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

[Xinliang David Li via llvm-commits]

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 :)

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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