don't we have SIN_EXPR,
> > >> >> POW_EXPR (ok, I did argue and have patches for that in the past),
> > >> >> RINT_EXPR, SQRT_EXPR, etc?
> > >> >
> > >> > Yeah, it doesn't sound so bad to me :-)  The choice of what's a function
> > >> > in C and what's inherent is pretty arbitrary.  E.g. % on doubles could
> > >> > have implemented fmod() or remainder().  Casts from double to int could
> > >> > have used the current rounding mode, but instead they truncate and
> > >> > conversions using the rounding mode need to go through something like
> > >> > (l)lrint().  Like you say, pow() could have been an operator (and is in
> > >> > many languages), but instead it's a function.
> > >> >
> > >> >> This patch starts to go down that route which is why I ask for the
> > >> >> whole picture to be considered and hinted at the alternative implementation
> > >> >> which follows existing practice.  Add a expander in builtins.c, add an optab,
> > >> >> and eventual support to vectorized_function.
> > >> >>
> > >> >> See for example ix86_builtin_vectorized_function which handles
> > >> >> sqrt, floor, ceil, etc. and even FMA (we only fold FMA to FMA_EXPR
> > >> >> if the target supports it for the scalar mode, so not sure if there is
> > >> >> any x86 ISA where it has vectorized FMA but not scalar FMA).
> > >> >
> > >> > Yeah.  TBH I'm really against doing that unless (a) there's good reason
> > >> > to believe that the concept really is specific to one target and
> > >> > wouldn't be implemented on others or (b) there really is a function
> > >> > rather than an instruction underneath (usually the case for sin, etc.).
> > >> > But (b) could also be handled by the optab support library mechanism.
> > >> >
> > >> > Reasons against using target-specific builtins for operations that
> > >> > have direct support in the ISA:
> > >> >
> > >> > 1. Like you say, in practice vector ops only tend to be supported if the
> > >> >    associated scalar op is also supported.  Sticking to this approach
> > >> >    means that vector ops follow a different path from scalar ops whereas
> > >> >    (for example) division follows the same path for both.  It just seems
> > >> >    confusing to have some floating-point optabs that support both scalar
> > >> >    and vector operands and others that only support scalar operands.
> > >> >
> > >> > 2. Once converted to a target-specific function, the target-independent
> > >> >    code has no idea what the function does or how expensive it is.
> > >> >    We might start out with just one hook to convert a scalar operation
> > >> >    to a target-dependent built-in function, but I bet over time we'll
> > >> >    grow other hooks to query properties about the function, such as
> > >> >    costs.
> > >> >
> > >> > 3. builtin_vectorized_function returns a decl rather than a call.
> > >> >    If the target's vector API doesn't already have a built-in for the
> > >> >    operation we need, with the exact types and arguments that we expect,
> > >> >    the target needs to define one, presumably marked so that it isn't
> > >> >    callable by input code.
> > >> >
> > >> >    E.g. on targets where FP conversion instructions allow an explicit
> > >> >    rounding mode to be specified as an operand, it's reasonable for a
> > >> >    target's vector API to expose that operand as a constant argument to
> > >> >    the API function.  There'd then be one API function for all vector-
> > >> >    float-to-vector-float integer rounding operations, rather than one
> > >> >    for vector rint(), one for vector ceil(), etc.  (I'm thinking of
> > >> >    System z instructions here, although I don't know offhand what the
> > >> >    vector API is there.)  IMO it doesn't make sense to force the target
> > >> >    to define "fake" built-in functions for all those possibilities
> > >> >    purely for the sake of the target hook.  It's a lot of extra code,
> > >> >    and it's extra code that would be duplicated on any target that needs
> > >> >    to do this.
> > >> >
> > >> > IMO optabs are the best way for the target to tell the target-independent
> > >> > code what it can do.  If it supports sqrt on df it defines sqrtdf and
> > >> > if it supports vector sqrt on v2df it defines sqrtv2df.  These patterns
> > >> > will often be a single define_expand or define_insn template -- the
> > >> > vectorness often comes "for free" in terms of writing the pattern.
> > >> >
> > >> >>>> > 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.
> > >> >>>
> > >> >>> Even better :-)
> > >> >>
> > >> >> Yes.
> > >> >>
> > >> >> I'm not against adding a corresponding tree code for all math builtin functions,
> > >> >> we just have to decide whether this is the way to go (and of course support
> > >> >> expanding those back to libcalls to libc/m rather than libgcc).  There are
> > >> >> also constraints on what kind of STRICT_FMIN_EXPR the compiler may
> > >> >> generate as the target may not be able to expand the long double variant
> > >> >> directly but needs a libcall but libm might not be linked or may not
> > >> >> have support
> > >> >> for it.  That would be a new thing compared to libgcc providing a fallback
> > >> >> for all other tree codes.
> > >> >
> > >> > True, but that doesn't seem too bad.  The constraints would be the same
> > >> > if we're operating on built-in functions rather than codes.  I suppose
> > >> > built-in functions make this more explicit, but at the end of the day
> > >> > it's a costing decision.  We should no more be converting a cheap
> > >> > operation into an expensive libgcc function than converting a cheap
> > >> > operation into an expensive libm function, even if the libgcc conversion
> > >> > links.
> > >> >
> > >> > There's certainly precedent for introducing calls to things that libgcc
> > >> > doesn't define.  E.g. we already introduce calls to memcpy in things
> > >> > like loop distribution, even though we don't provide a fallback memcpy
> > >> > in libgcc.
> > >>
> > >> As an additional point for many math functions we have to support errno
> > >> which means, like, BUILT_IN_SQRT can be rewritten to SQRT_EXPR
> > >> only if -fno-math-errno is in effect.  But then code has to handle
> > >> both variants for things like constant folding and expression combining.
> > >> That's very unfortunate and something we want to avoid (one reason
> > >> the POW_EXPR thing didn't fly when I tried).  STRICT_FMIN/MAX_EXPR
> > >> is an example where this doesn't apply, of course (but I detest the name,
> > >> just use FMIN/FMAX_EXPR?).  Still you'd need to handle both,
> > >> FMIN_EXPR and BUILT_IN_FMIN, in code doing analysis/transform.
> > >>
> > >> Richard.
> > >>
> > >>
> > >> > Thanks,
> > >> > Richard
> > >> >
> > >
>