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

[Sean Silva via llvm-dev]

On Wed, May 25, 2016 at 9:22 AM, Xinliang David Li <davidxl at google.com>
wrote:

> 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.
>

One question for your David, related to merging indexed profiles. The
requirement that indexed profiles from different compiler versions can be
merged (with the intended result) implies that the CFG hash for a given
function must be the same (this is the change that actually breaks merging
IRPGO vs FEPGO). But the same issue in principle applies to IRPGO across
compiler versions. How likely do you think it will be that this hash may
change? I can think of two sources:
a) we decide to walk the CFG differently for computing the hash or decide
to use a different hash function etc.
b) pre-instrumentation passes change across compiler versions (e.g. inliner
makes different decisions in a newer compiler version)

`b)` definitely seems like it will be a problem. What do you think about a)?

-- Sean Silva


>
> 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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