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

Fri Feb 3 23:28:07 PST 2017

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. 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).

>
> 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)

> 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.

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).

>
>
>
>>
>> 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 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?

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. That seems more robust than a path name.

-- Sean Silva

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