llvm dev - Nov 2017 - RFC: [X86] Introducing command line options to prefer narrower vector instructions even when wider instructions are available

If skylake is that bad at AVX2 it belongs in -mcpu / -march IMO.    Most
people will build for the standard x86_64-pc-linux or whatever anyway,  and
completely ignore the change. This will mainly affect those who build their
own software and optimize for their system, and lots there have probably
caught on to this already.  I always thought that's what -march was made
for, really.

GNOMETOYS

On Sat, Nov 11, 2017 at 10:25 AM, Sanjay Patel via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Yes - I was thinking of FeatureFastScalarFSQRT / FeatureFastVectorFSQRT
> which are used by isFsqrtCheap(). These were added to override the default
> x86 sqrt estimate codegen with:
> https://reviews.llvm.org/D21379
>
> But I'm not sure we really need that kind of hack. Can we adjust the
> attribute in clang based on the target cpu? Ie, if you have something like:
> $ clang -O2 -march=skylake-avx512 foo.c
>
> Then you can detect that in the clang driver and pass
> -mprefer-vector-width=256 to clang codegen as an option? Clang codegen then
> adds that function attribute to everything it outputs. Then, the
> vectorizers and/or backend detect that attribute and adjust their behavior
> based on it.
>
> So I don't think we should be messing with any kind of type legality
> checking because that stuff should all be correct already. We're just
> choosing a vector size based on a pref. I think we should even allow the
> pref to go bigger than a legal type. This came up somewhere on llvm-dev or
> in a bug recently in the context of vector reductions.
>
>
>
> On Fri, Nov 10, 2017 at 6:04 PM, Craig Topper <craig.topper at
gmail.com>
> wrote:
>
>> Are you referring to the X86TargetLowering::isFsqrtCheap hook?
>>
>> ~Craig
>>
>> On Fri, Nov 10, 2017 at 7:39 AM, Sanjay Patel <spatel at
rotateright.com>
>> wrote:
>>
>>> We can tie a user preference / override to a CPU model. We do
something
>>> like that for square root estimates already (although it does use a
>>> SubtargetFeature currently for x86; ideally, we'd key that off
of something
>>> in the CPU scheduler model).
>>>
>>>
>>> On Thu, Nov 9, 2017 at 4:21 PM, Craig Topper <craig.topper at
gmail.com>
>>> wrote:
>>>
>>>> I agree that a less x86 specific command line makes sense.
I've been
>>>> having an internal discussions with gcc folks and their
evaluating
>>>> switching to something like
-mprefer-vector-width=128/256/512/none
>>>>
>>>> Based on the current performance data we're seeing, we
think we need to
>>>> ultimately default skylake-avx512 to -mprefer-vector-width=256.
If we go
>>>> with a target independent option/implementation is there
someway we could
>>>> still affect the default behavior in a target specific way?
>>>>
>>>> ~Craig
>>>>
>>>> On Tue, Nov 7, 2017 at 9:06 AM, Sanjay Patel <spatel at
rotateright.com>
>>>> wrote:
>>>>
>>>>> It's clear from the Intel docs how this has evolved,
but from a
>>>>> compiler perspective, this isn't a Skylake
"feature" :) ... nor an Intel
>>>>> feature, nor an x86 feature.
>>>>>
>>>>> It's a generic programmer hint for any target with
multiple potential
>>>>> vector lengths.
>>>>>
>>>>> On x86, there's already a potential use case for this
hint with a
>>>>> different starting motivation: re-vectorization. That's
where we take C
>>>>> code that uses 128-bit vector intrinsics and selectively
widen it to 256-
>>>>> or 512-bit vector ops based on a newer CPU target than the
code was
>>>>> originally written for.
>>>>>
>>>>> I think it's just a matter of time before a customer
requests the same
>>>>> ability for another target (maybe they already have and I
don't know about
>>>>> it). So we should have a solution that recognizes that
possibility.
>>>>>
>>>>> Note that having a target-independent implementation in the
optimizer
>>>>> doesn't preclude a flag alias in clang to maintain
compatibility with gcc.
>>>>>
>>>>>
>>>>>
>>>>> On Tue, Nov 7, 2017 at 2:02 AM, Tobias Grosser via llvm-dev
<
>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>
>>>>>> On Fri, Nov 3, 2017, at 05:47, Craig Topper via
llvm-dev wrote:
>>>>>> > That's a very good point about the ordering of
the command line
>>>>>> options.
>>>>>> > gcc's current implementation treats
-mprefer-avx256 has "prefer 256
>>>>>> over
>>>>>> > 512" and -mprefer-avx128 as "prefer 128
over 256". Which feels
>>>>>> weird for
>>>>>> > other reasons, but has less of an ordering
ambiguity.
>>>>>> >
>>>>>> > -mprefer-avx128 has been in gcc for many years and
predates the
>>>>>> creation
>>>>>> > of
>>>>>> > avx512. -mprefer-avx256 was added a couple months
ago.
>>>>>> >
>>>>>> > We've had an internal conversation with the
implementor of
>>>>>> > -mprefer-avx256
>>>>>> > in gcc about making -mprefer-avx128 affect 512-bit
vectors as well.
>>>>>> I'll
>>>>>> > bring up the ambiguity issue with them.
>>>>>> >
>>>>>> > Do we want to be compatible with gcc here?
>>>>>>
>>>>>> I certainly believe we would want to be compatible with
gcc (if we use
>>>>>> the same names).
>>>>>>
>>>>>> Best,
>>>>>> Tobias
>>>>>>
>>>>>> >
>>>>>> > ~Craig
>>>>>> >
>>>>>> > On Thu, Nov 2, 2017 at 7:18 PM, Eric Christopher
<
>>>>>> echristo at gmail.com>
>>>>>> > wrote:
>>>>>> >
>>>>>> > >
>>>>>> > >
>>>>>> > > On Thu, Nov 2, 2017 at 7:05 PM James Y Knight
via llvm-dev <
>>>>>> > > llvm-dev at lists.llvm.org> wrote:
>>>>>> > >
>>>>>> > >> On Wed, Nov 1, 2017 at 7:35 PM, Craig
Topper via llvm-dev <
>>>>>> > >> llvm-dev at lists.llvm.org> wrote:
>>>>>> > >>
>>>>>> > >>> Hello all,
>>>>>> > >>>
>>>>>> > >>>
>>>>>> > >>>
>>>>>> > >>> I would like to propose adding the
-mprefer-avx256 and
>>>>>> -mprefer-avx128
>>>>>> > >>> command line flags supported by
latest GCC to clang. These
>>>>>> flags will be
>>>>>> > >>> used to limit the vector register
size presented by TTI to the
>>>>>> vectorizers.
>>>>>> > >>> The backend will still be able to use
wider registers for code
>>>>>> written
>>>>>> > >>> using the instrinsics in x86intrin.h.
And the backend will
>>>>>> still be able to
>>>>>> > >>> use AVX512VL instructions and the
additional XMM16-31 and
>>>>>> YMM16-31
>>>>>> > >>> registers.
>>>>>> > >>>
>>>>>> > >>>
>>>>>> > >>>
>>>>>> > >>> Motivation:
>>>>>> > >>>
>>>>>> > >>> -Using 512-bit operations on some
Intel CPUs may cause a
>>>>>> decrease in CPU
>>>>>> > >>> frequency that may offset the gains
from using the wider
>>>>>> register size. See
>>>>>> > >>> section 15.26 of Intel® 64 and IA-32
Architectures Optimization
>>>>>> Reference
>>>>>> > >>> Manual published October 2017.
>>>>>> > >>>
>>>>>> > >>
>>>>>> > >> I note the doc mentions that 256-bit AVX
operations also have
>>>>>> the same
>>>>>> > >> issue with reducing the CPU frequency,
which is nice to see
>>>>>> documented!
>>>>>> > >>
>>>>>> > >> There's also the issues discussed
here <http://www.agner.org/
>>>>>> > >> optimize/blog/read.php?i=165> (and
elsewhere) related to warm-up
>>>>>> time
>>>>>> > >> for the 256-bit execution pipeline, which
is another issue with
>>>>>> using
>>>>>> > >> wide-vector ops.
>>>>>> > >>
>>>>>> > >>
>>>>>> > >> -The vector ALUs on ports 0 and 1 of the
Skylake Server
>>>>>> microarchitecture
>>>>>> > >>> are only 256-bits wide. 512-bit
instructions using these ALUs
>>>>>> must use both
>>>>>> > >>> ports. See section 2.1 of Intel® 64
and IA-32 Architectures
>>>>>> Optimization
>>>>>> > >>> Reference Manual published October
2017.
>>>>>> > >>>
>>>>>> > >>
>>>>>> > >>
>>>>>> > >>>  Implementation Plan:
>>>>>> > >>>
>>>>>> > >>> -Add prefer-avx256 and prefer-avx128
as SubtargetFeatures in
>>>>>> X86.td not
>>>>>> > >>> mapped to any CPU.
>>>>>> > >>>
>>>>>> > >>> -Add mprefer-avx256 and
mprefer-avx128 and the corresponding
>>>>>> > >>> -mno-prefer-avx128/256 options to
clang's driver Options.td
>>>>>> file. I believe
>>>>>> > >>> this will allow clang to pass these
straight through to the
>>>>>> -target-feature
>>>>>> > >>> attribute in IR.
>>>>>> > >>>
>>>>>> > >>> -Modify
X86TTIImpl::getRegisterBitWidth to only return 512 if
>>>>>> AVX512 is
>>>>>> > >>> enabled and prefer-avx256 and
prefer-avx128 is not set.
>>>>>> Similarly return
>>>>>> > >>> 256 if AVX is enabled and
prefer-avx128 is not set.
>>>>>> > >>>
>>>>>> > >>
>>>>>> > >> Instead of multiple flags that have
difficult to understand
>>>>>> intersecting
>>>>>> > >> behavior, one flag with a value would be
better. E.g., what
>>>>>> should
>>>>>> > >> "-mprefer-avx256 -mprefer-avx128
-mno-prefer-avx256" do? No
>>>>>> matter the
>>>>>> > >> answer, it's confusing. (Similarly
with other such
>>>>>> combinations). Just a
>>>>>> > >> single arg
"-mprefer-avx={128/256/512}" (with no "no" version)
>>>>>> seems easier
>>>>>> > >> to understand to me (keeping the same
behavior as you mention:
>>>>>> asking to
>>>>>> > >> prefer a larger width than is supported
by your architecture
>>>>>> should be fine
>>>>>> > >> but ignored).
>>>>>> > >>
>>>>>> > >>
>>>>>> > > I agree with this. It's a little more
plumbing as far as subtarget
>>>>>> > > features etc (represent via an optional value
or just various
>>>>>> "set the avx
>>>>>> > > width" features - the latter being
easier, but uglier), however,
>>>>>> it's
>>>>>> > > probably the right thing to do.
>>>>>> > >
>>>>>> > > I was looking at this myself just a couple
weeks ago and think
>>>>>> this is the
>>>>>> > > right direction (when and how to turn things
off) - and probably
>>>>>> makes
>>>>>> > > sense to be a default for these
architectures? We might end up
>>>>>> needing to
>>>>>> > > check a couple of additional TTI places, but
it sounds like
>>>>>> you're on top
>>>>>> > > of it. :)
>>>>>> > >
>>>>>> > > Thanks very much for doing this work.
>>>>>> > >
>>>>>> > > -eric
>>>>>> > >
>>>>>> > >
>>>>>> > >>
>>>>>> > >>
>>>>>> > >> There may be some other backend changes
needed, but I plan to
>>>>>> address
>>>>>> > >>> those as we find them.
>>>>>> > >>>
>>>>>> > >>>
>>>>>> > >>> At a later point, consider making
-mprefer-avx256 the default
>>>>>> for
>>>>>> > >>> Skylake Server due to the above
mentioned performance
>>>>>> considerations.
>>>>>> > >>>
>>>>>> > >>
>>>>>> > >>
>>>>>> > >>
>>>>>> > >>
>>>>>> > >>
>>>>>> > >>>
>>>>>> > >> Does this sound reasonable?
>>>>>> > >>>
>>>>>> > >>>
>>>>>> > >>>
>>>>>> > >>> *Latest Intel Optimization manual
available here:
>>>>>> > >>>
https://software.intel.com/en-us/articles/intel-sdm#optimiza
>>>>>> tion
>>>>>> > >>>
>>>>>> > >>>
>>>>>> > >>> -Craig Topper
>>>>>> > >>>
>>>>>> > >>>
_______________________________________________
>>>>>> > >>> LLVM Developers mailing list
>>>>>> > >>> llvm-dev at lists.llvm.org
>>>>>> > >>>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>> > >>>
>>>>>> > >>>
_______________________________________________
>>>>>> > >> LLVM Developers mailing list
>>>>>> > >> llvm-dev at lists.llvm.org
>>>>>> > >>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>> > >>
>>>>>> > >
>>>>>> > _______________________________________________
>>>>>> > LLVM Developers mailing list
>>>>>> > llvm-dev at lists.llvm.org
>>>>>> >
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>> _______________________________________________
>>>>>> LLVM Developers mailing list
>>>>>> llvm-dev at lists.llvm.org
>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>
>>>>>
>>>>>
>>>>
>>>
>>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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On 11/11/2017 09:52 PM, UE US via llvm-dev wrote:
> If skylake is that bad at AVX2
I don't think this says anything negative about AVX2, but AVX-512.
> it belongs in -mcpu / -march IMO.
No. We'd still want to enable the architectural features for vector 
intrinsics and the like.
> Based on the current performance data we're seeing, we think we need 
> to ultimately default skylake-avx512 to -mprefer-vector-width=256.
Craig, is this for both integer and floating-point code?

  -Hal
>    Most people will build for the standard x86_64-pc-linux or whatever 
> anyway,  and completely ignore the change. This will mainly affect 
> those who build their own software and optimize for their system, and 
> lots there have probably caught on to this already.  I always thought 
> that's what -march was made for, really.
>
> GNOMETOYS
>
> On Sat, Nov 11, 2017 at 10:25 AM, Sanjay Patel via llvm-dev 
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>> wrote:
>
>     Yes - I was thinking of FeatureFastScalarFSQRT /
>     FeatureFastVectorFSQRT which are used by isFsqrtCheap(). These
>     were added to override the default x86 sqrt estimate codegen with:
>     https://reviews.llvm.org/D21379 <https://reviews.llvm.org/D21379>
>
>     But I'm not sure we really need that kind of hack. Can we adjust
>     the attribute in clang based on the target cpu? Ie, if you have
>     something like:
>     $ clang -O2 -march=skylake-avx512 foo.c
>
>     Then you can detect that in the clang driver and pass
>     -mprefer-vector-width=256 to clang codegen as an option? Clang
>     codegen then adds that function attribute to everything it
>     outputs. Then, the vectorizers and/or backend detect that
>     attribute and adjust their behavior based on it.
>
>     So I don't think we should be messing with any kind of type
>     legality checking because that stuff should all be correct
>     already. We're just choosing a vector size based on a pref. I
>     think we should even allow the pref to go bigger than a legal
>     type. This came up somewhere on llvm-dev or in a bug recently in
>     the context of vector reductions.
>
>
>
>     On Fri, Nov 10, 2017 at 6:04 PM, Craig Topper
>     <craig.topper at gmail.com <mailto:craig.topper at
gmail.com>> wrote:
>
>         Are you referring to the X86TargetLowering::isFsqrtCheap hook?
>
>         ~Craig
>
>         On Fri, Nov 10, 2017 at 7:39 AM, Sanjay Patel
>         <spatel at rotateright.com <mailto:spatel at
rotateright.com>> wrote:
>
>             We can tie a user preference / override to a CPU model. We
>             do something like that for square root estimates already
>             (although it does use a SubtargetFeature currently for
>             x86; ideally, we'd key that off of something in the CPU
>             scheduler model).
>
>
>             On Thu, Nov 9, 2017 at 4:21 PM, Craig Topper
>             <craig.topper at gmail.com <mailto:craig.topper at
gmail.com>>
>             wrote:
>
>                 I agree that a less x86 specific command line makes
>                 sense. I've been having an internal discussions with
>                 gcc folks and their evaluating switching to something
>                 like -mprefer-vector-width=128/256/512/none
>
>                 Based on the current performance data we're seeing, we
>                 think we need to ultimately default skylake-avx512 to
>                 -mprefer-vector-width=256. If we go with a target
>                 independent option/implementation is there someway we
>                 could still affect the default behavior in a target
>                 specific way?
>
>                 ~Craig
>
>                 On Tue, Nov 7, 2017 at 9:06 AM, Sanjay Patel
>                 <spatel at rotateright.com
>                 <mailto:spatel at rotateright.com>> wrote:
>
>                     It's clear from the Intel docs how this has
>                     evolved, but from a compiler perspective, this
>                     isn't a Skylake "feature" :) ... nor an
Intel
>                     feature, nor an x86 feature.
>
>                     It's a generic programmer hint for any target with
>                     multiple potential vector lengths.
>
>                     On x86, there's already a potential use case for
>                     this hint with a different starting motivation:
>                     re-vectorization. That's where we take C code that
>                     uses 128-bit vector intrinsics and selectively
>                     widen it to 256- or 512-bit vector ops based on a
>                     newer CPU target than the code was originally
>                     written for.
>
>                     I think it's just a matter of time before a
>                     customer requests the same ability for another
>                     target (maybe they already have and I don't know
>                     about it). So we should have a solution that
>                     recognizes that possibility.
>
>                     Note that having a target-independent
>                     implementation in the optimizer doesn't preclude a
>                     flag alias in clang to maintain compatibility with
>                     gcc.
>
>
>
>                     On Tue, Nov 7, 2017 at 2:02 AM, Tobias Grosser via
>                     llvm-dev <llvm-dev at lists.llvm.org
>                     <mailto:llvm-dev at lists.llvm.org>> wrote:
>
>                         On Fri, Nov 3, 2017, at 05:47, Craig Topper
>                         via llvm-dev wrote:
>                         > That's a very good point about the
ordering
>                         of the command line options.
>                         > gcc's current implementation treats
>                         -mprefer-avx256 has "prefer 256 over
>                         > 512" and -mprefer-avx128 as "prefer
128 over
>                         256". Which feels weird for
>                         > other reasons, but has less of an ordering
>                         ambiguity.
>                         >
>                         > -mprefer-avx128 has been in gcc for many
>                         years and predates the creation
>                         > of
>                         > avx512. -mprefer-avx256 was added a couple
>                         months ago.
>                         >
>                         > We've had an internal conversation with
the
>                         implementor of
>                         > -mprefer-avx256
>                         > in gcc about making -mprefer-avx128 affect
>                         512-bit vectors as well. I'll
>                         > bring up the ambiguity issue with them.
>                         >
>                         > Do we want to be compatible with gcc here?
>
>                         I certainly believe we would want to be
>                         compatible with gcc (if we use
>                         the same names).
>
>                         Best,
>                         Tobias
>
>                         >
>                         > ~Craig
>                         >
>                         > On Thu, Nov 2, 2017 at 7:18 PM, Eric
>                         Christopher <echristo at gmail.com
>                         <mailto:echristo at gmail.com>>
>                         > wrote:
>                         >
>                         > >
>                         > >
>                         > > On Thu, Nov 2, 2017 at 7:05 PM James Y
>                         Knight via llvm-dev <
>                         > > llvm-dev at lists.llvm.org
>                         <mailto:llvm-dev at lists.llvm.org>>
wrote:
>                         > >
>                         > >> On Wed, Nov 1, 2017 at 7:35 PM, Craig
>                         Topper via llvm-dev <
>                         > >> llvm-dev at lists.llvm.org
>                         <mailto:llvm-dev at lists.llvm.org>>
wrote:
>                         > >>
>                         > >>> Hello all,
>                         > >>>
>                         > >>>
>                         > >>>
>                         > >>> I would like to propose adding
the
>                         -mprefer-avx256 and -mprefer-avx128
>                         > >>> command line flags supported by
latest
>                         GCC to clang. These flags will be
>                         > >>> used to limit the vector register
size
>                         presented by TTI to the vectorizers.
>                         > >>> The backend will still be able to
use
>                         wider registers for code written
>                         > >>> using the instrinsics in
x86intrin.h.
>                         And the backend will still be able to
>                         > >>> use AVX512VL instructions and the
>                         additional XMM16-31 and YMM16-31
>                         > >>> registers.
>                         > >>>
>                         > >>>
>                         > >>>
>                         > >>> Motivation:
>                         > >>>
>                         > >>> -Using 512-bit operations on some
Intel
>                         CPUs may cause a decrease in CPU
>                         > >>> frequency that may offset the
gains from
>                         using the wider register size. See
>                         > >>> section 15.26 of Intel® 64 and
IA-32
>                         Architectures Optimization Reference
>                         > >>> Manual published October 2017.
>                         > >>>
>                         > >>
>                         > >> I note the doc mentions that 256-bit
AVX
>                         operations also have the same
>                         > >> issue with reducing the CPU
frequency,
>                         which is nice to see documented!
>                         > >>
>                         > >> There's also the issues discussed
here
>                         <http://www.agner.org/
>                         > >> optimize/blog/read.php?i=165> (and
>                         elsewhere) related to warm-up time
>                         > >> for the 256-bit execution pipeline,
which
>                         is another issue with using
>                         > >> wide-vector ops.
>                         > >>
>                         > >>
>                         > >> -The vector ALUs on ports 0 and 1 of
the
>                         Skylake Server microarchitecture
>                         > >>> are only 256-bits wide. 512-bit
>                         instructions using these ALUs must use both
>                         > >>> ports. See section 2.1 of Intel®
64 and
>                         IA-32 Architectures Optimization
>                         > >>> Reference Manual published
October 2017.
>                         > >>>
>                         > >>
>                         > >>
>                         > >>> Implementation Plan:
>                         > >>>
>                         > >>> -Add prefer-avx256 and
prefer-avx128 as
>                         SubtargetFeatures in X86.td not
>                         > >>> mapped to any CPU.
>                         > >>>
>                         > >>> -Add mprefer-avx256 and
mprefer-avx128
>                         and the corresponding
>                         > >>> -mno-prefer-avx128/256 options to
>                         clang's driver Options.td file. I believe
>                         > >>> this will allow clang to pass
these
>                         straight through to the -target-feature
>                         > >>> attribute in IR.
>                         > >>>
>                         > >>> -Modify
X86TTIImpl::getRegisterBitWidth
>                         to only return 512 if AVX512 is
>                         > >>> enabled and prefer-avx256 and
>                         prefer-avx128 is not set. Similarly return
>                         > >>> 256 if AVX is enabled and
prefer-avx128
>                         is not set.
>                         > >>>
>                         > >>
>                         > >> Instead of multiple flags that have
>                         difficult to understand intersecting
>                         > >> behavior, one flag with a value would
be
>                         better. E.g., what should
>                         > >> "-mprefer-avx256 -mprefer-avx128
>                         -mno-prefer-avx256" do? No matter the
>                         > >> answer, it's confusing.
(Similarly with
>                         other such combinations). Just a
>                         > >> single arg
"-mprefer-avx={128/256/512}"
>                         (with no "no" version) seems easier
>                         > >> to understand to me (keeping the same
>                         behavior as you mention: asking to
>                         > >> prefer a larger width than is
supported
>                         by your architecture should be fine
>                         > >> but ignored).
>                         > >>
>                         > >>
>                         > > I agree with this. It's a little more
>                         plumbing as far as subtarget
>                         > > features etc (represent via an optional
>                         value or just various "set the avx
>                         > > width" features - the latter being
easier,
>                         but uglier), however, it's
>                         > > probably the right thing to do.
>                         > >
>                         > > I was looking at this myself just a
couple
>                         weeks ago and think this is the
>                         > > right direction (when and how to turn
>                         things off) - and probably makes
>                         > > sense to be a default for these
>                         architectures? We might end up needing to
>                         > > check a couple of additional TTI places,
>                         but it sounds like you're on top
>                         > > of it. :)
>                         > >
>                         > > Thanks very much for doing this work.
>                         > >
>                         > > -eric
>                         > >
>                         > >
>                         > >>
>                         > >>
>                         > >> There may be some other backend
changes
>                         needed, but I plan to address
>                         > >>> those as we find them.
>                         > >>>
>                         > >>>
>                         > >>> At a later point, consider making
>                         -mprefer-avx256 the default for
>                         > >>> Skylake Server due to the above
>                         mentioned performance considerations.
>                         > >>>
>                         > >>
>                         > >>
>                         > >>
>                         > >>
>                         > >>
>                         > >>>
>                         > >> Does this sound reasonable?
>                         > >>>
>                         > >>>
>                         > >>>
>                         > >>> *Latest Intel Optimization manual
>                         available here:
>                         > >>>
>                        
https://software.intel.com/en-us/articles/intel-sdm#optimization
>                        
<https://software.intel.com/en-us/articles/intel-sdm#optimization>
>                         > >>>
>                         > >>>
>                         > >>> -Craig Topper
>                         > >>>
>                         > >>>
>                         _______________________________________________
>                         > >>> LLVM Developers mailing list
>                         > >>> llvm-dev at lists.llvm.org
>                         <mailto:llvm-dev at lists.llvm.org>
>                         > >>>
>                        
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>                        
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>                         > >>>
>                         > >>>
>                         _______________________________________________
>                         > >> LLVM Developers mailing list
>                         > >> llvm-dev at lists.llvm.org
>                         <mailto:llvm-dev at lists.llvm.org>
>                         > >>
>                        
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>                        
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>                         > >>
>                         > >
>                         >
_______________________________________________
>                         > LLVM Developers mailing list
>                         > llvm-dev at lists.llvm.org
>                         <mailto:llvm-dev at lists.llvm.org>
>                         >
>                        
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>                        
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Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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On Sat, Nov 11, 2017 at 8:52 PM, Hal Finkel via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
>
> On 11/11/2017 09:52 PM, UE US via llvm-dev wrote:
>
> If skylake is that bad at AVX2
>
>
> I don't think this says anything negative about AVX2, but AVX-512.
>
> it belongs in -mcpu / -march IMO.
>
>
> No. We'd still want to enable the architectural features for vector
> intrinsics and the like.
>
I took this to mean that the feature should be enabled by default for
-march=skylake-avx512.


>
>
> Based on the current performance data we're seeing, we think we need to
> ultimately default skylake-avx512 to -mprefer-vector-width=256.
>
>
> Craig, is this for both integer and floating-point code?
>
I believe so, but I'll try to get confirmation from the people with more
data.

>
>
>  -Hal
>
>    Most people will build for the standard x86_64-pc-linux or whatever
> anyway,  and completely ignore the change. This will mainly affect those
> who build their own software and optimize for their system, and lots there
> have probably caught on to this already.  I always thought that's what
> -march was made for, really.
>
> GNOMETOYS
>
> On Sat, Nov 11, 2017 at 10:25 AM, Sanjay Patel via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Yes - I was thinking of FeatureFastScalarFSQRT / FeatureFastVectorFSQRT
>> which are used by isFsqrtCheap(). These were added to override the
default
>> x86 sqrt estimate codegen with:
>> https://reviews.llvm.org/D21379
>>
>> But I'm not sure we really need that kind of hack. Can we adjust
the
>> attribute in clang based on the target cpu? Ie, if you have something
like:
>> $ clang -O2 -march=skylake-avx512 foo.c
>>
>> Then you can detect that in the clang driver and pass
>> -mprefer-vector-width=256 to clang codegen as an option? Clang codegen
then
>> adds that function attribute to everything it outputs. Then, the
>> vectorizers and/or backend detect that attribute and adjust their
behavior
>> based on it.
>>
>
Do we have a precedent for setting a target independent flag from a target
specific cpu string in the clang driver? Want to make sure I understand
what the processing on such a thing would look like. Particularly to get
the order right so the user can override it.

>
>> So I don't think we should be messing with any kind of type
legality
>> checking because that stuff should all be correct already. We're
just
>> choosing a vector size based on a pref. I think we should even allow
the
>> pref to go bigger than a legal type. This came up somewhere on llvm-dev
or
>> in a bug recently in the context of vector reductions.
>>
>>
>>
>> On Fri, Nov 10, 2017 at 6:04 PM, Craig Topper <craig.topper at
gmail.com>
>> wrote:
>>
>>> Are you referring to the X86TargetLowering::isFsqrtCheap hook?
>>>
>>> ~Craig
>>>
>>> On Fri, Nov 10, 2017 at 7:39 AM, Sanjay Patel <spatel at
rotateright.com>
>>> wrote:
>>>
>>>> We can tie a user preference / override to a CPU model. We do
something
>>>> like that for square root estimates already (although it does
use a
>>>> SubtargetFeature currently for x86; ideally, we'd key that
off of something
>>>> in the CPU scheduler model).
>>>>
>>>>
>>>> On Thu, Nov 9, 2017 at 4:21 PM, Craig Topper <craig.topper
at gmail.com>
>>>> wrote:
>>>>
>>>>> I agree that a less x86 specific command line makes sense.
I've been
>>>>> having an internal discussions with gcc folks and their
evaluating
>>>>> switching to something like
-mprefer-vector-width=128/256/512/none
>>>>>
>>>>> Based on the current performance data we're seeing, we
think we need
>>>>> to ultimately default skylake-avx512 to
-mprefer-vector-width=256. If we go
>>>>> with a target independent option/implementation is there
someway we could
>>>>> still affect the default behavior in a target specific way?
>>>>>
>>>>> ~Craig
>>>>>
>>>>> On Tue, Nov 7, 2017 at 9:06 AM, Sanjay Patel <spatel at
rotateright.com>
>>>>> wrote:
>>>>>
>>>>>> It's clear from the Intel docs how this has
evolved, but from a
>>>>>> compiler perspective, this isn't a Skylake
"feature" :) ... nor an Intel
>>>>>> feature, nor an x86 feature.
>>>>>>
>>>>>> It's a generic programmer hint for any target with
multiple potential
>>>>>> vector lengths.
>>>>>>
>>>>>> On x86, there's already a potential use case for
this hint with a
>>>>>> different starting motivation: re-vectorization.
That's where we take C
>>>>>> code that uses 128-bit vector intrinsics and
selectively widen it to 256-
>>>>>> or 512-bit vector ops based on a newer CPU target than
the code was
>>>>>> originally written for.
>>>>>>
>>>>>> I think it's just a matter of time before a
customer requests the
>>>>>> same ability for another target (maybe they already
have and I don't know
>>>>>> about it). So we should have a solution that recognizes
that possibility.
>>>>>>
>>>>>> Note that having a target-independent implementation in
the optimizer
>>>>>> doesn't preclude a flag alias in clang to maintain
compatibility with gcc.
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Tue, Nov 7, 2017 at 2:02 AM, Tobias Grosser via
llvm-dev <
>>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>>
>>>>>>> On Fri, Nov 3, 2017, at 05:47, Craig Topper via
llvm-dev wrote:
>>>>>>> > That's a very good point about the
ordering of the command line
>>>>>>> options.
>>>>>>> > gcc's current implementation treats
-mprefer-avx256 has "prefer
>>>>>>> 256 over
>>>>>>> > 512" and -mprefer-avx128 as "prefer
128 over 256". Which feels
>>>>>>> weird for
>>>>>>> > other reasons, but has less of an ordering
ambiguity.
>>>>>>> >
>>>>>>> > -mprefer-avx128 has been in gcc for many years
and predates the
>>>>>>> creation
>>>>>>> > of
>>>>>>> > avx512. -mprefer-avx256 was added a couple
months ago.
>>>>>>> >
>>>>>>> > We've had an internal conversation with
the implementor of
>>>>>>> > -mprefer-avx256
>>>>>>> > in gcc about making -mprefer-avx128 affect
512-bit vectors as
>>>>>>> well. I'll
>>>>>>> > bring up the ambiguity issue with them.
>>>>>>> >
>>>>>>> > Do we want to be compatible with gcc here?
>>>>>>>
>>>>>>> I certainly believe we would want to be compatible
with gcc (if we
>>>>>>> use
>>>>>>> the same names).
>>>>>>>
>>>>>>> Best,
>>>>>>> Tobias
>>>>>>>
>>>>>>> >
>>>>>>> > ~Craig
>>>>>>> >
>>>>>>> > On Thu, Nov 2, 2017 at 7:18 PM, Eric
Christopher <
>>>>>>> echristo at gmail.com>
>>>>>>> > wrote:
>>>>>>> >
>>>>>>> > >
>>>>>>> > >
>>>>>>> > > On Thu, Nov 2, 2017 at 7:05 PM James Y
Knight via llvm-dev <
>>>>>>> > > llvm-dev at lists.llvm.org> wrote:
>>>>>>> > >
>>>>>>> > >> On Wed, Nov 1, 2017 at 7:35 PM, Craig
Topper via llvm-dev <
>>>>>>> > >> llvm-dev at lists.llvm.org> wrote:
>>>>>>> > >>
>>>>>>> > >>> Hello all,
>>>>>>> > >>>
>>>>>>> > >>>
>>>>>>> > >>>
>>>>>>> > >>> I would like to propose adding
the -mprefer-avx256 and
>>>>>>> -mprefer-avx128
>>>>>>> > >>> command line flags supported by
latest GCC to clang. These
>>>>>>> flags will be
>>>>>>> > >>> used to limit the vector register
size presented by TTI to the
>>>>>>> vectorizers.
>>>>>>> > >>> The backend will still be able to
use wider registers for code
>>>>>>> written
>>>>>>> > >>> using the instrinsics in
x86intrin.h. And the backend will
>>>>>>> still be able to
>>>>>>> > >>> use AVX512VL instructions and the
additional XMM16-31 and
>>>>>>> YMM16-31
>>>>>>> > >>> registers.
>>>>>>> > >>>
>>>>>>> > >>>
>>>>>>> > >>>
>>>>>>> > >>> Motivation:
>>>>>>> > >>>
>>>>>>> > >>> -Using 512-bit operations on some
Intel CPUs may cause a
>>>>>>> decrease in CPU
>>>>>>> > >>> frequency that may offset the
gains from using the wider
>>>>>>> register size. See
>>>>>>> > >>> section 15.26 of Intel® 64 and
IA-32 Architectures
>>>>>>> Optimization Reference
>>>>>>> > >>> Manual published October 2017.
>>>>>>> > >>>
>>>>>>> > >>
>>>>>>> > >> I note the doc mentions that 256-bit
AVX operations also have
>>>>>>> the same
>>>>>>> > >> issue with reducing the CPU
frequency, which is nice to see
>>>>>>> documented!
>>>>>>> > >>
>>>>>>> > >> There's also the issues discussed
here <http://www.agner.org/
>>>>>>> > >> optimize/blog/read.php?i=165> (and
elsewhere) related to
>>>>>>> warm-up time
>>>>>>> > >> for the 256-bit execution pipeline,
which is another issue with
>>>>>>> using
>>>>>>> > >> wide-vector ops.
>>>>>>> > >>
>>>>>>> > >>
>>>>>>> > >> -The vector ALUs on ports 0 and 1 of
the Skylake Server
>>>>>>> microarchitecture
>>>>>>> > >>> are only 256-bits wide. 512-bit
instructions using these ALUs
>>>>>>> must use both
>>>>>>> > >>> ports. See section 2.1 of Intel®
64 and IA-32 Architectures
>>>>>>> Optimization
>>>>>>> > >>> Reference Manual published
October 2017.
>>>>>>> > >>>
>>>>>>> > >>
>>>>>>> > >>
>>>>>>> > >>>  Implementation Plan:
>>>>>>> > >>>
>>>>>>> > >>> -Add prefer-avx256 and
prefer-avx128 as SubtargetFeatures in
>>>>>>> X86.td not
>>>>>>> > >>> mapped to any CPU.
>>>>>>> > >>>
>>>>>>> > >>> -Add mprefer-avx256 and
mprefer-avx128 and the corresponding
>>>>>>> > >>> -mno-prefer-avx128/256 options to
clang's driver Options.td
>>>>>>> file. I believe
>>>>>>> > >>> this will allow clang to pass
these straight through to the
>>>>>>> -target-feature
>>>>>>> > >>> attribute in IR.
>>>>>>> > >>>
>>>>>>> > >>> -Modify
X86TTIImpl::getRegisterBitWidth to only return 512 if
>>>>>>> AVX512 is
>>>>>>> > >>> enabled and prefer-avx256 and
prefer-avx128 is not set.
>>>>>>> Similarly return
>>>>>>> > >>> 256 if AVX is enabled and
prefer-avx128 is not set.
>>>>>>> > >>>
>>>>>>> > >>
>>>>>>> > >> Instead of multiple flags that have
difficult to understand
>>>>>>> intersecting
>>>>>>> > >> behavior, one flag with a value would
be better. E.g., what
>>>>>>> should
>>>>>>> > >> "-mprefer-avx256 -mprefer-avx128
-mno-prefer-avx256" do? No
>>>>>>> matter the
>>>>>>> > >> answer, it's confusing.
(Similarly with other such
>>>>>>> combinations). Just a
>>>>>>> > >> single arg
"-mprefer-avx={128/256/512}" (with no "no" version)
>>>>>>> seems easier
>>>>>>> > >> to understand to me (keeping the same
behavior as you mention:
>>>>>>> asking to
>>>>>>> > >> prefer a larger width than is
supported by your architecture
>>>>>>> should be fine
>>>>>>> > >> but ignored).
>>>>>>> > >>
>>>>>>> > >>
>>>>>>> > > I agree with this. It's a little more
plumbing as far as
>>>>>>> subtarget
>>>>>>> > > features etc (represent via an optional
value or just various
>>>>>>> "set the avx
>>>>>>> > > width" features - the latter being
easier, but uglier), however,
>>>>>>> it's
>>>>>>> > > probably the right thing to do.
>>>>>>> > >
>>>>>>> > > I was looking at this myself just a
couple weeks ago and think
>>>>>>> this is the
>>>>>>> > > right direction (when and how to turn
things off) - and probably
>>>>>>> makes
>>>>>>> > > sense to be a default for these
architectures? We might end up
>>>>>>> needing to
>>>>>>> > > check a couple of additional TTI places,
but it sounds like
>>>>>>> you're on top
>>>>>>> > > of it. :)
>>>>>>> > >
>>>>>>> > > Thanks very much for doing this work.
>>>>>>> > >
>>>>>>> > > -eric
>>>>>>> > >
>>>>>>> > >
>>>>>>> > >>
>>>>>>> > >>
>>>>>>> > >> There may be some other backend
changes needed, but I plan to
>>>>>>> address
>>>>>>> > >>> those as we find them.
>>>>>>> > >>>
>>>>>>> > >>>
>>>>>>> > >>> At a later point, consider making
-mprefer-avx256 the default
>>>>>>> for
>>>>>>> > >>> Skylake Server due to the above
mentioned performance
>>>>>>> considerations.
>>>>>>> > >>>
>>>>>>> > >>
>>>>>>> > >>
>>>>>>> > >>
>>>>>>> > >>
>>>>>>> > >>
>>>>>>> > >>>
>>>>>>> > >> Does this sound reasonable?
>>>>>>> > >>>
>>>>>>> > >>>
>>>>>>> > >>>
>>>>>>> > >>> *Latest Intel Optimization manual
available here:
>>>>>>> > >>>
https://software.intel.com/en-us/articles/intel-sdm#optimiza
>>>>>>> tion
>>>>>>> > >>>
>>>>>>> > >>>
>>>>>>> > >>> -Craig Topper
>>>>>>> > >>>
>>>>>>> > >>>
_______________________________________________
>>>>>>> > >>> LLVM Developers mailing list
>>>>>>> > >>> llvm-dev at lists.llvm.org
>>>>>>> > >>>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>> > >>>
>>>>>>> > >>>
_______________________________________________
>>>>>>> > >> LLVM Developers mailing list
>>>>>>> > >> llvm-dev at lists.llvm.org
>>>>>>> > >>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>> > >>
>>>>>>> > >
>>>>>>> >
_______________________________________________
>>>>>>> > LLVM Developers mailing list
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>>>>>>> >
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>> _______________________________________________
>>>>>>> LLVM Developers mailing list
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>>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
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> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
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