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

Fri Feb 26 13:33:25 PST 2016

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.

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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