[PATCH] D17654: [PGO] Remove duplicate profile entries for functions with available_externally linkage

Fri Feb 26 14:35:03 PST 2016

Rightio - thanks for walking me through it. Think I'm following/understand.
Though the long comment you're adding confuses me in a few ways, but I'll
write that off as lack of familiarity with this code & leave you to it.

(Guess there's no sane way to emit the counter code such that it just
no-ops if the counter isn't linked in from the external def - that'd seem
ideal, but I can't think of any linker/linkage tricks that would produce
that behavior)

(please consider my issues closed & carry on with review from anyone
else/more domain-specific reviewers)

On Fri, Feb 26, 2016 at 2:22 PM, Xinliang David Li <davidxl at google.com>
wrote:

>
>
> On Fri, Feb 26, 2016 at 1:57 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>>
>>
>> On Fri, Feb 26, 2016 at 1:41 PM, Xinliang David Li <davidxl at google.com>
>> wrote:
>>
>>>
>>>
>>> On Fri, Feb 26, 2016 at 1:39 PM, Xinliang David Li <davidxl at google.com>
>>> wrote:
>>>
>>>>
>>>>
>>>> On Fri, Feb 26, 2016 at 1:36 PM, David Blaikie <dblaikie at gmail.com>
>>>> wrote:
>>>>
>>>>>
>>>>>
>>>>> On Fri, Feb 26, 2016 at 1:33 PM, Xinliang David Li <davidxl at google.com
>>>>> > wrote:
>>>>>
>>>>>>
>>>>>>
>>>>>> On Fri, Feb 26, 2016 at 1:27 PM, David Blaikie <dblaikie at gmail.com>
>>>>>> wrote:
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Fri, Feb 26, 2016 at 1:18 PM, Xinliang David Li <
>>>>>>> davidxl at google.com> wrote:
>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> On Fri, Feb 26, 2016 at 1:07 PM, David Blaikie <dblaikie at gmail.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On Fri, Feb 26, 2016 at 12:32 PM, David Li via llvm-commits <
>>>>>>>>> llvm-commits at lists.llvm.org> wrote:
>>>>>>>>>
>>>>>>>>>> davidxl created this revision.
>>>>>>>>>> davidxl added a reviewer: vsk.
>>>>>>>>>> davidxl added a subscriber: llvm-commits.
>>>>>>>>>>
>>>>>>>>>> The per function data and counter variables for available
>>>>>>>>>> externally functions are created with linkonce linkage (to prevent compiler
>>>>>>>>>> from dropping counter vars). However on ELF based systems, linkonce symbols
>>>>>>>>>> are created as weak symbols and the duplicated entries are not removed by
>>>>>>>>>> the linker.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> That ^ seems surprising/strange/confusing/problematic - any idea
>>>>>>>>> what's going on there? Is that a bug in the compiler? linker? specification
>>>>>>>>> of some kind?
>>>>>>>>>
>>>>>>>>
>>>>>>>> I am not sure what you questions is. Duplicate definitions of weak
>>>>>>>> symbols are allowed so linker won't discard the duplicate copies. The
>>>>>>>> symbol reference will be resolved the strong definition by the linker.
>>>>>>>>
>>>>>>>
>>>>>>> Generally, I'm trying to understand why the counter is handled
>>>>>>> differently from the function itself.
>>>>>>>
>>>>>>> The comment mentions "To avoid link errors, profile counters for
>>>>>>> function with available_externally linkage needs to be changed to
>>>>>>> linkonce linkage. "
>>>>>>> Why is that? Can we not rely on the counter to be emitted where the
>>>>>>> function is emitted? (where the external definition of the function is)
>>>>>>>
>>>>>>>
>>>>>>
>>>>>> This is a good question. This is a design choice made long ago. For
>>>>>> function with regular external linkage, the profile counter's linkage is
>>>>>> set to 'internal', possibly to reduce the size overhead (by eliminating
>>>>>> symbol table entries). Because of this, the available_external function's
>>>>>> profile counter will no longer have a backup definition to be resolved to
>>>>>> so their counter's linkage gets changed to linkonce.
>>>>>>
>>>>>
>>>>> It seems like the compiler's in control of available_externally
>>>>> definitions, would it be possible that it would be in control of not
>>>>> internalizing counters it knows it may use available_externally elsewhere?
>>>>> (ie: when emitting the external definition of a thing that the compiler may
>>>>> emit available_externally definitions for, do not internalize the counter?)
>>>>>
>>>>
>>>> I am not aware of a way to check this condition -- if there is one I
>>>> would certainly like to use it :)
>>>>
>>>
>> I believe it's knowable - there may not be a single function that works
>> as a perfect predicate for this now, but all I'm saying is: we/the compiler
>> knows which entities we emit as available_externally, so we can know this
>> when emitting the external definition too.
>>
>
> I guess FE needs to pass that down to IR in some way.
>
>
>>
>>>
>>> Actually there is another complicacy here -- user may decide not to
>>> instrument the module with the strong definition -- thus this can not
>>> really be depended upon.
>>>
>>
>> Do we support this, generally? (what do we deal with inline functions and
>> their associated counter?)
>>
>
> Yes, we do support this. We have customers with legit reason that some
> modules can not be instrumented. What is lost is only the counter values
> for the out of line copy.
>
> A more general case is actually that the available_externally function is
> strongly defined in a standard library (libc++.so) for instance.  The
> example I mentioned in the patch is such a case.
>
> David
>
>>
>>
>>>
>>> David
>>>
>>>
>>>>
>>>> David
>>>>
>>>>
>>>>
>>>>
>>>>>
>>>>>
>>>>>>
>>>>>> David
>>>>>>
>>>>>>
>>>>>>
>>>>>>>
>>>>>>>> David
>>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>> For bad consequences it causes, see comments in the code. One
>>>>>>>>>> example, the profile counts for _ZNKSs7_M_dataEv method is duplicated 655
>>>>>>>>>> times in raw profile data of clang. In the merged indexed profile, the
>>>>>>>>>> counter values are magnified 655x -- totally dwarfed other functions --
>>>>>>>>>> this also distorted profile summary a lot leading to not useful profile
>>>>>>>>>> data.
>>>>>>>>>>
>>>>>>>>>> http://reviews.llvm.org/D17654
>>>>>>>>>>
>>>>>>>>>> Files:
>>>>>>>>>>   lib/Transforms/Instrumentation/InstrProfiling.cpp
>>>>>>>>>>   test/Instrumentation/InstrProfiling/linkage.ll
>>>>>>>>>>
>>>>>>>>>> Index: test/Instrumentation/InstrProfiling/linkage.ll
>>>>>>>>>>
>>>>>>>>>> ===================================================================
>>>>>>>>>> --- test/Instrumentation/InstrProfiling/linkage.ll
>>>>>>>>>> +++ test/Instrumentation/InstrProfiling/linkage.ll
>>>>>>>>>> @@ -7,6 +7,7 @@
>>>>>>>>>>  @__profn_foo_weak = weak hidden constant [8 x i8] c"foo_weak"
>>>>>>>>>>  @"__profn_linkage.ll:foo_internal" = internal constant [23 x i8]
>>>>>>>>>> c"linkage.ll:foo_internal"
>>>>>>>>>>  @__profn_foo_inline = linkonce_odr hidden constant [10 x i8]
>>>>>>>>>> c"foo_inline"
>>>>>>>>>> + at __profn_foo_extern = linkonce_odr hidden constant [10 x i8]
>>>>>>>>>> c"foo_extern"
>>>>>>>>>>
>>>>>>>>>>  ; COMMON: @__profc_foo = hidden global
>>>>>>>>>>  ; COMMON: @__profd_foo = hidden global
>>>>>>>>>> @@ -36,6 +37,15 @@
>>>>>>>>>>    ret void
>>>>>>>>>>  }
>>>>>>>>>>
>>>>>>>>>> +; LINUX: @__profc_foo_extern = linkonce_odr hidden global
>>>>>>>>>> {{.*}}section "__llvm_prf_cnts", comdat($__profv_foo_extern), align 8
>>>>>>>>>> +; LINUX: @__profd_foo_extern = linkonce_odr hidden global
>>>>>>>>>> {{.*}}section "__llvm_prf_data", comdat($__profv_foo_extern), align 8
>>>>>>>>>> +; OTHER: @__profc_foo_extern = linkonce_odr hidden global
>>>>>>>>>> +; OTHER: @__profd_foo_extern = linkonce_odr hidden global
>>>>>>>>>> +define available_externally void @foo_extern() {
>>>>>>>>>> +  call void @llvm.instrprof.increment(i8* getelementptr inbounds
>>>>>>>>>> ([10 x i8], [10 x i8]* @__profn_foo_extern, i32 0, i32 0), i64 0, i32 1,
>>>>>>>>>> i32 0)
>>>>>>>>>> +  ret void
>>>>>>>>>> +}
>>>>>>>>>> +
>>>>>>>>>>  declare void @llvm.instrprof.increment(i8*, i64, i32, i32)
>>>>>>>>>>
>>>>>>>>>>  ; OTHER: @__llvm_profile_runtime = external global i32
>>>>>>>>>> Index: lib/Transforms/Instrumentation/InstrProfiling.cpp
>>>>>>>>>>
>>>>>>>>>> ===================================================================
>>>>>>>>>> --- lib/Transforms/Instrumentation/InstrProfiling.cpp
>>>>>>>>>> +++ lib/Transforms/Instrumentation/InstrProfiling.cpp
>>>>>>>>>> @@ -286,8 +286,38 @@
>>>>>>>>>>    return F->hasAddressTaken();
>>>>>>>>>>  }
>>>>>>>>>>
>>>>>>>>>> -static inline Comdat *getOrCreateProfileComdat(Module &M,
>>>>>>>>>> +static inline bool needsComdatForCounter(Function &F, Module &M)
>>>>>>>>>> {
>>>>>>>>>> +
>>>>>>>>>> +  if (F.hasComdat())
>>>>>>>>>> +    return true;
>>>>>>>>>> +
>>>>>>>>>> +  Triple TT(M.getTargetTriple());
>>>>>>>>>> +  if (!TT.isOSBinFormatELF())
>>>>>>>>>> +    return false;
>>>>>>>>>> +
>>>>>>>>>> +  // See createPGOFuncNameVar for more details. To avoid link
>>>>>>>>>> errors, profile
>>>>>>>>>> +  // counters for function with available_externally linkage
>>>>>>>>>> needs to be changed
>>>>>>>>>> +  // to linkonce linkage. On ELF based systems, this leads to
>>>>>>>>>> weak symbols to be
>>>>>>>>>> +  // created. Without using comdat, duplicate entries won't be
>>>>>>>>>> removed by the
>>>>>>>>>> +  // linker leading to increased data segement size and raw
>>>>>>>>>> profile size. Even
>>>>>>>>>> +  // worse, since the referenced counter from profile
>>>>>>>>>> per-function data object
>>>>>>>>>> +  // will be resolved to the common strong definition, the
>>>>>>>>>> profile counts for
>>>>>>>>>> +  // available_externally functions will end up being duplicated
>>>>>>>>>> in raw profile
>>>>>>>>>> +  // data. This can result in distorted profile as the counts of
>>>>>>>>>> those dups
>>>>>>>>>> +  // will be accumulated by the profile merger.
>>>>>>>>>> +  GlobalValue::LinkageTypes Linkage = F.getLinkage();
>>>>>>>>>> +  if (Linkage != GlobalValue::ExternalWeakLinkage &&
>>>>>>>>>> +      Linkage != GlobalValue::AvailableExternallyLinkage)
>>>>>>>>>> +    return false;
>>>>>>>>>> +
>>>>>>>>>> +  return true;
>>>>>>>>>> +}
>>>>>>>>>> +
>>>>>>>>>> +static inline Comdat *getOrCreateProfileComdat(Module &M,
>>>>>>>>>> Function &F,
>>>>>>>>>>
>>>>>>>>>> InstrProfIncrementInst *Inc) {
>>>>>>>>>> +  if (!needsComdatForCounter(F, M))
>>>>>>>>>> +    return nullptr;
>>>>>>>>>> +
>>>>>>>>>>    // COFF format requires a COMDAT section to have a key symbol
>>>>>>>>>> with the same
>>>>>>>>>>    // name. The linker targeting COFF also requires that the
>>>>>>>>>> COMDAT
>>>>>>>>>>    // a section is associated to must precede the associating
>>>>>>>>>> section. For this
>>>>>>>>>> @@ -315,8 +345,7 @@
>>>>>>>>>>    // linking.
>>>>>>>>>>    Function *Fn = Inc->getParent()->getParent();
>>>>>>>>>>    Comdat *ProfileVarsComdat = nullptr;
>>>>>>>>>> -  if (Fn->hasComdat())
>>>>>>>>>> -    ProfileVarsComdat = getOrCreateProfileComdat(*M, Inc);
>>>>>>>>>> +  ProfileVarsComdat = getOrCreateProfileComdat(*M, *Fn, Inc);
>>>>>>>>>>
>>>>>>>>>>    uint64_t NumCounters = Inc->getNumCounters()->getZExtValue();
>>>>>>>>>>    LLVMContext &Ctx = M->getContext();
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> _______________________________________________
>>>>>>>>>> llvm-commits mailing list
>>>>>>>>>> llvm-commits at lists.llvm.org
>>>>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
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