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

[Bob Wilson via llvm-dev]

> On Jun 2, 2016, at 5:30 PM, Sean Silva <chisophugis at gmail.com> wrote:
>  
> This also means that if the consensus is that -fprofile-instr-generate should really change its meaning to mean IRPGO, I’m open to having this internal patch on our side. 
> 
> Yeah, it sounds like someone is going to have to keep a "private patch" to change the default. At that point doing a switch on the triple in upstream seems preferable though :/

I don’t see why a private patch would be needed.

> So I propose the following (which is equivalent to what you proposed, but starting to put specific option names):
> 1. Add -fprofile-instr-generate=stable and -fprofile-instr-generate=unstable

That sounds like the right approach. I don’t have a strong opinion about the =stable/unstable names.

> a) Indexed profiles for -fprofile-instr-generate=stable are guaranteed to be compatible across compiler releases (which will include existing releases). Additionally, -fprofile-instr-generate=stable can be used in combination with -fcoverage-mapping.
> b) Indexed profiles for -fprofile-instr-generate=unstable are not guaranteed to be compatible across compiler releases. It is an error to use this in conjunction with -fcoverage-mapping
> c) -fprofile-instr-generate defaults to one of the two in a way that meets vendor needs, via private patches or an upstream switch on the triple or whatever. It can eventually be deprecated or whatever.

Typically we handle things like this by staging the change over a release or two. If we add those two new options (=stable, =unstable), we can have one release where the default matches the old behavior, and then change the default for the following release. Someone relying on the current behavior would then be able to change their builds to use the =stable option so that they won’t be broken when the default switches.

> 
> -fprofile-instr-generate=stable would basically correspond to FEPGO / FE coverage instrumentation. -fprofile-instr-generate=unstable would basically correspond to IRPGO.
> 
> What do you think?
> 
> -- Sean Silva
>  
> 
> I also think a new option is cleaner (again I don’t care about the name), but if people feel that -fprofile-instr-generate=[llvm|clang] is easier, then this works for me too.
> 
> Fred
> 
>> 
>> -- 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 <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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