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

Fri Feb 26 13:57:36 PST 2016

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.

>
> 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?)

>
> 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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