llvm dev - Mar 2016 - LLD: Possible optimization for TargetInfo

On Wed, Mar 30, 2016 at 4:20 PM, Sean Silva <chisophugis at gmail.com>
wrote:
> I believe the relocation stuff that Rafael is currently working on will
> make this a non-issue (it will make relocation application much friendlier
> for the CPU).
>
I don't think Rafael's patch would make this a non-issue. He's
making
scanRelocs to create data, which would reduce the number of calls to the
virtual functions, but it wouldn't be reduced to zero.

However, even in the current scheme, since the target is fixed, all
the
> indirect call sites should be monomorphic and so there shouldn't be
much
> branch-prediction cost (certainly nothing that would cause 1.8% performance
> delta for the entire link).
>
Agreed. We could template functions that call TargetInfo's member functions
for each target to eliminate the virtual function calls.

> Notice that 1.8% is smaller than the performance variation from r263227
> which is a very innocuous-looking change but caused ~2-3% slowdown for
> ScyllaDB (see the thread "LLD performance w.r.t. local symbols (and
> --build-id)").
>
> -- Sean Silva
>
> On Wed, Mar 30, 2016 at 3:39 PM, Rui Ueyama via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> I was wandering how much is the overhead of virtual function calls of
>> TargetInfo member functions. TargetInfo handles platform-specific
details,
>> and we have target-specific subclasses of that class. The subclasses
>> override functions defined in TargetInfo.
>>
>> The TargetInfo member functions are called multiple times for each
>> relocation. So the cost of virtual function calls may be non-neglible.
That
>> is a motication to do the following test.
>>
>> As a test, I removed all TargetInfo subclasses except for x86-64, move
>> all code from X86_64TargetInfo to TargetInfo, and remove `virtual` from
>> TargetInfo.
>>
>> The original LLD links itself (with debug info) in 7.499 seconds. The
>> de-virtualized version did the same thing in 7.364 seconds. So it can
>> improve it by 1.8%.
>>
>> I'm just pointing out that there's room there to improve
performance, and
>> I'm not suggesting we do something for this right now. We probably
>> shouldn't do anything for this because the current code is pretty
>> straightforward. But I'd expect that we will eventually want do
something
>> for this in future.
>>
>> _______________________________________________
>> 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 Mar 30, 2016, at 4:25 PM, Rui Ueyama via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> On Wed, Mar 30, 2016 at 4:20 PM, Sean Silva <chisophugis at gmail.com
<mailto:chisophugis at gmail.com>> wrote:
> I believe the relocation stuff that Rafael is currently working on will
make this a non-issue (it will make relocation application much friendlier for
the CPU).
> 
> I don't think Rafael's patch would make this a non-issue. He's
making scanRelocs to create data, which would reduce the number of calls to the
virtual functions, but it wouldn't be reduced to zero.
> 
> However, even in the current scheme, since the target is fixed, all the
indirect call sites should be monomorphic and so there shouldn't be much
branch-prediction cost (certainly nothing that would cause 1.8% performance
delta for the entire link).
> 
> Agreed. We could template functions that call TargetInfo's member
functions for each target to eliminate the virtual function calls.
Are you measuring with a build of LLD with LTO+PGO
(+ICP+WholeDevirtualization...)?
We have compiler technologies to address these problems, I think we'd better
leave it to them when they can handle it.

--
 Mehdi


>  
> Notice that 1.8% is smaller than the performance variation from r263227
which is a very innocuous-looking change but caused ~2-3% slowdown for ScyllaDB
(see the thread "LLD performance w.r.t. local symbols (and
--build-id)").
> 
> -- Sean Silva
> 
> On Wed, Mar 30, 2016 at 3:39 PM, Rui Ueyama via llvm-dev <llvm-dev at
lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> I was wandering how much is the overhead of virtual function calls of
TargetInfo member functions. TargetInfo handles platform-specific details, and
we have target-specific subclasses of that class. The subclasses override
functions defined in TargetInfo.
> 
> The TargetInfo member functions are called multiple times for each
relocation. So the cost of virtual function calls may be non-neglible. That is a
motication to do the following test.
> 
> As a test, I removed all TargetInfo subclasses except for x86-64, move all
code from X86_64TargetInfo to TargetInfo, and remove `virtual` from TargetInfo.
> 
> The original LLD links itself (with debug info) in 7.499 seconds. The
de-virtualized version did the same thing in 7.364 seconds. So it can improve it
by 1.8%.
> 
> I'm just pointing out that there's room there to improve
performance, and I'm not suggesting we do something for this right now. We
probably shouldn't do anything for this because the current code is pretty
straightforward. But I'd expect that we will eventually want do something
for this in future.
> 
> _______________________________________________
> 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
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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On Wed, Mar 30, 2016 at 4:28 PM, Mehdi Amini <mehdi.amini at apple.com>
wrote:
>
> On Mar 30, 2016, at 4:25 PM, Rui Ueyama via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> On Wed, Mar 30, 2016 at 4:20 PM, Sean Silva <chisophugis at
gmail.com> wrote:
>
>> I believe the relocation stuff that Rafael is currently working on will
>> make this a non-issue (it will make relocation application much
friendlier
>> for the CPU).
>>
>
> I don't think Rafael's patch would make this a non-issue. He's
making
> scanRelocs to create data, which would reduce the number of calls to the
> virtual functions, but it wouldn't be reduced to zero.
>
> However, even in the current scheme, since the target is fixed, all the
>> indirect call sites should be monomorphic and so there shouldn't be
much
>> branch-prediction cost (certainly nothing that would cause 1.8%
performance
>> delta for the entire link).
>>
>
> Agreed. We could template functions that call TargetInfo's member
> functions for each target to eliminate the virtual function calls.
>
>
> Are you measuring with a build of LLD with LTO+PGO
> (+ICP+WholeDevirtualization...)?
> We have compiler technologies to address these problems, I think we'd
> better leave it to them when they can handle it.
>
I didn't measure it with them, but can it devirtualize these function calls?

TargetInfo is a class to handle target-specific stuff. We create an
instance of its subclass at startup and call its member functions. For each
linker invocation, there is only one TargetInfo instantiated, but the types
of TargetInfos for two linker invocations are not guaranteed to be the same
(if you are cross-linking a binary, it's different from the default.)

In order to devirtualize that, I think the compiler needs to deduct that we
create only one TargetInfo in a program and that will never change once it
is created. Is it possible?

--
>  Mehdi
>
>
>
>
>
>> Notice that 1.8% is smaller than the performance variation from r263227
>> which is a very innocuous-looking change but caused ~2-3% slowdown for
>> ScyllaDB (see the thread "LLD performance w.r.t. local symbols
(and
>> --build-id)").
>>
>> -- Sean Silva
>>
>> On Wed, Mar 30, 2016 at 3:39 PM, Rui Ueyama via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> I was wandering how much is the overhead of virtual function calls
of
>>> TargetInfo member functions. TargetInfo handles platform-specific
details,
>>> and we have target-specific subclasses of that class. The
subclasses
>>> override functions defined in TargetInfo.
>>>
>>> The TargetInfo member functions are called multiple times for each
>>> relocation. So the cost of virtual function calls may be
non-neglible. That
>>> is a motication to do the following test.
>>>
>>> As a test, I removed all TargetInfo subclasses except for x86-64,
move
>>> all code from X86_64TargetInfo to TargetInfo, and remove `virtual`
from
>>> TargetInfo.
>>>
>>> The original LLD links itself (with debug info) in 7.499 seconds.
The
>>> de-virtualized version did the same thing in 7.364 seconds. So it
can
>>> improve it by 1.8%.
>>>
>>> I'm just pointing out that there's room there to improve
performance,
>>> and I'm not suggesting we do something for this right now. We
probably
>>> shouldn't do anything for this because the current code is
pretty
>>> straightforward. But I'd expect that we will eventually want do
something
>>> for this in future.
>>>
>>> _______________________________________________
>>> 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 Wed, Mar 30, 2016 at 4:25 PM, Rui Ueyama <ruiu at google.com> wrote:
> On Wed, Mar 30, 2016 at 4:20 PM, Sean Silva <chisophugis at
gmail.com> wrote:
>
>> I believe the relocation stuff that Rafael is currently working on will
>> make this a non-issue (it will make relocation application much
friendlier
>> for the CPU).
>>
>
> I don't think Rafael's patch would make this a non-issue. He's
making
> scanRelocs to create data, which would reduce the number of calls to the
> virtual functions, but it wouldn't be reduced to zero.
>
> However, even in the current scheme, since the target is fixed, all the
>> indirect call sites should be monomorphic and so there shouldn't be
much
>> branch-prediction cost (certainly nothing that would cause 1.8%
performance
>> delta for the entire link).
>>
>
> Agreed. We could template functions that call TargetInfo's member
> functions for each target to eliminate the virtual function calls.
>
>From what has been presented I would not conclude that virtual calls are
actually the problem (or a problem at all). A root-cause analysis is
necessary. As r263227 shows, the relocation application loop is very
sensitive to small changes.

One quick thing you may also want to try as a sanity check is inserting
nops in different places in the function. I suspect you'll find that the
performance swings (both speedups and slowdowns) from doing that are
similar in magnitude to what you are seeing. You may also want to try
editing the indirect call instruction to a direct call without otherwise
modifying the binary; if that reproduces the 1.8% speedup then it will be
convincing.

If you haven't read it, I think you would enjoy this paper:
http://static.googleusercontent.com/media/research.google.com/en//pubs/archive/37077.pdf

-- Sean Silva

>
>
>> Notice that 1.8% is smaller than the performance variation from r263227
>> which is a very innocuous-looking change but caused ~2-3% slowdown for
>> ScyllaDB (see the thread "LLD performance w.r.t. local symbols
(and
>> --build-id)").
>>
>> -- Sean Silva
>>
>> On Wed, Mar 30, 2016 at 3:39 PM, Rui Ueyama via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> I was wandering how much is the overhead of virtual function calls
of
>>> TargetInfo member functions. TargetInfo handles platform-specific
details,
>>> and we have target-specific subclasses of that class. The
subclasses
>>> override functions defined in TargetInfo.
>>>
>>> The TargetInfo member functions are called multiple times for each
>>> relocation. So the cost of virtual function calls may be
non-neglible. That
>>> is a motication to do the following test.
>>>
>>> As a test, I removed all TargetInfo subclasses except for x86-64,
move
>>> all code from X86_64TargetInfo to TargetInfo, and remove `virtual`
from
>>> TargetInfo.
>>>
>>> The original LLD links itself (with debug info) in 7.499 seconds.
The
>>> de-virtualized version did the same thing in 7.364 seconds. So it
can
>>> improve it by 1.8%.
>>>
>>> I'm just pointing out that there's room there to improve
performance,
>>> and I'm not suggesting we do something for this right now. We
probably
>>> shouldn't do anything for this because the current code is
pretty
>>> straightforward. But I'd expect that we will eventually want do
something
>>> for this in future.
>>>
>>> _______________________________________________
>>> 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 Wed, Mar 30, 2016 at 5:34 PM, Sean Silva <chisophugis at gmail.com>
wrote:
>
>
> On Wed, Mar 30, 2016 at 4:25 PM, Rui Ueyama <ruiu at google.com>
wrote:
>
>> On Wed, Mar 30, 2016 at 4:20 PM, Sean Silva <chisophugis at
gmail.com>
>> wrote:
>>
>>> I believe the relocation stuff that Rafael is currently working on
will
>>> make this a non-issue (it will make relocation application much
friendlier
>>> for the CPU).
>>>
>>
>> I don't think Rafael's patch would make this a non-issue.
He's making
>> scanRelocs to create data, which would reduce the number of calls to
the
>> virtual functions, but it wouldn't be reduced to zero.
>>
>> However, even in the current scheme, since the target is fixed, all the
>>> indirect call sites should be monomorphic and so there
shouldn't be much
>>> branch-prediction cost (certainly nothing that would cause 1.8%
performance
>>> delta for the entire link).
>>>
>>
>> Agreed. We could template functions that call TargetInfo's member
>> functions for each target to eliminate the virtual function calls.
>>
>
> From what has been presented I would not conclude that virtual calls are
> actually the problem (or a problem at all). A root-cause analysis is
> necessary. As r263227 shows, the relocation application loop is very
> sensitive to small changes.
>
> One quick thing you may also want to try as a sanity check is inserting
> nops in different places in the function. I suspect you'll find that
the
> performance swings (both speedups and slowdowns) from doing that are
> similar in magnitude to what you are seeing. You may also want to try
> editing the indirect call instruction to a direct call without otherwise
> modifying the binary; if that reproduces the 1.8% speedup then it will be
> convincing.
>
Honestly I was somewhat skeptical about what you wrote here, but I observed
0.4% *slowdown* when I used gcc to compile it, so looks like I was wrong.
It is possible that devirtualization might have been effective for
clang-generated code, but it is more likely that that was a result of some
performance deviation caused by some other factor.

The relocation handling loop is really a tight loop and therefore sensitive
to small changes. How can we optimize this? Maybe PGO?

If you haven't read it, I think you would enjoy this
paper:
>
http://static.googleusercontent.com/media/research.google.com/en//pubs/archive/37077.pdf
>
> -- Sean Silva
>
>
>>
>>
>>> Notice that 1.8% is smaller than the performance variation from
r263227
>>> which is a very innocuous-looking change but caused ~2-3% slowdown
for
>>> ScyllaDB (see the thread "LLD performance w.r.t. local symbols
(and
>>> --build-id)").
>>>
>>> -- Sean Silva
>>>
>>> On Wed, Mar 30, 2016 at 3:39 PM, Rui Ueyama via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> I was wandering how much is the overhead of virtual function
calls of
>>>> TargetInfo member functions. TargetInfo handles
platform-specific details,
>>>> and we have target-specific subclasses of that class. The
subclasses
>>>> override functions defined in TargetInfo.
>>>>
>>>> The TargetInfo member functions are called multiple times for
each
>>>> relocation. So the cost of virtual function calls may be
non-neglible. That
>>>> is a motication to do the following test.
>>>>
>>>> As a test, I removed all TargetInfo subclasses except for
x86-64, move
>>>> all code from X86_64TargetInfo to TargetInfo, and remove
`virtual` from
>>>> TargetInfo.
>>>>
>>>> The original LLD links itself (with debug info) in 7.499
seconds. The
>>>> de-virtualized version did the same thing in 7.364 seconds. So
it can
>>>> improve it by 1.8%.
>>>>
>>>> I'm just pointing out that there's room there to
improve performance,
>>>> and I'm not suggesting we do something for this right now.
We probably
>>>> shouldn't do anything for this because the current code is
pretty
>>>> straightforward. But I'd expect that we will eventually
want do something
>>>> for this in future.
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>>
>>>
>>
>
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