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

Wed Feb 8 11:43:41 PST 2017

On Tue, Feb 7, 2017 at 6:55 PM, Sean Silva <chisophugis at gmail.com> wrote:

>
>
> On Mon, Feb 6, 2017 at 10:27 AM, Rong Xu <xur at google.com> wrote:
>
>>
>>
>> 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-prefi
>>>>> x.
>>>>>
>>>>>
>>>>>> 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?
>>>
>>
>> Another disadvantage is this is less tolerable to source changes: if the
>> use adds another exported symbol in the use-compilation (which has nothing
>> to do with another of the existing functions), the suddenly voids all the
>> static function's profile.  This is not happening in current path name
>> based scheme.
>>
>
> You are arguing that this is a correctness issue, and I gave an example
> where the path based scheme is incorrect. Conservatively invalidating
> profile info seems preferable, no?
>
>
I'm not sure what you are referring to here. If it is the option of
-static-func-full-module-prefix, yes, this is a correctness issue. This
generates bad code in instrumentation compilation that seg-fault the binary
If you were talking the hash-based uniquely ID for static function, I don't
mean  this is a correction issue.
On the other hand, I also don't think path based scheme is of correctness
issues either (refers to your "incorrect").

For both method, the worst case will be drop of the profiles.

Also let's look at your example above, function PreprocessHelper has 2
implementations under MACRO USE_AVX, and I assume you mean profile-use and
profile-generate have different code path because of this.
Is't this the same as the user changes the source after profile-generate?
This applies to non-static function also.
We meant to use function hash to catch this case. IMO, this has little to
do with static function naming.

Also what if there are no exported symbols of foo_avx() and foo_noavx(). It
will generate the same ID in hash-based method.

> -- Sean Silva
>
>
>>
>>
>>
>>>
>>> David
>>>
>>>
>>>> -- Sean Silva
>>>>
>>>>
>>>>>
>>>>> thanks,
>>>>>
>>>>> David
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> https://reviews.llvm.org/D29512
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
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