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

Fri Feb 26 13:41:21 PST 2016

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 :)
>

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.

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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