[PATCH] D29512: [PGO] Directory name stripping in global identifier for static functions

Tue Feb 7 19:40:28 PST 2017

On Tue, Feb 7, 2017 at 7:34 PM, Xinliang David Li <davidxl at google.com>
wrote:

>
>
> On Tue, Feb 7, 2017 at 7:25 PM, Sean Silva <chisophugis at gmail.com> wrote:
>
>>
>>
>> On Tue, Feb 7, 2017 at 7:19 PM, Xinliang David Li <davidxl at google.com>
>> wrote:
>>
>>>
>>>
>>> On Tue, Feb 7, 2017 at 6:57 PM, Sean Silva <chisophugis at gmail.com>
>>> wrote:
>>>
>>>>
>>>>
>>>> On Mon, Feb 6, 2017 at 9:54 AM, Rong Xu <xur at google.com> wrote:
>>>>
>>>>> When I wrote IRPGO for PGOFuncName, I intentionally chose to use the
>>>>> source path with directory name. This is different from the Clang PGO where
>>>>> the base name is used. I believe I mentioned this in the comment or commit
>>>>> message. This is to avoid the name clashing like this.
>>>>>
>>>>> So the change in D22028 was a regression.
>>>>>
>>>>> Surely, this will not solve all the name clashing: if the user could
>>>>> have used the same source name string (with directory) in the build. In
>>>>> this case, there still could be name clashing. But I don't think it is
>>>>> common. I haven't encountered one in the real applications. On the other
>>>>> hand, using the base name only breaks many of out applications.  In
>>>>> addition, no matter how complicate the scheme is, like Hash, there is
>>>>> still a chance for the naming clash (unless, of course, using global
>>>>> module ID in thin-lto or lto).
>>>>>
>>>>
>>>> So from what you're saying it seems like we already have a global
>>>> module ID that could be used to disambiguate static functions? Why don't we
>>>> use that?
>>>>
>>>
>>> It is using the same scheme as PGO.
>>>
>>
>> I thought ThinLTO hashed the exported symbols? (sorry, I guess I'm out of
>> the loop)
>>
>
> For function importing, the same scheme is used as PGO.  Regarding global
> Module Id, do not take my word for it :) I will let Teresa/Medhi comment on
> it.
>

We use the same name scheme as PGO as the key in the ThinLTO combined index
for importing - this is strategic so that we can enable importing of
profiled hot indirect call targets which are encoded in the profile with
the PGO name scheme.

We use a hash of the module's IR for something else, namely for creating a
unique name when promoting statics to global scope when necessary.

Thanks,
Teresa

> Thanks,
>
> David
>
>
>
>>
>>
>>>
>>>> Figuring out how many paths to strip from the source dir path seems
>>>> like a pain that we shouldn't push on users if we already have a solution.
>>>> We want PGO to be as easy to use as possible.
>>>>
>>>
>>> The idea is that most users (who do not generate source into different
>>> dirs or use relative source paths) never need to use/worry about it.
>>>
>>
>> Okay. I'll take your word on that for now.
>>
>> -- Sean Silva
>>
>>>
>>> David
>>>
>>>>
>>>> -- Sean Silva
>>>>
>>>>
>>>>> I still think using a path name is a good choice.
>>>>>
>>>>> Second, I don't think D22028's use case is common. The typical use
>>>>> case for FDO is to have the same command line options and just replacing
>>>>> -fprofile-generate with -fprofile-use. D22028 has different source string,
>>>>> I believe the user has a better position to deal with this than the
>>>>> compiler.
>>>>>
>>>>> -Rong
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On Sun, Feb 5, 2017 at 12:11 AM, Xinliang David Li <davidxl at google.com
>>>>> > wrote:
>>>>>
>>>>>>
>>>>>>
>>>>>> On Fri, Feb 3, 2017 at 11:28 PM, Sean Silva <chisophugis at gmail.com>
>>>>>> wrote:
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Fri, Feb 3, 2017 at 10:18 PM, Xinliang David Li <
>>>>>>> davidxl at google.com> wrote:
>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> On Fri, Feb 3, 2017 at 8:31 PM, Sean Silva via Phabricator <
>>>>>>>> reviews at reviews.llvm.org> wrote:
>>>>>>>>
>>>>>>>>> silvas added a comment.
>>>>>>>>>
>>>>>>>>> This change does two things (as you mention in the description):
>>>>>>>>>
>>>>>>>>> 1. Adding -static-func-strip-dirname-prefix which provides a way
>>>>>>>>> to have more control when `-static-func-full-module-prefix=true`
>>>>>>>>> is specified.
>>>>>>>>>
>>>>>>>>
>>>>>>>> This is actually a more general form of
>>>>>>>> -static-func-full-mdoule-prefix.
>>>>>>>>
>>>>>>>>
>>>>>>>>> 2. Changing the default of -static-func-full-module-prefix to
>>>>>>>>> true.
>>>>>>>>>
>>>>>>>>> IIRC, -static-func-full-module-prefix defaults to false because
>>>>>>>>> it caused issues when set to true (in fact, it was introduced to avoid
>>>>>>>>> these issues). The default value of -static-func-strip-dirname-prefix
>>>>>>>>> introduced in this patch (i.e. 0) is effectively a no-op; so ignore 1. for
>>>>>>>>> now. This means that the net effect of this patch is that compilation will,
>>>>>>>>> by default, have a regression on the issue fixed by r275193 /
>>>>>>>>> http://reviews.llvm.org/D22028, which is not a good idea. I think
>>>>>>>>> that the default behavior (which is user-visible) should not be changed in
>>>>>>>>> this patch.
>>>>>>>>>
>>>>>>>>
>>>>>>>> I disagree. The original default behavior was to preserve the full
>>>>>>>> path which was also user visible :)
>>>>>>>>
>>>>>>>
>>>>>>> And yet we found a compelling-enough use case to change it.
>>>>>>>
>>>>>>
>>>>>> It was probably better to introduce the option but not flipping the
>>>>>> default the first time. The stripping-path-fully mode had not been widely
>>>>>> tested at that time.
>>>>>>
>>>>>> I am a little curious about the use case for D22028. The pgo name of
>>>>>> static function is only affected by source module path. Why would that be
>>>>>> different for pgo-gen/use builds? In most common setup I saw, the source
>>>>>> paths should remain the same.
>>>>>>
>>>>>>
>>>>>>> We may need to revisit that decision, but clearly the current
>>>>>>> default is intentional and part of changing away from that is explaining
>>>>>>> why we no longer care about that use case (or care about it less than some
>>>>>>> other thing).
>>>>>>>
>>>>>>
>>>>>> We care about all use cases, which is why the more general form of
>>>>>> option is introduced -- it makes sure the use case in D22028 can also be
>>>>>> handled but more safely (stripping all prefix will bound to cause problems).
>>>>>>
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> The whole rationale for changing the current default is that it is
>>>>>>>> generally not safe -- mainly problem #1 because of counter variables for
>>>>>>>> static functions can not guaranteed to be unique when full path is stripped.
>>>>>>>>
>>>>>>>
>>>>>>> Can the counter variables be static to match the static nature of
>>>>>>> the functions they describe? (there would still be collisions when indexing
>>>>>>> the profile data though; the function CFG hash could be included in the
>>>>>>> "name" to avoid this)
>>>>>>>
>>>>>>
>>>>>> For simple functions, cfg hash collision is also very likely, so the
>>>>>> first line of defense is always the name key.
>>>>>>
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>> The ThinLTO issue is secondary (probably irrelevant here because of
>>>>>>>> other bugs).
>>>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> The issue addressed in D22028 is actually not common -- the source
>>>>>>>> module paths should generally match in profile-gen and profile-use phases,
>>>>>>>> so using internal option for that use case seems more reasonable to me.
>>>>>>>>
>>>>>>>
>>>>>>> This is subjective, but I think it is quite reasonable to assume
>>>>>>> that each build will use a different output directory. Hence any build that
>>>>>>> generates .cpp files into the output directory (which seems reasonable too)
>>>>>>> is susceptible.
>>>>>>>
>>>>>>
>>>>>> so the case is for generated source files? Should they be accessed
>>>>>> with relative paths?
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> Overall, requiring a user to use a compiler-internal option for
>>>>>>> something that seems to happen in practice (e.g. back when I was a
>>>>>>> PlayStation we actually ran into it and spent time fixing it) is a pretty
>>>>>>> poor experience. I think we should aim to do better (though we might settle
>>>>>>> for less if that proves challenging).
>>>>>>>
>>>>>>
>>>>>> I agree in general. However I think it is reasonable for a user to
>>>>>> use an internal option for corner use cases.  Another choice is to
>>>>>> introduce an external option for this which user can rely on.
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>> Overall, it sounds like this approach of relying on users to tweak
>>>>>>>>> internal compiler options (-mllvm) to get correct behavior in their
>>>>>>>>> environment is not the kind of user experience we want to deliver (or the
>>>>>>>>> kind of implementation that we want to maintain). IIRC, when we added
>>>>>>>>> -static-func-full-module-prefix, it was with the understanding
>>>>>>>>> that it was a simple hack for working around the larger issue of relying on
>>>>>>>>> the module name which we knew was not very robust. The further addition of
>>>>>>>>> the "InLTO" complicates things even further. It seems like a code smell
>>>>>>>>> that we do not have a Single Point Of Truth.
>>>>>>>>>
>>>>>>>>> I proposed a solution at one point https://groups.google.com/d/ms
>>>>>>>>> g/llvm-dev/s_VZbFTWbVs/d0b4Zh80CgAJ though it may no longer be
>>>>>>>>> applicable. It seems like ThinLTO already has to solve a problem of finding
>>>>>>>>> unique identifiers for all functions (even static), so we may want to
>>>>>>>>> piggy-back on that mechanism (this is just a high-level thought; haven't
>>>>>>>>> looked into the details).
>>>>>>>>>
>>>>>>>>>
>>>>>>>> For LTO/ThinLTO,  we solved the issue by using meta data which uses
>>>>>>>> getPGOFuncName as singe source of truth.
>>>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>> So:
>>>>>>>>>
>>>>>>>>> - I specifically object to changing user-visible defaults in this
>>>>>>>>> patch. Those changes should be isolated, and I don't think we have
>>>>>>>>> justification to change those defaults anyway.
>>>>>>>>>
>>>>>>>>
>>>>>>>> See my reply about the safety issue of keeping the current default.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>> - I'm slightly opposed to adding the -static-func-strip-dirname-prefix
>>>>>>>>> flag, since it seems like a workaround (among others that have already
>>>>>>>>> piled up) for a more fundamental issue. This is a frog-in-boiling-water
>>>>>>>>> situation; if solving the fundamental issue would be a huge amount of work,
>>>>>>>>> then adding the new flag is probably fine for now, but we need to keep in
>>>>>>>>> mind the larger situation. IIUC, defaulting `-static-func-strip-dirname-prefix=-1`
>>>>>>>>> would emulate the current default behavior, so
>>>>>>>>> -static-func-full-module-prefix could just be removed in the same
>>>>>>>>> patch.
>>>>>>>>>
>>>>>>>>
>>>>>>>> The current -static-func-full-module-path=false is simply a
>>>>>>>> special case of the new option. For users who rely on this option may hit
>>>>>>>> the correctness issue, they won't have any fallback without the new option.
>>>>>>>>
>>>>>>>>
>>>>>>>>> - I would encourage brainstorming/discussion of alternative
>>>>>>>>> solutions that solve the fundamental problem (which seems to be more about
>>>>>>>>> having a stable globally unique identifier than being specifically about
>>>>>>>>> preserving/mangling the "name" per se).
>>>>>>>>>
>>>>>>>>
>>>>>>>> The problem itself is simple: handle name conflicts between
>>>>>>>>
>>>>>>>> /a/b/c/foo.c:static_func
>>>>>>>> /e/f/g/foo.c:static_func
>>>>>>>>
>>>>>>>> Path info is a natural choice. Note that FE instrumentation also
>>>>>>>> uses module path to uniquely identify static_func as well.
>>>>>>>>
>>>>>>>
>>>>>>> Yes, the problem was inherited from FE instrumentation. I remember
>>>>>>> that when I explained to Justin the issue, he said that it was clearly
>>>>>>> buggy and not intentional (an oversight when implementing FEPGO).
>>>>>>>
>>>>>>
>>>>>> It works very well in practice -- though it is not guaranteed to be
>>>>>> 100% free of conflict. I won't label it as buggy.
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> It seems that the fundamental issue is coming up with a unique
>>>>>>> identifier for the current TU that is stable across compiler invocations.
>>>>>>> How do other compilers handle this?
>>>>>>>
>>>>>>> GCC does not suffer from the problem because it does not dump
>>>>>> profile into one file but one profile file per module. The profile data
>>>>>> file tree structure mirrors the build output file structure so there will
>>>>>> be problem if profile-gen and use do not share the same structure. Runtime
>>>>>> options are provided to strip prefixes from output directories. Compiler
>>>>>> time option is also provided to relocate profile data (e.g. pointing to
>>>>>> different root).
>>>>>>
>>>>>>
>>>>>>
>>>>>>> For example, path names are not enough. E.g. a user may build
>>>>>>> /a/b/c/foo.c with two different sets of compiler options, yet static
>>>>>>> functions of the same name must still be treated as separate. A file like:
>>>>>>>
>>>>>>> foo.c:
>>>>>>>
>>>>>>> static void PreprocessHelper(/* something */) {
>>>>>>> #ifdef USE_AVX
>>>>>>>   // something
>>>>>>> #else
>>>>>>>   // something else
>>>>>>> #endif
>>>>>>> }
>>>>>>>
>>>>>>> #ifdef USE_AVX
>>>>>>> void foo_avx(/* something */) {
>>>>>>> #else
>>>>>>> void foo_noavx(/* something */) {
>>>>>>> #endif
>>>>>>>   PreprocessHelper(/* something */);
>>>>>>>   // something
>>>>>>> }
>>>>>>>
>>>>>>>
>>>>>>> IIRC, one option (suggested by pcc if I remember correctly) is to
>>>>>>> use a hash of the TU's exported symbols (or something like that) to
>>>>>>> uniquely identify the TU.
>>>>>>>
>>>>>>
>>>>>> Should be limited to user defined public symbols.
>>>>>>
>>>>>>
>>>>>>> That seems more robust than a path name.
>>>>>>>
>>>>>>
>>>>>> I believe so.
>>>>>>
>>>>>> Content based ID has its advantage but has disadvantages too. For
>>>>>> instance more expensive to compute, less readable names. Using path based
>>>>>> naming, we can immediately identify where the static function is defined.
>>>>>> Perhaps we can use base name plus content hash.
>>>>>>
>>>>>> Teresa, Rong, do you see a situation when  module ID needs to be
>>>>>> identified but it is difficult or  too expensive to access the module's
>>>>>> content?
>>>>>>
>>>>>> David
>>>>>>
>>>>>>
>>>>>>> -- Sean Silva
>>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> thanks,
>>>>>>>>
>>>>>>>> David
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> https://reviews.llvm.org/D29512
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>

-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |  408-460-2413
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