[llvm-dev] The state of IRPGO (3 remaining work items)

Sean Silva via llvm-dev llvm-dev at lists.llvm.org
Thu Jun 2 00:10:45 PDT 2016

On Wed, Jun 1, 2016 at 5:46 PM, Frédéric Riss <friss at apple.com> wrote:

> On Jun 1, 2016, at 1:46 PM, Sean Silva <chisophugis at gmail.com> wrote:
> On Tue, May 31, 2016 at 6:02 PM, Frédéric Riss <friss at apple.com> wrote:
>> 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> 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.
>> Sorry it took me so long.
> Hi Fred,
> My understanding is that you were specifically investigating whether Apple
> needed compatibility for merging indexed profiles. Is that compatibility
> needed? The only compelling argument I have heard to continue to expose
> FEPGO is that Apple may have a compatibility requirement for merging
> indexed profiles from previous compiler versions.
> Sorry no, my comment had nothing to do with merging profiles. I understand
> that this will break, and it might very well be an issue for us, but I
> think there is a more fundamental issue with the proposed plan. As you
> bring it up though, this is a user visible breakage that shouldn’t be
> disregarded completely.

Merging with existing indexed profiles is the only user-visible breakage
AFAIK (this was discussed at length in http://reviews.llvm.org/D15829 and
the corresponding email thread). Please provide concrete examples where
things would break.

> Even if this is a requirement, then I still intend to make IRPGO the
> default and only PGO going forward, at least on PS4. I think that doing the
> same for all platforms in the upstream compiler probably makes sense as
> well, since an internal Apple vendor compatibility requirement should not
> penalize all users of the open source project.
> Again, I’m not expressing an Apple requirement, just trying to discuss the
> specifics of the proposed implementation. My goal is not to hinder
> anything, and I want our platforms to be able to use IRPGO reliably if
> users see the need for it.

What I'm saying is that besides reduced training overhead (and the
inability to merge with older indexed profiles, which AFAIK is the only
actual potential requirement that would need a deprecation period for
FEPGO), IRPGO is basically "just a better PGO", so adding a frontend one
(except as something purely during a deprecation period) is
pointless. "just a better PGO" is what IRPGO is for my users. I don't want
to have to have them deal with (and I don't want to support) FEPGO.

Anything that will cause the existing flag to continue to produce FEPGO on
PS4 is not something that I'm really okay with. The reduced overhead of
IRPGO is really important on PS4 (i.e. the difference between the
instrumented game being playable or not). I really don't want to have to
test the triple to determine the meaning of `-fprofile-instr-generate`
(without `-fcoverage-mapping`).

> 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.
> Bob already mentioned in the other thread that `-fprofile-instr-generate
> -fcoverage-mapping` was sufficiently different from
> `-fprofile-instr-generate` that `-fprofile-instr-generate
> -fcoverage-mapping` was not an acceptable workaround that could be used for
> enabling FEPGO during a transitionary period, so I'm not convinced that
> your argument here makes sense.
> I’m not sure what you’re referring to here, and I have a hard time parsing
> the sentence. I suppose “was not an acceptable” should read “was an
> acceptable”? I would be surprised that Bob ever agreed to completely
> transition away from FEPGO. I didn’t even understand that getting rid of
> FEPGO was on table as you seem to imply bellow.

No, it is written as intended. The backstory is in
http://reviews.llvm.org/D15829 (and the corresponding email thread). The
paragraph starting with "The coverage mapping adds considerable cost.".

> I also share David's opinion that this is not going to be an issue in
> practice. I think it makes sense for PGO and coverage to have different
> overheads. Coverage inherently has to trace all locations at source level,
> while PGO has more freedom.
> I’m sorry if I wasn’t clear, but I’m not talking about instrumentation
> overhead, I’m talking about the performance of the binary generated using
> the profiles. If we go the route of making the meaning of
> -fprofile-instr-generate depend on whether -fcoverage-mapping gets passed,
> then we change the kind of instrumentation and thus the input to the
> optimizations behind the user’s back. I wouldn’t be surprised that using
> profiles generated by FEPGO and IRPGO give you a final executable with
> measurably different performance characteristics.

I think the point is that given the effort being put into IRPGO, the IRPGO
version will always be a faster final executable. Why provide a "worse" PGO

> If you’re tracking your performance, this can be really painful. Recently
> we wasted days investigating performance regressions that were due to buggy
> profiles. I strongly believe having an option seemingly unrelated to PGO
> change this behavior is wrong and can cause actual pain for our end users.

After a deprecation period we can force `-fprofile-instr-generate` and
`-fcoverage-mapping` to be mutually exclusive if necessary. Does this solve
your problem?

Actually, I think it makes a lot of sense in some respects for
`-fprofile-instr-generate` and `-fprofile-instr-generate
-fcoverage-mapping` to be IRPGO and FEPGO/coverage. The difference from a
user's perspective is basically "is the instrumentation inserted by the
compiler constrained to have source-level coverage, or does the compiler
not have this restriction". Although as I've said, I'm not a fan of
supporting FEPGO in the long-term due to maintenance issues.

Also note that things like the context-sensitivity obtained through
pre-inlining (see Rong's original RFC) is simply not obtainable within a
source-level instrumentation paradigm (even if we did something like the
counter fusion discussed in "[llvm-dev] RFC: Pass to prune redundant
profiling instrumentation" to reduce the overhead to that of IRPGO with
pre-inlining). Thus FEPGO a.k.a. "coverage-level PGO" would nonetheless be
at an inherent disadvantage.

> Also, David's point about redundant work on FEPGO is a good one. We don't
> want to continue maintaining two different PGO’s.
> Are you implying that LLVM should drop FEPGO? It’s a totally sensible
> thing to do to use your tests as training data for your profile generation.
> It’s also a very valid thing to do to use your tests to do coverage. Xcode
> does both of these things. I would see it a a big regression to not support
> doing both at the same time (this would mean doubling compile+testing time
> for users of both).

As David pointed out, training runs for PGO and coverage have different
goals. I'm very skeptical of any testing that tries to do both at the same
time, but this will continue to work (albeit without benefitting from any
of the effort being put into IRPGO).

> As the instrumentation needs to stay there for coverage anyway, I expect
> FEPGO to stay there and maintained (we care a lot about coverage). I’m not
> saying that all the work going into IRPGO should be duplicated in FEPGO,
> but what’s there and working should keep working.

For my users the reduced overhead of IRPGO is an important feature, and
making it the default is important for that reason. Since most of the
effort going into PGO is focused on IRPGO, this will lead to users using
FEPGO ending up as a second-tier PGO, which Vedant said he specifically
wanted to avoid. The only option to avoid this is for users to not be using

> Also, FEPGO has a lot of nice characteristics like resilience to IRGEN
> changes. If you have archived profiles, then when you switch compilers your
> performance shouldn’t degrade with FEPGO (modulo optimization bugs), while
> it’s much more likely to degrade with IRPGO.

Note that this use case continues to work. I.e. we continue to apply
existing frontend profiles correctly. including frontend profiles generated
with -fcoverage-mapping, so that collecting coverage and PGO at the same
time (although not advisable) still works. The only use case that breaks is
merging existing indexed profiles, which is why we are specifically waiting
for an answer on whether this is a requirement for you guys at Apple, which
will determine what kind of deprecation period etc. will be needed before
we can default it.

> It overall looks like a much better option for people who do not need the
> lower instrumentation overhead.

This is not just about lower instrumentation overhead. Things like the
recently added static VP node allocation (which will e.g. make indirect
callsite promotion for LTO'd kernels work) are other things are being
missed out on.

> I would actually make the IRPGO mode completely incompatible with the
>> -fcoverage-mapping flag.
> I'm not sure what you mean by this. Nobody is proposing anything that
> would make -fcoverage-mapping do anything related to IRPGO.
> What I mean is that -f<whatever enables IRPGO> should error out when
> passed at the same time as -fcoverage-mapping.

I think you're coming into this with the mindset that FEPGO will still be a
possibility (outside of a build that is used for coverage mapping). I'm not
convinced that we actually need to continue exposing that except as a weird
thing in conjunction with coverage (and possibly for a deprecation period
if users want to merge indexed profiles).

-- Sean Silva

> Thanks!
> Fred
> -- Sean Silva
>> 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 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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