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

Tue Jan 22 21:17:36 PST 2019

On Tue, Jan 22, 2019 at 5:51 PM Teresa Johnson <tejohnson at google.com> wrote:

>
>
> On Tue, Jan 22, 2019 at 5:03 PM David Blaikie <dblaikie at gmail.com> wrote:
>
>>
>>
>> On Tue, Jan 22, 2019 at 4:10 PM Rong Xu <xur at google.com> wrote:
>>
>>> On Tue, Jan 22, 2019 at 2:44 PM David Blaikie <dblaikie at gmail.com>
>>> wrote:
>>>
>>>>
>>>>
>>>> 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.
>>>>
>>>> For Google applications, we use distributed thinlto build. The patch
>>> won't affect them as the IR files (with .o suffix) is passed the post LTO
>>> compilation. This patch will treat the .o as the source files.
>>>
>>
> This is not correct. The source file name (the module id when we compile
> from cc to bitcode .o) is encoded in the bitcode file:
> https://llvm.org/docs/LangRef.html#source-filename
>
> When the .o bitcode file is processed through the LTO backend, regardless
> of whether it is in process or distributed, that will be saved in the
> sourceFileName field on the module, which is what is being accessed here.
> So it will always be the actual source .cc file name - the same as the
> module id in the .cc->.o compile step, which is why this change is a no-op
> for existing PGO which is done during the compile step.
>
> We are talking different things: Teresa is talking the callers to this
function for indirectly promotion or importing. In these callers, the
parameter of InLTO is set. So we will use the metadata.
What I was referring to is in the context sensitive patch, where the caller
to this function does not set InLTO flag.
The reason for this is many functions are internaliazed after
PGOInstrumentaiton pass (where the metadata is set).
When we call from there, we will use the IR module name (.o) as the source
name. If you check the profile for context sensitive profile, there are
many entries like bar.o:foo()

> Teresa
>
>
>>>
>>>> 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.
>>>>
>>>> This only affects the compilation through linker plugin. Without this
>>> patch, if we call that function after LTO linking, all the static function
>>> will have a mismatch. As the profile-use and profile-gen have different
>>> module name (with a unique temporary string in the file name). This is much
>>> worse than the collision.
>>>
>>
>> Ah, OK - so what's the source name if the input to the linker is an IR
>> file (with .o suffix) as would usually be the case (but with linker plugin
>> rather than distributed thinlto) - is it still the .o file name passed to
>> the linekr? or is it now the user source code file name? If it's still the
>> .o file name, that'd still avoid a collision, I think?
>>
>> - Dave
>>
>>
>>>
>>>
>>>> - 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);
>>>>>>>    }
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> llvm-commits mailing list
>>>>>>> llvm-commits at lists.llvm.org
>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>>>>>>
>>>>>>
>>>>>>
>>>>>> --
>>>>>> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>>>>>>
>>>>>
>
> --
> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>
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