[llvm] r350671 - [PGO] Use SourceFileName rather module name in PGOFuncName

[David Blaikie via llvm-commits]

On Tue, Jan 22, 2019 at 2:39 PM Rong Xu <xur at google.com> wrote:

> On Tue, Jan 22, 2019 at 1:24 PM Teresa Johnson <tejohnson at google.com>
> wrote:
>
>> On Tue, Jan 22, 2019 at 1:06 PM David Blaikie <dblaikie at gmail.com> wrote:
>> >
>> > Hey Rong, Teresa,
>> >
>> > This seems like it might be problematic to me - Couldn't multiple
>> modules have the same source file name (built with different preprocessor
>> defines, etc) - at least I think that's the case for some projects at
>> Google.
>> >
>> > Does this identifier need to be unique? What are the ramifications if
>> multiple modules had the same source file name & this situation?
>>
>> Rong will know for sure, but I think the profiles will be merged and it
>> is possible there will be a profile mismatch.
>>
>> However,  note that this is what already happens for PGO - this change
>> retains the status quo since PGO is matched during the compile from source
>> where the module name (F.getParent()->getName()) is the same as
>> F.getParent()->getSourceFileName(). So any uniqueness issue already exists
>> with PGO.
>>
>> This change is to enable context sensitive PGO which must be done in the
>> *LTO backends, where we have recorded the original module name in the
>> SourceFileName in the bitcode (in the *LTO backends the module name is the
>> .o bitcode file name).
>>
>>
> Thanks for Teresa explanation and the background. I just want to add one
> more thing: we use both filename and function's CFG checksum in the static
> function's PGOFuncName. If multiple functions have the same source name,
> but with different CFG checksums, they can coexist in the profile.
> Profile-use compilation will pick the exact match one. In theory, there
> still exist some collisions, but the chance are very small.
>

So if this was the original source file name, it wouldn't be too hard to
collide, even with the CFG checksum mixed in, I think. In Google, at least,
it's not too uncommon (I've seen it a few times, at least) for a source
file to be built into multiple libraries, either by mistake or
intentionally (if it's intentional, then the builds probably use different
parameters - different #defines and the like) - and a file-local static
function in such a file might not depend on those #defines or other
parameters, so it would be the same across files - while still being
distinct functions.

But if the filename being used is the name of the intermediate .o file
rather than the original source file, well that filename has to be unique
for the linker to be able to read both .o files (even if they came from the
same source file) & so it should provide a good uniqueness.

- Dave


>
>
>> Teresa
>>
>> >
>> > - Dave
>>
>> On Tue, Jan 8, 2019 at 2:43 PM Rong Xu via llvm-commits <
>> llvm-commits at lists.llvm.org> wrote:
>>
>>> Author: xur
>>> Date: Tue Jan  8 14:39:47 2019
>>> New Revision: 350671
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=350671&view=rev
>>> Log:
>>> [PGO] Use SourceFileName rather module name in PGOFuncName
>>>
>>> In LTO or Thin-lto mode (though linker plugin), the module
>>> names are of temp file names which are different for
>>> different compilations. Using SourceFileName avoids the issue.
>>> This should not change any functionality for current PGO as
>>> all the current callers of getPGOFuncName() is before LTO.
>>>
>>> Differential Revision: https://reviews.llvm.org/D56327
>>>
>>> Modified:
>>>     llvm/trunk/lib/ProfileData/InstrProf.cpp
>>>
>>> Modified: llvm/trunk/lib/ProfileData/InstrProf.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/ProfileData/InstrProf.cpp?rev=350671&r1=350670&r2=350671&view=diff
>>>
>>> ==============================================================================
>>> --- llvm/trunk/lib/ProfileData/InstrProf.cpp (original)
>>> +++ llvm/trunk/lib/ProfileData/InstrProf.cpp Tue Jan  8 14:39:47 2019
>>> @@ -252,11 +252,12 @@ static StringRef stripDirPrefix(StringRe
>>>  // data, its original linkage must be non-internal.
>>>  std::string getPGOFuncName(const Function &F, bool InLTO, uint64_t
>>> Version) {
>>>    if (!InLTO) {
>>> -    StringRef FileName = (StaticFuncFullModulePrefix
>>> -                              ? F.getParent()->getName()
>>> -                              :
>>> sys::path::filename(F.getParent()->getName()));
>>> -    if (StaticFuncFullModulePrefix && StaticFuncStripDirNamePrefix != 0)
>>> -      FileName = stripDirPrefix(FileName, StaticFuncStripDirNamePrefix);
>>> +    StringRef FileName(F.getParent()->getSourceFileName());
>>> +    uint32_t StripLevel = StaticFuncFullModulePrefix ? 0 : (uint32_t)-1;
>>> +    if (StripLevel < StaticFuncStripDirNamePrefix)
>>> +      StripLevel = StaticFuncStripDirNamePrefix;
>>> +    if (StripLevel)
>>> +      FileName = stripDirPrefix(FileName, StripLevel);
>>>      return getPGOFuncName(F.getName(), F.getLinkage(), FileName,
>>> Version);
>>>    }
>>>
>>>
>>>
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>>>
>>
>>
>> --
>> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>>
>
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