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

[Vedant Kumar via llvm-commits]

IMO, it is a little surprising that specifying 'linkonce' on a symbol can result in it showing up in multiple locations in the binary. I think @dblaikie's question was -- is that the expected behavior on ELF?

Caveat: I'm really not a linker expert, so I might have the wrong idea here.

vedant


> 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.
> 
> 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();
> 
> 
> 
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