Just a quick heads up -- removing BBVectorize from LLVM (and Clang)

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Just a quick heads up -- removing BBVectorize from LLVM (and Clang)

Xin Wang via cfe-dev
If you don't use BBVectorize at all, you can ignore this.

Hal suggested this in a thread in 2014:

None objected then, and I don't think any new uses have arisen so I plan to just remove it. It is causing maintenance burden, complexity, and is a set of features I'd rather not port to the new PM.

Just an FYI email to folks so they aren't confused when the patches land.

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Re: Just a quick heads up -- removing BBVectorize from LLVM (and Clang)

Xin Wang via cfe-dev

On 06/29/2017 05:42 PM, Chandler Carruth wrote:
> If you don't use BBVectorize at all, you can ignore this.
>
> Hal suggested this in a thread in 2014:
> http://lists.llvm.org/pipermail/llvm-dev/2014-November/079091.html
>
> None objected then, and I don't think any new uses have arisen so I
> plan to just remove it. It is causing maintenance burden, complexity,
> and is a set of features I'd rather not port to the new PM.

Thanks, Chandler. Indeed, I've not seen any "-fslp-vectorize-aggressive
makes my code faster" reports in a long time.

  -Hal

>
> Just an FYI email to folks so they aren't confused when the patches land.

--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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Re: Just a quick heads up -- removing BBVectorize from LLVM (and Clang)

Xin Wang via cfe-dev
In reply to this post by Xin Wang via cfe-dev
On Thu, Jun 29, 2017 at 3:42 PM, Chandler Carruth via cfe-dev
<[hidden email]> wrote:

> If you don't use BBVectorize at all, you can ignore this.
>
> Hal suggested this in a thread in 2014:
> http://lists.llvm.org/pipermail/llvm-dev/2014-November/079091.html
>
> None objected then, and I don't think any new uses have arisen so I plan to
> just remove it. It is causing maintenance burden, complexity, and is a set
> of features I'd rather not port to the new PM.
>
> Just an FYI email to folks so they aren't confused when the patches land.

Maybe worth mentioning in the release notes?
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Re: Just a quick heads up -- removing BBVectorize from LLVM (and Clang)

Xin Wang via cfe-dev
Already added in the commit (I think)

On Fri, Jun 30, 2017 at 3:58 PM Hans Wennborg <[hidden email]> wrote:
On Thu, Jun 29, 2017 at 3:42 PM, Chandler Carruth via cfe-dev
<[hidden email]> wrote:
> If you don't use BBVectorize at all, you can ignore this.
>
> Hal suggested this in a thread in 2014:
> http://lists.llvm.org/pipermail/llvm-dev/2014-November/079091.html
>
> None objected then, and I don't think any new uses have arisen so I plan to
> just remove it. It is causing maintenance burden, complexity, and is a set
> of features I'd rather not port to the new PM.
>
> Just an FYI email to folks so they aren't confused when the patches land.

Maybe worth mentioning in the release notes?

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Re: Just a quick heads up -- removing BBVectorize from LLVM (and Clang)

Xin Wang via cfe-dev
I don't know how others feel, but it seems this was removed more by feeling and less by actual data points. It doesn't mean it wasn't the correct choice, but just because nobody spoke up could mean that they silently enjoy the benefits and maybe some end user far away isn't subscribed. With this in mind - I hope others would consider turning optimizations into an Assert, Warning or Error if they will be removed - at least for a release. That way there's some visible warning that users had a chance to see and potentially relay the feedback. The middle ground could be disabling the transformation and just warning on the analysis pass. "Foo optimization has been removed and it may impact your code performance"...

Too much work?

On Sat, Jul 1, 2017 at 9:32 AM, Chandler Carruth via cfe-dev <[hidden email]> wrote:
Already added in the commit (I think)

On Fri, Jun 30, 2017 at 3:58 PM Hans Wennborg <[hidden email]> wrote:
On Thu, Jun 29, 2017 at 3:42 PM, Chandler Carruth via cfe-dev
<[hidden email]> wrote:
> If you don't use BBVectorize at all, you can ignore this.
>
> Hal suggested this in a thread in 2014:
> http://lists.llvm.org/pipermail/llvm-dev/2014-November/079091.html
>
> None objected then, and I don't think any new uses have arisen so I plan to
> just remove it. It is causing maintenance burden, complexity, and is a set
> of features I'd rather not port to the new PM.
>
> Just an FYI email to folks so they aren't confused when the patches land.

Maybe worth mentioning in the release notes?

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Re: Just a quick heads up -- removing BBVectorize from LLVM (and Clang)

Xin Wang via cfe-dev


On 06/30/2017 09:01 PM, C Bergström via cfe-dev wrote:
I don't know how others feel, but it seems this was removed more by feeling and less by actual data points. It doesn't mean it wasn't the correct choice, but just because nobody spoke up could mean that they silently enjoy the benefits and maybe some end user far away isn't subscribed. With this in mind - I hope others would consider turning optimizations into an Assert, Warning or Error if they will be removed - at least for a release.

They'll get a warning about an unused command-line argument. Also, I think there's been some discussion about adding a more-specific warning for this case.

Thanks again,
Hal

That way there's some visible warning that users had a chance to see and potentially relay the feedback. The middle ground could be disabling the transformation and just warning on the analysis pass. "Foo optimization has been removed and it may impact your code performance"...

Too much work?

On Sat, Jul 1, 2017 at 9:32 AM, Chandler Carruth via cfe-dev <[hidden email]> wrote:
Already added in the commit (I think)

On Fri, Jun 30, 2017 at 3:58 PM Hans Wennborg <[hidden email]> wrote:
On Thu, Jun 29, 2017 at 3:42 PM, Chandler Carruth via cfe-dev
<[hidden email]> wrote:
> If you don't use BBVectorize at all, you can ignore this.
>
> Hal suggested this in a thread in 2014:
> http://lists.llvm.org/pipermail/llvm-dev/2014-November/079091.html
>
> None objected then, and I don't think any new uses have arisen so I plan to
> just remove it. It is causing maintenance burden, complexity, and is a set
> of features I'd rather not port to the new PM.
>
> Just an FYI email to folks so they aren't confused when the patches land.

Maybe worth mentioning in the release notes?

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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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Re: [llvm-dev] Just a quick heads up -- removing BBVectorize from LLVM (and Clang)

Xin Wang via cfe-dev
In reply to this post by Xin Wang via cfe-dev
On Fri, Jun 30, 2017 at 7:01 PM, C Bergström via llvm-dev
<[hidden email]> wrote:

> I don't know how others feel, but it seems this was removed more by feeling
> and less by actual data points. It doesn't mean it wasn't the correct
> choice, but just because nobody spoke up could mean that they silently enjoy
> the benefits and maybe some end user far away isn't subscribed. With this in
> mind - I hope others would consider turning optimizations into an Assert,
> Warning or Error if they will be removed - at least for a release. That way
> there's some visible warning that users had a chance to see and potentially
> relay the feedback. The middle ground could be disabling the transformation
> and just warning on the analysis pass. "Foo optimization has been removed
> and it may impact your code performance"...
>
> Too much work?
>

FWIW, people have independently found issues/miscompiles in the BB
Vectorizer. I picked up this only recently, but some of them have been
reported on bugzilla and been there forever without anybody looking at
them (see Zhendong's reports).
I don't think resurrecting would be hard if anybody really wants to.
It requires some amount of effort nobody is apparently able to put at
the moment. We removed loadcombine for a similar reason recently.

--
Davide
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