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

Wed Feb 8 11:49:01 PST 2017

On Wed, Feb 8, 2017 at 11:43 AM, Rong Xu <xur at google.com> wrote:

>
>
> 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.
>

What Sean meant is that full path can not resolve conflict 100%. The use
case of the small example is like:

clang -DUSE_AVX -o test_avx.o  -c /a/b/c/test.c
clang -o test_noavx.o  -c /a/b/c/test.c

basically same source file producing two different modules.

>
> Also what if there are no exported symbols of foo_avx() and foo_noavx().
> It will generate the same ID in hash-based method.
>
>
One global symbol (data or function) needs to be defined and reference the
static function to make it live.

David

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