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

[Xinliang David Li via llvm-commits]

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 :)

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.  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.



>
> 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/
> msg/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.

thanks,

David




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> https://reviews.llvm.org/D29512
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