[cfe-dev] Disable integer promotion (Dilan Manatunga via cfe-dev)
Norman Rink via cfe-dev
cfe-dev at lists.llvm.org
Mon May 30 07:11:14 PDT 2016
Hi all,
I realize this is potentially only tangent to the ongoing discussion, but
does anyone have significant experience with how integer promotion interacts
with vectorization? When I looked into this interaction, I did not have the
time to conduct a careful analysis, but I have reason to believe that
integer promotion can get in the way of vectorization, thereby limiting its
benefits. Can anyone comment? Thanks.
Best,
Norman
From: "Martin J. O'Riordan" <martin.oriordan at movidius.com>
Organization: Movidius Ltd.
Date: Monday 30 May 2016 14:53
To: 'James Molloy' <james at jamesmolloy.co.uk>, 'David Majnemer'
<david.majnemer at gmail.com>, 'Dilan Manatunga' <manatunga at gmail.com>
Cc: 'Clang Dev' <cfe-dev at lists.llvm.org>, Norman Rink
<norman.rink at tu-dresden.de>
Subject: RE: [cfe-dev] Disable integer promotion (Dilan Manatunga via
cfe-dev)
Hi James and thanks for pointing out the existence of this transformation,
we were quite unaware of it.
As it happens, I am highly allergic to re-invention and avoid doing so
whenever possible; the only reason an already overburdened team of 2
developers will re-invent is because they are unaware of an existing
solution which is not difficult given the scope and complexity of LLVM.
So far as I can tell, ‘truncateToMinimalBitwidths’ is always enabled, so it
is not a target specific selection and our target should automatically reap
the rewards of this optimisation pass. I certainly cannot find a switch to
enable or disable it. But in fact we are not seeing anywhere near the
benefits we would expect.
void InnerLoopVectorizer::truncateToMinimalBitwidths() {
// For every instruction `I` in MinBWs, truncate the operands, create a
// truncated version of `I` and reextend its result. InstCombine runs
// later and will remove any ext/trunc pairs.
This appears to only run on inner-loops, and it appear to insert
narrowings/truncations and subsequent widenings/extendings into the IR
chains.
The DataLayout for our target includes “-n8:16:32”, so it should see the
benefits of optimisations for multiple native integer support. We also
provide both 32-bit SIMD and 128-bit SIMD native support.
The pass that we wrote is quite different. It is run as a machine pass
prior to loop-unrolling and vectorisation, and instead of pre-truncating and
post-extending IR chains, it removes the existing pre-extending and
post-truncating that brackets a sequence of IR operations if it can prove
that the outcome is the same. The results are actually very good and match
what our expectations are from such a transformation, which makes me wonder
“why does ‘truncateToMinimalBitwidths’ not already produce comparable
results?”.
Our observations are that with the new pass, a significant majority of
vectorised code showed some improvement, with results as high as 40X faster
than without. Of the small number of tests that regressed in performance,
adding a ‘#pragma clang unroll_count(N)’ eliminated the loss. This could
probably be eliminate too by better tuning of the cost-models.
The re-invention is inadvertent, but in any event our new pass appears to
provide considerable additional performance improvements that are not
currently happening with the stock LLVM transformations.
I will have to contrive some tests to see why ‘truncateToMinimalBitwidths’
is not already doing this, and if there is something that we have done wrong
in our target that is breaking it, I will happily revert to an existing
solution.
MartinO
From: James Molloy [mailto:james at jamesmolloy.co.uk]
Sent: 28 May 2016 19:58
To: Martin.ORiordan at movidius.com; David Majnemer; Dilan Manatunga
Cc: Clang Dev; Norman Rink
Subject: Re: [cfe-dev] Disable integer promotion (Dilan Manatunga via
cfe-dev)
Hi,
X86 has native support for i8 and i16. Aarch64 and ARM have native i8 and
i16 vector operations that are lowered and analysed using
truncateToMinimalBitwidths in LoopVectorize. Similarly for scalar code on
x86 truncation is done in instcombine.
Why do you need to reinvent this?
Cheers,
James
On Sat, 28 May 2016 at 19:02, Martin J. O'Riordan via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
>
> Instead of suppressing the integer promotion rules which are part of the ISO
> C/C++ Standards, we wrote a new pass that analyses the IR to see if the input
> values and output value were of an integer type that was narrower than the
> promoted types used in the IR, and if we could prove that the outcome would be
> identical if the type was unpromoted, then we reduced the IR to use the
> narrower form.
>
> In our case the motive was to enhance vectorisation because our vector ALU can
> work with 8-, 16- and 32-bit integers natively, and handling ‘vXi8’ vectors
> ended was actually being promoted to multiple ‘v4i32’ vectors requiring 4
> times as many instructions as were necessary, or worse still, fully
> scalarized.
>
> This pass was presented by my colleague Stephen Rogers in a “Lighting Talk” at
> the October 2015 LLVM Conference in San Jose and titled “Integer Vector
> Optimizations and “Usual Arithmetic Conversions””. I can’t find the paper or
> slides on the LLVM Meetings page, perhaps these are not archived for Lightning
> Talks (?), but as they are not large I have attached them here.
>
> This approach allowed us to gain the optimisations that are possible with our
> architecture which supports 8-, 16- and 32-bit native integer computations
> (scalar and vector), while also respecting the ISO C and C++ Standards. I am
> a lot more nervous of a front-end switch for this, as it will lead to
> non-compliant programs, and in the presence of overloading and
> template-instantiation it could also lead to very different programs, and
> would recommend that we do not add a front-end switch which alters the
> semantics of the language in this way.
>
> It is my intention to publish this pass if it is of general interest, and
> since it is target independent there are no particular blocking issue for me
> (Patents, IP, etc.) to doing so. I do have to catch-up on the HEAD revision
> to ensure that it still works correctly, but it was working perfectly at SVN
> #262824 and it will be a month before I have enough time to catch up on the
> HEAD revision as we are busy with a product release that takes precedence.
>
> All the best,
>
> MartinO
>
> From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] On Behalf Of David
> Majnemer via cfe-dev
> Sent: 27 May 2016 19:55
> To: Dilan Manatunga <manatunga at gmail.com>
> Cc: clang developer list <cfe-dev at lists.llvm.org>; Norman Rink
> <norman.rink at tu-dresden.de>; cfe-dev-request at lists.llvm.org
> Subject: Re: [cfe-dev] Disable integer promotion (Dilan Manatunga via cfe-dev)
>
>
>
> You could set IntWidth to 16 or 8 in clang, not unlike what MSP430 does:
>
> https://github.com/llvm-mirror/clang/blob/3317d0fa0bd1f5c5adc14bcc6adc2a38acc9
> 064b/lib/Basic/Targets.cpp#L6823
>
>
>
> On Fri, May 27, 2016 at 10:32 AM, Dilan Manatunga via cfe-dev
> <cfe-dev at lists.llvm.org> wrote:
>>
>> I need disabling this feature because I am researching architectures where
>> 8-bit or 16-bit adds are preferred to 32-bit. So, integer promotion kinda
>> mucks everything up. I was hoping there was a way in clang to disable it,
>> instead of having to implement an LLVM pass to coalesce unnecessary
>> promotions.
>>
>>
>>
>> Thanks for catching the IR mistake. Should have double checked that. This
>> should be the correct version:
>>
>> nt8_t a = 1;
>>
>> int8_t b = 2;
>>
>> int8_t c = a + b
>>
>>
>>
>> The LLVM IR will be:
>>
>> %x = sext i8 %a to i32
>>
>> %y = sext i8 %b to i32
>>
>> %z = add nsw i32 %x, %y
>>
>> %c = trunc i32 %z to i8
>>
>>
>>
>> Instead, it would simply compile to:
>>
>> $c = add nsw i8 %z, $y
>>
>>
>>
>> -Dilan
>>
>>
>>
>>
>>
>> On Fri, May 27, 2016 at 5:30 AM Norman Rink via cfe-dev
>> <cfe-dev at lists.llvm.org> wrote:
>>> Hi Dilan,
>>>
>>> I would like to second your request for an option to disable integer
>>> promotion. What do you need it for?
>>>
>>> As far as I am aware, there is no such option and the code that implements
>>> integer promotion is somewhat scattered across ³SemaExpr.cpp².
>>>
>>> Also, I think your example code snippet contains a few ³i32²s too many. It
>>> will be clearer to people what you are looking for if your code example is
>>> consistent with your question.
>>>
>>> Best,
>>>
>>> Norman
>>>
>>>
>>>> >Message: 1
>>>> >Date: Fri, 27 May 2016 01:50:12 +0000
>>>> >From: Dilan Manatunga via cfe-dev <cfe-dev at lists.llvm.org>
>>>> >To: cfe-dev at lists.llvm.org
>>>> >Subject: [cfe-dev] Disable integer promotion
>>>> >Message-ID:
>>>> > <CAHpgGu4=jFC9ohQQZZMp2NMG3Hw0sE5U4-Lqrgb+6gcXv9SEtQ at mail.gmail.com
>>>> <mailto:jFC9ohQQZZMp2NMG3Hw0sE5U4-Lqrgb%2B6gcXv9SEtQ at mail.gmail.com> >
>>>> >Content-Type: text/plain; charset="utf-8"
>>>> >
>>>> >Is there a way to disable integer promotion when performing math
>>>> >operations. For example, when compiling a statement such as this:
>>>> >int8_t a = 1;
>>>> >int8_t b = 2;
>>>> >int8_t c = a + b
>>>> >
>>>> >The LLVM IR will be:
>>>> >%x = sext i32 %a to i32
>>>> >%y = sext i32 %b to i32
>>>> >%z = add nsw i32 %x, %y
>>>> >%c = trunc i32 %z to i16
>>>> >
>>>> >Instead, it would simply compile to:
>>>> >$c = add nsw i32 %z, $y
>>>> >
>>>> >-Dilan Manatunga
>>>> >-------------- next part --------------
>>>> >An HTML attachment was scrubbed...
>>>> >URL:
>>>> ><http://lists.llvm.org/pipermail/cfe-dev/attachments/20160527/4a7920ab/att
>>>> >achment-0001.html>
>>>> >
>>>> >------------------------------
>>>
>>>
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20160530/547eeb02/attachment.html>
More information about the cfe-dev
mailing list