llvm dev - Jun 2016 - The state of IRPGO (3 remaining work items)

> On May 23, 2016, at 8:56 PM, Xinliang David Li <davidxl at
google.com> wrote:
> 
> On Mon, May 23, 2016 at 8:23 PM, Sean Silva <chisophugis at
gmail.com> wrote:
> Jake and I have been integrating IRPGO on PS4, and we've identified 3
remaining work items.
> 
> Sean, thanks for the write up. It matches very well with what we think as
well.
+ 1

> - Driver changes
> 
> We'd like to make IRPGO the default on PS4. We also think that it would
be beneficial to make IRPGO the default PGO on all platforms (coverage would
continue to use FE instr as it does currently, of course). In previous
conversations (e.g. http://reviews.llvm.org/D15829) it has come up that Apple
have requirements that would prevent them from moving to IRPGO as the default
PGO, at least without a deprecation period of one or two releases.
Sean pointed out the problematic scenario in D15829 (in plan "C"):

```
All existing user workflows continue to work, except for workflows that attempt
to llvm-profdata merge some old frontend profile data (e.g. they have checked-in
to version control and represents some special workload) with the profile data
from new binaries.
```

We can address this issue by (1) making sure llvm-profdata emits a helpful
warning when merging an FE-based profile with an IR-based one, and (2) keeping
an option to use FE instrumentation for PGO. Having (2) helps people who
can't (or don't want) to switch to IR PGO.

> I'd like to get consensus on a path forward.
> As a point of discussion, how about we make IRPGO the default on all
platforms except Apple platforms.
I'd really rather not introduce this inconsistency. I'm worried that it
might lead to Darwin becoming a second-tier platform for PGO.

Fred (CC'd) is following up with some of our internal users to check if we
can change the default behavior of -fprofile-instr-generate. He should be able
to chime in on this soon.

At its core I don't think -fprofile-instr-generate *implies* FE-based
instrumentation. So, I'd like to see the driver do this (on all platforms):

  * -fprofile-instr-generate: IR instrumentation
  * -fprofile-instr-generate=IR: IR instrumentation
  * -fprofile-instr-generate=FE: FE instrumentation
  * -fprofile-instr-generate -fcoverage-mapping: FE + coverage instrumentation

It's a bit ugly because the meaning of -fprofile-instr-generate becomes
context-sensitive. But, (1) it doesn't break existing common workflows and
(2) it makes it easier to ship IRPGO. The big caveat here is that we'll need
to wait a bit and see if our internal users are OK with this.

One alternative is to introduce a separate driver flag for IRPGO. This might not
work well for Sony's existing users. I'd be interested in any feedback
about this approach.

> I really don't like fragmenting things like this (e.g. if a third-party
tests "clang's" PGO they will get something different depending on
the platform), but I don't see another way given Apple's constraints.
> 
> I'd like to see IRPGO to be the default as well, but the first thing we
need is a driver level option to make the switch (prof-gen) -- currently we rely
on -Xclang option to switch between two modes, which is less than ideal.
> 
> If the concern from Apple is that the old profile still need to work, then
this is problem already solved. The reason is that -fprofile-instr-use can
automatically detect the type of the profile and switch the mode.
It's not just that. As Sean pointed out, we're concerned about old
profiles inter-operating poorly with new ones.

thanks,
vedant

> - Pre-instrumentation passes
> 
> Pre-instrumentation optimization has been critical for reducing the
overhead of PGO for the PS4 games we tested (as expected). However, in our
measurements (and we are glad to provide more info) the main benefit was
inlining (also as expected). A simple pass of inlining at threshold 100 appeared
to give all the benefits. Even inlining at threshold 0 gave almost all the
benefits. For example, the passes initially proposed in
http://reviews.llvm.org/D15828did not improve over just inlining with threshold
100.
> 
> (due to PR27299 we also need to add simplifycfg after inlining to clean up,
but this doesn't affect the instrumentation overhead in our measurements)
> 
> Bottom line: for our use cases, inlining does all the work, but we're
not opposed to having more passes, which might be beneficial for non-game
workloads (which is most code).
> 
> 
> 
> Yes, Rong is re-collecting performance data before submitting the patch.
>  
> - Warnings
> 
> We identified 3 classes of issues which manifest as spammy warnings when
applying profile data with IRPGO (these affect FEPGO also I believe, but we
looked in depth at IRPGO):
> 
> 1. The main concerning one is that getPGOFuncName mangles the filename into
the counter name. This causes us to get instrprof_error::unknown_function when
the pgo-use build is done in a different build directory from the training build
(which is a reasonable thing to support). In this situation, PGO data is useless
for all `static` functions (and as a byproduct results in a huge volume of
warnings).
> 
> This can be enhanced with an user option to override the behavior. Can you
help filing a tracking bug?
>  
> 
> 2. In different TU's, pre-instr inlining might make different inlining
decisions (for example, different functions may be available for inlining),
causing hash mismatch errors (instrprof_error::hash_mismatch). In building a
large game, we only saw 8 instance of this, so it is not as severe as 1, but
would be good to fix.
> 
> 
> Rong has a patch addressing that -- will submit after cleanup pass change
is done.
> 
>  
> 3. A .cpp file may be compiled and put into an archive, but then not
selected by the linker and will therefore not result in a counter in the
profraw. When compiling this file with pgo-use,
instrprof_error::unknown_function will result and a warning will be emitted.
> 
> yes -- this is a common problem to other compilers as well. 
> 
>  
> 
> Case 1 can be fixed using a function hash or other unique identifier
instead of a file path. David, in D20195 you mentioned that Rong was working on
a patch that would fix 2; we are looking forward to that.
> 
> 
> Right.
>  
> For 3, I unfortunately do not know of any solution. I don't think there
is a way for us to make this warning reliable in the face of this circumstance.
So my conclusion is that instrprof_error::unknown_function at least must be
defaulted to off unfortunately.
> 
> yes, this can be annoying. If the warnings can be buffered, then the
compiler can check if this is due to missing profile for the whole file and can
reduce the warnings into one single warning (source file has no profile data). 
Making it off by default sounds fine to me too if it is too noisy.
> 
> thanks,
> 
> David
>  
> 
> -- Sean Silva

Zooming into the command-line option bike-shed:
> On 2016-May-24, at 15:41, Vedant Kumar via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> At its core I don't think -fprofile-instr-generate *implies* FE-based
instrumentation. So, I'd like to see the driver do this (on all platforms):
> 
>  * -fprofile-instr-generate: IR instrumentation
>  * -fprofile-instr-generate=IR: IR instrumentation
>  * -fprofile-instr-generate=FE: FE instrumentation
>  * -fprofile-instr-generate -fcoverage-mapping: FE + coverage
instrumentation
I feel like this would be simpler:
  * -fcoverage-mapping: -fprofile-instr-generate=FE + coverage instrumentation

Maybe there's a downside I'm not seeing though?

Also, I don't like "FE".  Maybe "source"?  And instead
of "IR", "llvm-ir" or something?
> It's a bit ugly because the meaning of -fprofile-instr-generate becomes
context-sensitive. But, (1) it doesn't break existing common workflows and
(2) it makes it easier to ship IRPGO. The big caveat here is that we'll need
to wait a bit and see if our internal users are OK with this.
> 
> One alternative is to introduce a separate driver flag for IRPGO. This
might not work well for Sony's existing users. I'd be interested in any
feedback about this approach.
My first thought is `-mprofile-instr-generate`, since if it's not in the
frontend then "-f" doesn't really make sense...

On Tue, May 24, 2016 at 3:50 PM, Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:
> Zooming into the command-line option bike-shed:
>
> > On 2016-May-24, at 15:41, Vedant Kumar via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > At its core I don't think -fprofile-instr-generate *implies*
FE-based
> instrumentation. So, I'd like to see the driver do this (on all
platforms):
> >
> >  * -fprofile-instr-generate: IR instrumentation
> >  * -fprofile-instr-generate=IR: IR instrumentation
> >  * -fprofile-instr-generate=FE: FE instrumentation
> >  * -fprofile-instr-generate -fcoverage-mapping: FE + coverage
> instrumentation
>
> I feel like this would be simpler:
>   * -fcoverage-mapping: -fprofile-instr-generate=FE + coverage
> instrumentation
>
> Maybe there's a downside I'm not seeing though?
>

I proposed the same change in proposal B in the review thread.

B. Proposed new behavior:
 -fprofile-instr-generate turns on IR late instrumentation
 -fcoverage-mapping turns on FE instrumentation and coverage-mapping
 -fprofile-instr-generate -fcoverage-mapping result in compiler warning
 -fprofile-instr-use=<> will automatically determine how to use the

The upside is that -fcoverage-mapping itself does not do anything by itself
today. This change will simplify its usage (without user specifying
-fprofile-instr-generate)

The downside Sean mentioned is that this changes the existing behavior of
-fcoverage-mapping which can be a surprise to users (though I wonder why
would a user depend on this old behavior).

>
> Also, I don't like "FE".  Maybe "source"?  And
instead of "IR", "llvm-ir"
> or something?
>
Perhaps clang vs LLVM (similar to the cc1 option we have).  The downside is
'Clang' is clearly tied to Clang, but not any other FEs.


David
>
> > It's a bit ugly because the meaning of -fprofile-instr-generate
becomes
> context-sensitive. But, (1) it doesn't break existing common workflows
and
> (2) it makes it easier to ship IRPGO. The big caveat here is that we'll
> need to wait a bit and see if our internal users are OK with this.
> >
> > One alternative is to introduce a separate driver flag for IRPGO. This
> might not work well for Sony's existing users. I'd be interested in
any
> feedback about this approach.
>
> My first thought is `-mprofile-instr-generate`, since if it's not in
the
> frontend then "-f" doesn't really make sense...
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On Tue, May 24, 2016 at 3:41 PM, Vedant Kumar <vsk at apple.com> wrote:
>
> > On May 23, 2016, at 8:56 PM, Xinliang David Li <davidxl at
google.com>
> wrote:
> >
> > On Mon, May 23, 2016 at 8:23 PM, Sean Silva <chisophugis at
gmail.com>
> wrote:
> > Jake and I have been integrating IRPGO on PS4, and we've
identified 3
> remaining work items.
> >
> > Sean, thanks for the write up. It matches very well with what we think
> as well.
>
> + 1
>
>
> > - Driver changes
> >
> > We'd like to make IRPGO the default on PS4. We also think that it
would
> be beneficial to make IRPGO the default PGO on all platforms (coverage
> would continue to use FE instr as it does currently, of course). In
> previous conversations (e.g. http://reviews.llvm.org/D15829) it has come
> up that Apple have requirements that would prevent them from moving to
> IRPGO as the default PGO, at least without a deprecation period of one or
> two releases.
>
> Sean pointed out the problematic scenario in D15829 (in plan
"C"):
>
> ```
> All existing user workflows continue to work, except for workflows that
> attempt to llvm-profdata merge some old frontend profile data (e.g. they
> have checked-in to version control and represents some special workload)
> with the profile data from new binaries.
> ```
>
> We can address this issue by (1) making sure llvm-profdata emits a helpful
> warning when merging an FE-based profile with an IR-based one, and (2)
> keeping an option to use FE instrumentation for PGO. Having (2) helps
> people who can't (or don't want) to switch to IR PGO.
>
>
> > I'd like to get consensus on a path forward.
> > As a point of discussion, how about we make IRPGO the default on all
> platforms except Apple platforms.
>
> I'd really rather not introduce this inconsistency. I'm worried
that it
> might lead to Darwin becoming a second-tier platform for PGO.
>
> Fred (CC'd) is following up with some of our internal users to check if
we
> can change the default behavior of -fprofile-instr-generate. He should be
> able to chime in on this soon.
>
Great!

>
> At its core I don't think -fprofile-instr-generate *implies* FE-based
> instrumentation. So, I'd like to see the driver do this (on all
platforms):
>
>   * -fprofile-instr-generate: IR instrumentation
>   * -fprofile-instr-generate=IR: IR instrumentation
>   * -fprofile-instr-generate=FE: FE instrumentation
>   * -fprofile-instr-generate -fcoverage-mapping: FE + coverage
> instrumentation
>
> It's a bit ugly because the meaning of -fprofile-instr-generate becomes
> context-sensitive. But, (1) it doesn't break existing common workflows
and
> (2) it makes it easier to ship IRPGO. The big caveat here is that we'll
> need to wait a bit and see if our internal users are OK with this.
>
Is there a reason to even have the possibility for FEPGO in the long
run?
>From what I can tell, at most we would add a -fuse-the-old-pgo-because-i-
want-to-merge-with-old-profiles option to hold people over until they can
regenerate their profiles with the current compiler. We can add a flag to
control what pre-instrumentation is done to retain the source-level
robustness of FEPGO (e.g. -fpgo-no-simplify-before-instrumenting or
something).

> One alternative is to introduce a separate driver flag for IRPGO. This
> might not work well for Sony's existing users. I'd be interested in
any
> feedback about this approach.
>
Personally, I would prefer to maintaining command line compatibility for
PGO in Clang (i.e. users don't have to modify their build systems).


-- Sean Silva

>
>
> > I really don't like fragmenting things like this (e.g. if a
third-party
> tests "clang's" PGO they will get something different
depending on the
> platform), but I don't see another way given Apple's constraints.
> >
> > I'd like to see IRPGO to be the default as well, but the first
thing we
> need is a driver level option to make the switch (prof-gen) -- currently we
> rely on -Xclang option to switch between two modes, which is less than
> ideal.
> >
> > If the concern from Apple is that the old profile still need to work,
> then this is problem already solved. The reason is that -fprofile-instr-use
> can automatically detect the type of the profile and switch the mode.
>
> It's not just that. As Sean pointed out, we're concerned about old
> profiles inter-operating poorly with new ones.
>
> thanks,
> vedant
>
>
> > - Pre-instrumentation passes
> >
> > Pre-instrumentation optimization has been critical for reducing the
> overhead of PGO for the PS4 games we tested (as expected). However, in our
> measurements (and we are glad to provide more info) the main benefit was
> inlining (also as expected). A simple pass of inlining at threshold 100
> appeared to give all the benefits. Even inlining at threshold 0 gave almost
> all the benefits. For example, the passes initially proposed in
> http://reviews.llvm.org/D15828did not improve over just inlining with
> threshold 100.
> >
> > (due to PR27299 we also need to add simplifycfg after inlining to
clean
> up, but this doesn't affect the instrumentation overhead in our
> measurements)
> >
> > Bottom line: for our use cases, inlining does all the work, but
we're
> not opposed to having more passes, which might be beneficial for non-game
> workloads (which is most code).
> >
> >
> >
> > Yes, Rong is re-collecting performance data before submitting the
patch.
> >
> > - Warnings
> >
> > We identified 3 classes of issues which manifest as spammy warnings
when
> applying profile data with IRPGO (these affect FEPGO also I believe, but we
> looked in depth at IRPGO):
> >
> > 1. The main concerning one is that getPGOFuncName mangles the filename
> into the counter name. This causes us to get
> instrprof_error::unknown_function when the pgo-use build is done in a
> different build directory from the training build (which is a reasonable
> thing to support). In this situation, PGO data is useless for all `static`
> functions (and as a byproduct results in a huge volume of warnings).
> >
> > This can be enhanced with an user option to override the behavior. Can
> you help filing a tracking bug?
> >
> >
> > 2. In different TU's, pre-instr inlining might make different
inlining
> decisions (for example, different functions may be available for inlining),
> causing hash mismatch errors (instrprof_error::hash_mismatch). In building
> a large game, we only saw 8 instance of this, so it is not as severe as 1,
> but would be good to fix.
> >
> >
> > Rong has a patch addressing that -- will submit after cleanup pass
> change is done.
> >
> >
> > 3. A .cpp file may be compiled and put into an archive, but then not
> selected by the linker and will therefore not result in a counter in the
> profraw. When compiling this file with pgo-use,
> instrprof_error::unknown_function will result and a warning will be
emitted.
> >
> > yes -- this is a common problem to other compilers as well.
> >
> >
> >
> > Case 1 can be fixed using a function hash or other unique identifier
> instead of a file path. David, in D20195 you mentioned that Rong was
> working on a patch that would fix 2; we are looking forward to that.
> >
> >
> > Right.
> >
> > For 3, I unfortunately do not know of any solution. I don't think
there
> is a way for us to make this warning reliable in the face of this
> circumstance. So my conclusion is that instrprof_error::unknown_function at
> least must be defaulted to off unfortunately.
> >
> > yes, this can be annoying. If the warnings can be buffered, then the
> compiler can check if this is due to missing profile for the whole file and
> can reduce the warnings into one single warning (source file has no profile
> data).  Making it off by default sounds fine to me too if it is too noisy.
> >
> > thanks,
> >
> > David
> >
> >
> > -- Sean Silva
>
>
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On Tue, May 24, 2016 at 3:50 PM, Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:
> Zooming into the command-line option bike-shed:
>
Let's avoid bikeshedding until the exact requirements are clear.

-- Sean Silva

>
> > On 2016-May-24, at 15:41, Vedant Kumar via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > At its core I don't think -fprofile-instr-generate *implies*
FE-based
> instrumentation. So, I'd like to see the driver do this (on all
platforms):
> >
> >  * -fprofile-instr-generate: IR instrumentation
> >  * -fprofile-instr-generate=IR: IR instrumentation
> >  * -fprofile-instr-generate=FE: FE instrumentation
> >  * -fprofile-instr-generate -fcoverage-mapping: FE + coverage
> instrumentation
>
> I feel like this would be simpler:
>   * -fcoverage-mapping: -fprofile-instr-generate=FE + coverage
> instrumentation
>
> Maybe there's a downside I'm not seeing though?
>
> Also, I don't like "FE".  Maybe "source"?  And
instead of "IR", "llvm-ir"
> or something?
>
> > It's a bit ugly because the meaning of -fprofile-instr-generate
becomes
> context-sensitive. But, (1) it doesn't break existing common workflows
and
> (2) it makes it easier to ship IRPGO. The big caveat here is that we'll
> need to wait a bit and see if our internal users are OK with this.
> >
> > One alternative is to introduce a separate driver flag for IRPGO. This
> might not work well for Sony's existing users. I'd be interested in
any
> feedback about this approach.
>
> My first thought is `-mprofile-instr-generate`, since if it's not in
the
> frontend then "-f" doesn't really make sense...
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It sounds to me we are likely to converge on the following:

1) Making IR/llvm based PGO the default;
2) Enhance -fcoverage-mapping such that it automatically turns on FE based
instrumentation
3) if -fcoverage-mapping is used together with -fprofile-instr-generate,
-fcoverage-mapping serves as a switch to turn on FE based instrumetnation

All the above are transparent to users.

The following are for advanced usage:

4) have a new option to explicitly switch instrumentation flavor to be FE
based
5) have a new option to turn off part of pre-instrumentation
cleanup/simplification passes for users who want very stable profile for
stable library sources *

* 4 and 5 serves the same purpose so 5 may not be necessary.

thanks,


On Tue, May 24, 2016 at 5:21 PM, Sean Silva <chisophugis at gmail.com>
wrote:
>
>
> On Tue, May 24, 2016 at 3:41 PM, Vedant Kumar <vsk at apple.com>
wrote:
>
>>
>> > On May 23, 2016, at 8:56 PM, Xinliang David Li <davidxl at
google.com>
>> wrote:
>> >
>> > On Mon, May 23, 2016 at 8:23 PM, Sean Silva <chisophugis at
gmail.com>
>> wrote:
>> > Jake and I have been integrating IRPGO on PS4, and we've
identified 3
>> remaining work items.
>> >
>> > Sean, thanks for the write up. It matches very well with what we
think
>> as well.
>>
>> + 1
>>
>>
>> > - Driver changes
>> >
>> > We'd like to make IRPGO the default on PS4. We also think that
it would
>> be beneficial to make IRPGO the default PGO on all platforms (coverage
>> would continue to use FE instr as it does currently, of course). In
>> previous conversations (e.g. http://reviews.llvm.org/D15829) it has
come
>> up that Apple have requirements that would prevent them from moving to
>> IRPGO as the default PGO, at least without a deprecation period of one
or
>> two releases.
>>
>> Sean pointed out the problematic scenario in D15829 (in plan
"C"):
>>
>> ```
>> All existing user workflows continue to work, except for workflows that
>> attempt to llvm-profdata merge some old frontend profile data (e.g.
they
>> have checked-in to version control and represents some special
workload)
>> with the profile data from new binaries.
>> ```
>>
>> We can address this issue by (1) making sure llvm-profdata emits a
>> helpful warning when merging an FE-based profile with an IR-based one,
and
>> (2) keeping an option to use FE instrumentation for PGO. Having (2)
helps
>> people who can't (or don't want) to switch to IR PGO.
>>
>>
>> > I'd like to get consensus on a path forward.
>> > As a point of discussion, how about we make IRPGO the default on
all
>> platforms except Apple platforms.
>>
>> I'd really rather not introduce this inconsistency. I'm worried
that it
>> might lead to Darwin becoming a second-tier platform for PGO.
>>
>> Fred (CC'd) is following up with some of our internal users to
check if
>> we can change the default behavior of -fprofile-instr-generate. He
should
>> be able to chime in on this soon.
>>
>
> Great!
>
>
>>
>> At its core I don't think -fprofile-instr-generate *implies*
FE-based
>> instrumentation. So, I'd like to see the driver do this (on all
platforms):
>>
>>   * -fprofile-instr-generate: IR instrumentation
>>   * -fprofile-instr-generate=IR: IR instrumentation
>>   * -fprofile-instr-generate=FE: FE instrumentation
>>   * -fprofile-instr-generate -fcoverage-mapping: FE + coverage
>> instrumentation
>>
>> It's a bit ugly because the meaning of -fprofile-instr-generate
becomes
>> context-sensitive. But, (1) it doesn't break existing common
workflows and
>> (2) it makes it easier to ship IRPGO. The big caveat here is that
we'll
>> need to wait a bit and see if our internal users are OK with this.
>>
>
> Is there a reason to even have the possibility for FEPGO in the long run?
> From what I can tell, at most we would add a -fuse-the-old-pgo-because-i-
> want-to-merge-with-old-profiles option to hold people over until they can
> regenerate their profiles with the current compiler. We can add a flag to
> control what pre-instrumentation is done to retain the source-level
> robustness of FEPGO (e.g. -fpgo-no-simplify-before-instrumenting or
> something).
>
>
>> One alternative is to introduce a separate driver flag for IRPGO. This
>> might not work well for Sony's existing users. I'd be
interested in any
>> feedback about this approach.
>>
>
> Personally, I would prefer to maintaining command line compatibility for
> PGO in Clang (i.e. users don't have to modify their build systems).
>
>
> -- Sean Silva
>
>
>>
>>
>> > I really don't like fragmenting things like this (e.g. if a
third-party
>> tests "clang's" PGO they will get something different
depending on the
>> platform), but I don't see another way given Apple's
constraints.
>> >
>> > I'd like to see IRPGO to be the default as well, but the first
thing we
>> need is a driver level option to make the switch (prof-gen) --
currently we
>> rely on -Xclang option to switch between two modes, which is less than
>> ideal.
>> >
>> > If the concern from Apple is that the old profile still need to
work,
>> then this is problem already solved. The reason is that
-fprofile-instr-use
>> can automatically detect the type of the profile and switch the mode.
>>
>> It's not just that. As Sean pointed out, we're concerned about
old
>> profiles inter-operating poorly with new ones.
>>
>> thanks,
>> vedant
>>
>>
>> > - Pre-instrumentation passes
>> >
>> > Pre-instrumentation optimization has been critical for reducing
the
>> overhead of PGO for the PS4 games we tested (as expected). However, in
our
>> measurements (and we are glad to provide more info) the main benefit
was
>> inlining (also as expected). A simple pass of inlining at threshold 100
>> appeared to give all the benefits. Even inlining at threshold 0 gave
almost
>> all the benefits. For example, the passes initially proposed in
>> http://reviews.llvm.org/D15828did not improve over just inlining with
>> threshold 100.
>> >
>> > (due to PR27299 we also need to add simplifycfg after inlining to
clean
>> up, but this doesn't affect the instrumentation overhead in our
>> measurements)
>> >
>> > Bottom line: for our use cases, inlining does all the work, but
we're
>> not opposed to having more passes, which might be beneficial for
non-game
>> workloads (which is most code).
>> >
>> >
>> >
>> > Yes, Rong is re-collecting performance data before submitting the
patch.
>> >
>> > - Warnings
>> >
>> > We identified 3 classes of issues which manifest as spammy
warnings
>> when applying profile data with IRPGO (these affect FEPGO also I
believe,
>> but we looked in depth at IRPGO):
>> >
>> > 1. The main concerning one is that getPGOFuncName mangles the
filename
>> into the counter name. This causes us to get
>> instrprof_error::unknown_function when the pgo-use build is done in a
>> different build directory from the training build (which is a
reasonable
>> thing to support). In this situation, PGO data is useless for all
`static`
>> functions (and as a byproduct results in a huge volume of warnings).
>> >
>> > This can be enhanced with an user option to override the behavior.
Can
>> you help filing a tracking bug?
>> >
>> >
>> > 2. In different TU's, pre-instr inlining might make different
inlining
>> decisions (for example, different functions may be available for
inlining),
>> causing hash mismatch errors (instrprof_error::hash_mismatch). In
building
>> a large game, we only saw 8 instance of this, so it is not as severe as
1,
>> but would be good to fix.
>> >
>> >
>> > Rong has a patch addressing that -- will submit after cleanup pass
>> change is done.
>> >
>> >
>> > 3. A .cpp file may be compiled and put into an archive, but then
not
>> selected by the linker and will therefore not result in a counter in
the
>> profraw. When compiling this file with pgo-use,
>> instrprof_error::unknown_function will result and a warning will be
emitted.
>> >
>> > yes -- this is a common problem to other compilers as well.
>> >
>> >
>> >
>> > Case 1 can be fixed using a function hash or other unique
identifier
>> instead of a file path. David, in D20195 you mentioned that Rong was
>> working on a patch that would fix 2; we are looking forward to that.
>> >
>> >
>> > Right.
>> >
>> > For 3, I unfortunately do not know of any solution. I don't
think there
>> is a way for us to make this warning reliable in the face of this
>> circumstance. So my conclusion is that
instrprof_error::unknown_function at
>> least must be defaulted to off unfortunately.
>> >
>> > yes, this can be annoying. If the warnings can be buffered, then
the
>> compiler can check if this is due to missing profile for the whole file
and
>> can reduce the warnings into one single warning (source file has no
profile
>> data).  Making it off by default sounds fine to me too if it is too
noisy.
>> >
>> > thanks,
>> >
>> > David
>> >
>> >
>> > -- Sean Silva
>>
>>
>
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> On May 24, 2016, at 5:21 PM, Sean Silva <chisophugis at gmail.com>
wrote:
> 
> 
> 
> On Tue, May 24, 2016 at 3:41 PM, Vedant Kumar <vsk at apple.com
<mailto:vsk at apple.com>> wrote:
> 
> > On May 23, 2016, at 8:56 PM, Xinliang David Li <davidxl at
google.com <mailto:davidxl at google.com>> wrote:
> >
> > On Mon, May 23, 2016 at 8:23 PM, Sean Silva <chisophugis at
gmail.com <mailto:chisophugis at gmail.com>> wrote:
> > Jake and I have been integrating IRPGO on PS4, and we've
identified 3 remaining work items.
> >
> > Sean, thanks for the write up. It matches very well with what we think
as well.
> 
> + 1
> 
> 
> > - Driver changes
> >
> > We'd like to make IRPGO the default on PS4. We also think that it
would be beneficial to make IRPGO the default PGO on all platforms (coverage
would continue to use FE instr as it does currently, of course). In previous
conversations (e.g. http://reviews.llvm.org/D15829
<http://reviews.llvm.org/D15829>) it has come up that Apple have
requirements that would prevent them from moving to IRPGO as the default PGO, at
least without a deprecation period of one or two releases.
> 
> Sean pointed out the problematic scenario in D15829 (in plan
"C"):
> 
> ```
> All existing user workflows continue to work, except for workflows that
attempt to llvm-profdata merge some old frontend profile data (e.g. they have
checked-in to version control and represents some special workload) with the
profile data from new binaries.
> ```
> 
> We can address this issue by (1) making sure llvm-profdata emits a helpful
warning when merging an FE-based profile with an IR-based one, and (2) keeping
an option to use FE instrumentation for PGO. Having (2) helps people who
can't (or don't want) to switch to IR PGO.
> 
> 
> > I'd like to get consensus on a path forward.
> > As a point of discussion, how about we make IRPGO the default on all
platforms except Apple platforms.
> 
> I'd really rather not introduce this inconsistency. I'm worried
that it might lead to Darwin becoming a second-tier platform for PGO.
> 
> Fred (CC'd) is following up with some of our internal users to check if
we can change the default behavior of -fprofile-instr-generate. He should be
able to chime in on this soon.
Sorry it took me so long. I’ve discussed the change in behavior quiet
extensively, and I after having changed my mind a couple times, I would argue in
favor of keeping the current behavior for the existing flags. I think adding a
new switch for IRPGO is a better option. The argument that weighted most on my
opinion is the proposed interaction with -fcoverage-mapping, and it is not at
all platform specific. With the proposed new behavior, turning coverage on and
off in your build system will generate a binary with different performance
characteristics and this feels really wrong. I would actually make the IRPGO
mode completely incompatible with the -fcoverage-mapping flag.

Fred
>  
> 
> At its core I don't think -fprofile-instr-generate *implies* FE-based
instrumentation. So, I'd like to see the driver do this (on all platforms):
> 
>   * -fprofile-instr-generate: IR instrumentation
>   * -fprofile-instr-generate=IR: IR instrumentation
>   * -fprofile-instr-generate=FE: FE instrumentation
>   * -fprofile-instr-generate -fcoverage-mapping: FE + coverage
instrumentation
> 
> It's a bit ugly because the meaning of -fprofile-instr-generate becomes
context-sensitive. But, (1) it doesn't break existing common workflows and
(2) it makes it easier to ship IRPGO. The big caveat here is that we'll need
to wait a bit and see if our internal users are OK with this.
> 
> Is there a reason to even have the possibility for FEPGO in the long run?
From what I can tell, at most we would add a
-fuse-the-old-pgo-because-i-want-to-merge-with-old-profiles option to hold
people over until they can regenerate their profiles with the current compiler.
We can add a flag to control what pre-instrumentation is done to retain the
source-level robustness of FEPGO (e.g. -fpgo-no-simplify-before-instrumenting or
something).
> 
> 
> One alternative is to introduce a separate driver flag for IRPGO. This
might not work well for Sony's existing users. I'd be interested in any
feedback about this approach.
> 
> Personally, I would prefer to maintaining command line compatibility for
PGO in Clang (i.e. users don't have to modify their build systems).
> 
> 
> -- Sean Silva
>  
> 
> 
> > I really don't like fragmenting things like this (e.g. if a
third-party tests "clang's" PGO they will get something different
depending on the platform), but I don't see another way given Apple's
constraints.
> >
> > I'd like to see IRPGO to be the default as well, but the first
thing we need is a driver level option to make the switch (prof-gen) --
currently we rely on -Xclang option to switch between two modes, which is less
than ideal.
> >
> > If the concern from Apple is that the old profile still need to work,
then this is problem already solved. The reason is that -fprofile-instr-use can
automatically detect the type of the profile and switch the mode.
> 
> It's not just that. As Sean pointed out, we're concerned about old
profiles inter-operating poorly with new ones.
> 
> thanks,
> vedant
> 
> 
> > - Pre-instrumentation passes
> >
> > Pre-instrumentation optimization has been critical for reducing the
overhead of PGO for the PS4 games we tested (as expected). However, in our
measurements (and we are glad to provide more info) the main benefit was
inlining (also as expected). A simple pass of inlining at threshold 100 appeared
to give all the benefits. Even inlining at threshold 0 gave almost all the
benefits. For example, the passes initially proposed in
http://reviews.llvm.org/D15828did <http://reviews.llvm.org/D15828did> not
improve over just inlining with threshold 100.
> >
> > (due to PR27299 we also need to add simplifycfg after inlining to
clean up, but this doesn't affect the instrumentation overhead in our
measurements)
> >
> > Bottom line: for our use cases, inlining does all the work, but
we're not opposed to having more passes, which might be beneficial for
non-game workloads (which is most code).
> >
> >
> >
> > Yes, Rong is re-collecting performance data before submitting the
patch.
> >
> > - Warnings
> >
> > We identified 3 classes of issues which manifest as spammy warnings
when applying profile data with IRPGO (these affect FEPGO also I believe, but we
looked in depth at IRPGO):
> >
> > 1. The main concerning one is that getPGOFuncName mangles the filename
into the counter name. This causes us to get instrprof_error::unknown_function
when the pgo-use build is done in a different build directory from the training
build (which is a reasonable thing to support). In this situation, PGO data is
useless for all `static` functions (and as a byproduct results in a huge volume
of warnings).
> >
> > This can be enhanced with an user option to override the behavior. Can
you help filing a tracking bug?
> >
> >
> > 2. In different TU's, pre-instr inlining might make different
inlining decisions (for example, different functions may be available for
inlining), causing hash mismatch errors (instrprof_error::hash_mismatch). In
building a large game, we only saw 8 instance of this, so it is not as severe as
1, but would be good to fix.
> >
> >
> > Rong has a patch addressing that -- will submit after cleanup pass
change is done.
> >
> >
> > 3. A .cpp file may be compiled and put into an archive, but then not
selected by the linker and will therefore not result in a counter in the
profraw. When compiling this file with pgo-use,
instrprof_error::unknown_function will result and a warning will be emitted.
> >
> > yes -- this is a common problem to other compilers as well.
> >
> >
> >
> > Case 1 can be fixed using a function hash or other unique identifier
instead of a file path. David, in D20195 you mentioned that Rong was working on
a patch that would fix 2; we are looking forward to that.
> >
> >
> > Right.
> >
> > For 3, I unfortunately do not know of any solution. I don't think
there is a way for us to make this warning reliable in the face of this
circumstance. So my conclusion is that instrprof_error::unknown_function at
least must be defaulted to off unfortunately.
> >
> > yes, this can be annoying. If the warnings can be buffered, then the
compiler can check if this is due to missing profile for the whole file and can
reduce the warnings into one single warning (source file has no profile data). 
Making it off by default sounds fine to me too if it is too noisy.
> >
> > thanks,
> >
> > David
> >
> >
> > -- Sean Silva
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