"Reland "[pgo] Avoid introducing relocations by using private alias"

[paulkirth]

Summary

In many cases, we can use an alias to avoid a symbolic relocations,
instead of using the public, interposable symbol. When the instrumented
function is in a COMDAT, we can use a hidden alias, and still avoid
references to discarded sections.

Previous versions of this patch allowed the compiler to name the
generated alias, but that would only be valid when the functions were
local. Since the alias may be used across TUs we use a more
deterministic naming convention, and add a .local suffix to the alias
name just as we do for relative vtables aliases.

This should be safe to land after an incorrect LLD assertion was removed
in https://reviews.llvm.org/rG20894a478da224bdd69c91a22a5175b28bc08ed9
which caused assertion failures in LLD on Mac.

[paulkirth]

Missed readding the original review tag for a reland.

compiler-rt/test/profile/instrprof-discarded-comdat.cpp

+// Check that instrprof does not introduce references to discarded sections when
+// using comdats.
+//
+// Occasionally, it is possible that the same function can be compiled in
+// different TUs with slightly different linkages, e.g., due to different
+// compiler options. However, if these are comdat functions, a single
+// implementation will be chosen at link time. we want to ensure that the
+// profiling data does not contain a reference to the discarded section.
+
+// UNSUPPORTED: target={{.*windows.*}}
+
+// RUN: mkdir -p %t.d
+// RUN: %clangxx_pgogen -O2 -fPIC -ffunction-sections -fdata-sections -c %s -o %t.d/a1.o -DOBJECT_1 -mllvm -disable-preinline
+// RUN: %clangxx_pgogen -O2 -fPIC -ffunction-sections -fdata-sections -c %s -o %t.d/a2.o
+// RUN: %clangxx_pgogen -fPIC -shared -o %t.d/liba.so %t.d/a1.o %t.d/a2.o 2>&1 | FileCheck %s --allow-empty
+
+// Ensure that we don't get an error when linking
+// CHECK-NOT: relocation refers to a discarded section: .text._ZN1CIiE1fEi
+
+template <typename T> struct C {
+  void f(T x);
+  int g(T x) {
+    f(x);
+    return v;
+  }
+  int v;
+};
+
+template <typename T>
+#ifdef OBJECT_1
+__attribute__((weak))
+#else
+__attribute__((noinline))
+#endif
+void C<T>::f(T x) {
+  v += x;
+}
+
+#ifdef OBJECT_1
+int foo() {
+  C<int> c;
+  c.f(1);
+  return c.g(2);
+}
+#else
+int bar() {
+  C<int> c;
+  c.f(3);
+  return c.g(4);
+}
+#endif

llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp

static inline bool shouldRecordFunctionAddr(Function *F) {
   // exists, the vtable will only be emitted in the TU where the key method
   // is defined. In a TU where vtable is not available, the function won't
   // be 'addresstaken'. If its address is not recorded here, the profile data
   // with missing address may be picked by the linker leading  to missing
   // indirect call target info.
   return F->hasAddressTaken() || F->hasLinkOnceLinkage();
 }
 
+static inline Constant *getFuncAddrForProfData(Function *Fn) {
+  auto *Int8PtrTy = Type::getInt8PtrTy(Fn->getContext());
+  // Store a nullptr in __llvm_profd, if we shouldn't use a real address
+  if (!shouldRecordFunctionAddr(Fn))
+    return ConstantPointerNull::get(Int8PtrTy);
+
+  // If we can't use an alias, we must use the public symbol, even though this
+  // may require a symbolic relocation. When the function has local linkage, we
+  // can use the symbol directly without introducing relocations.
+  if (Fn->isDeclarationForLinker() || Fn->hasLocalLinkage())
+    return ConstantExpr::getBitCast(Fn, Int8PtrTy);
+
+  // When possible use a private alias to avoid symbolic relocations.
+  auto *GA = GlobalAlias::create(GlobalValue::LinkageTypes::PrivateLinkage,
+                                 Fn->getName() + ".local", Fn);
+
+  // When the instrumented function is a COMDAT function, we cannot use a
+  // private alias. If we did, we would create reference to a local label in
+  // this function's section. If this version of the function isn't selected by
+  // the linker, then the metadata would introduce a reference to a discarded
+  // section. So, for COMDAT functions, we need to adjust the linkage of the
+  // alias. Using hidden visibility avoids a dynamic relocation and an entry in
+  // the dynamic symbol table.
+  if (Fn->hasComdat()) {
+    GA->setLinkage(Fn->getLinkage());
+    GA->setVisibility(GlobalValue::VisibilityTypes::HiddenVisibility);
+  }
+  return ConstantExpr::getBitCast(GA, Int8PtrTy);
+}
+
 static bool needsRuntimeRegistrationOfSectionRange(const Triple &TT) {
   // Don't do this for Darwin.  compiler-rt uses linker magic.
   if (TT.isOSDarwin())
     return false;
   // Use linker script magic to get data/cnts/name start/end.
   if (TT.isOSAIX() || TT.isOSLinux() || TT.isOSFreeBSD() || TT.isOSNetBSD() ||
       TT.isOSSolaris() || TT.isOSFuchsia() || TT.isPS() || TT.isOSWindows())
     return false;
InstrProfiling::getOrCreateRegionCounters(InstrProfInstBase *Inc) {
   auto *Int16Ty = Type::getInt16Ty(Ctx);
   auto *Int16ArrayTy = ArrayType::get(Int16Ty, IPVK_Last + 1);
   Type *DataTypes[] = {
 #define INSTR_PROF_DATA(Type, LLVMType, Name, Init) LLVMType,
 #include "llvm/ProfileData/InstrProfData.inc"
   };
   auto *DataTy = StructType::get(Ctx, makeArrayRef(DataTypes));
 
-  Constant *FunctionAddr = shouldRecordFunctionAddr(Fn)
-                               ? ConstantExpr::getBitCast(Fn, Int8PtrTy)
-                               : ConstantPointerNull::get(Int8PtrTy);
+  Constant *FunctionAddr = getFuncAddrForProfData(Fn);
 
   Constant *Int16ArrayVals[IPVK_Last + 1];
   for (uint32_t Kind = IPVK_First; Kind <= IPVK_Last; ++Kind)
     Int16ArrayVals[Kind] = ConstantInt::get(Int16Ty, PD.NumValueSites[Kind]);
 
   // If the data variable is not referenced by code (if we don't emit
   // @llvm.instrprof.value.profile, NS will be 0), and the counter keeps the
   // data variable live under linker GC, the data variable can be private. This

llvm/test/Transforms/PGOProfile/comdat.ll

 ;; Test comdat functions.
 ; RUN: opt < %s -mtriple=x86_64-linux -passes=pgo-instr-gen,instrprof -S | FileCheck %s --check-prefixes=ELF
 ; RUN: opt < %s -mtriple=x86_64-windows -passes=pgo-instr-gen,instrprof -S | FileCheck %s --check-prefixes=COFF
 
 $linkonceodr = comdat any
 $weakodr = comdat any
+$weak = comdat any
+$linkonce = comdat any
 
 ;; profc/profd have hash suffixes. This definition doesn't have value profiling,
 ;; so definitions with the same name in other modules must have the same CFG and
 ;; cannot have value profiling, either. profd can be made private for ELF.
 ; ELF:   @__profc_linkonceodr.[[#]] = linkonce_odr hidden global {{.*}} comdat, align 8
 ; ELF:   @__profd_linkonceodr.[[#]] = private global {{.*}} comdat($__profc_linkonceodr.[[#]]), align 8
 ; COFF:  @__profc_linkonceodr.[[#]] = linkonce_odr hidden global {{.*}} comdat, align 8
 ; COFF:  @__profd_linkonceodr.[[#]] = linkonce_odr hidden global {{.*}} comdat, align 8
 define linkonce_odr void @linkonceodr() comdat {
   ret void
 }
 
 ;; weakodr in a comdat is not renamed. There is no guarantee that definitions in
 ;; other modules don't have value profiling. profd should be conservatively
 ;; non-private to prevent a caller from referencing a non-prevailing profd,
 ;; causing a linker error.
 ; ELF:   @__profc_weakodr = weak_odr hidden global {{.*}} comdat, align 8
 ; ELF:   @__profd_weakodr = weak_odr hidden global {{.*}} comdat($__profc_weakodr), align 8
 ; COFF:  @__profc_weakodr = weak_odr hidden global {{.*}} comdat, align 8
 ; COFF:  @__profd_weakodr = weak_odr hidden global {{.*}} comdat, align 8
 define weak_odr void @weakodr() comdat {
   ret void
 }
+
+;; weak in a comdat is not renamed. There is no guarantee that definitions in
+;; other modules don't have value profiling. profd should be conservatively
+;; non-private to prevent a caller from referencing a non-prevailing profd,
+;; causing a linker error.
+; ELF:   @__profc_weak = weak hidden global {{.*}} comdat, align 8
+; ELF:   @__profd_weak = weak hidden global {{.*}} comdat($__profc_weak), align 8
+; COFF:  @__profc_weak = weak hidden global {{.*}} comdat, align 8
+; COFF:  @__profd_weak = weak hidden global {{.*}} comdat, align 8
+define weak void @weak() comdat {
+  ret void
+}
+
+;; profc/profd have hash suffixes. This definition doesn't have value profiling,
+;; so definitions with the same name in other modules must have the same CFG and
+;; cannot have value profiling, either. profd can be made private for ELF.
+; ELF:   @__profc_linkonce.[[#]] = linkonce hidden global {{.*}} comdat, align 8
+; ELF:   @__profd_linkonce.[[#]] = private global {{.*}} comdat($__profc_linkonce.[[#]]), align 8
+; COFF:  @__profc_linkonce.[[#]] = linkonce hidden global {{.*}} comdat, align 8
+; COFF:  @__profd_linkonce.[[#]] = linkonce hidden global {{.*}} comdat, align 8
+define linkonce void @linkonce() comdat {
+  ret void
+}
+
+; Check that comdat aliases are hidden for all linkage types
+; ELF:   @linkonceodr.local = linkonce_odr hidden alias void (), ptr @linkonceodr
+; ELF:   @weakodr.local = weak_odr hidden alias void (), ptr @weakodr
+; ELF:   @weak.local = weak hidden alias void (), ptr @weak
+; ELF:   @linkonce.local = linkonce hidden alias void (), ptr @linkonce

llvm/test/Transforms/PGOProfile/prof_avoid_relocs.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --check-globals
+; RUN: opt -S -passes=pgo-instr-gen,instrprof < %s | FileCheck %s
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+;; Test that we use private aliases to reference function addresses inside profile data
+; CHECK: @__profd_foo = private global {{.*}}, ptr @foo.local,
+; CHECK-NOT: @__profd_foo = private global {{.*}}, ptr @foo,
+
+; CHECK: @[[__PROFC_WEAK:[a-zA-Z0-9_$"\\.-]+]] = weak hidden global [1 x i64] zeroinitializer, section "__llvm_prf_cnts", comdat, align 8
+; CHECK: @[[__PROFD_WEAK:[a-zA-Z0-9_$"\\.-]+]] = private global { i64, i64, i64, ptr, ptr, i32, [2 x i16] } { i64 -5028622335731970946, i64 742261418966908927, i64 sub (i64 ptrtoint (ptr @__profc_weak to i64), i64 ptrtoint (ptr @__profd_weak to i64)), ptr @weak.local, ptr null, i32 1, [2 x i16] zeroinitializer }, section "__llvm_prf_data", comdat($__profc_weak), align 8
+; CHECK: @[[__PROFC_LINKONCE:[a-zA-Z0-9_$"\\.-]+]] = linkonce hidden global [1 x i64] zeroinitializer, section "__llvm_prf_cnts", comdat, align 8
+; CHECK: @[[__PROFD_LINKONCE:[a-zA-Z0-9_$"\\.-]+]] = private global { i64, i64, i64, ptr, ptr, i32, [2 x i16] } { i64 -121947654961992603, i64 742261418966908927, i64 sub (i64 ptrtoint (ptr @__profc_linkonce to i64), i64 ptrtoint (ptr @__profd_linkonce to i64)), ptr @linkonce.local, ptr null, i32 1, [2 x i16] zeroinitializer }, section "__llvm_prf_data", comdat($__profc_linkonce), align 8
+; CHECK: @[[__PROFC_WEAKODR:[a-zA-Z0-9_$"\\.-]+]] = weak_odr hidden global [1 x i64] zeroinitializer, section "__llvm_prf_cnts", comdat, align 8
+; CHECK: @[[__PROFD_WEAKODR:[a-zA-Z0-9_$"\\.-]+]] = private global { i64, i64, i64, ptr, ptr, i32, [2 x i16] } { i64 -4807837289933096997, i64 742261418966908927, i64 sub (i64 ptrtoint (ptr @__profc_weakodr to i64), i64 ptrtoint (ptr @__profd_weakodr to i64)), ptr @weakodr.local, ptr null, i32 1, [2 x i16] zeroinitializer }, section "__llvm_prf_data", comdat($__profc_weakodr), align 8
+; CHECK: @[[__PROFC_LINKONCEODR:[a-zA-Z0-9_$"\\.-]+]] = linkonce_odr hidden global [1 x i64] zeroinitializer, section "__llvm_prf_cnts", comdat, align 8
+; CHECK: @[[__PROFD_LINKONCEODR:[a-zA-Z0-9_$"\\.-]+]] = private global { i64, i64, i64, ptr, ptr, i32, [2 x i16] } { i64 4214081367395809689, i64 742261418966908927, i64 sub (i64 ptrtoint (ptr @__profc_linkonceodr to i64), i64 ptrtoint (ptr @__profd_linkonceodr to i64)), ptr @linkonceodr.local, ptr null, i32 1, [2 x i16] zeroinitializer }, section "__llvm_prf_data", comdat($__profc_linkonceodr), align 8
+; CHECK: @[[__PROFC_AVAILABLE_EXTERNALLY_742261418966908927:[a-zA-Z0-9_$"\\.-]+]] = linkonce_odr hidden global [1 x i64] zeroinitializer, section "__llvm_prf_cnts", comdat, align 8
+; CHECK: @[[__PROFD_AVAILABLE_EXTERNALLY_742261418966908927:[a-zA-Z0-9_$"\\.-]+]] = private global { i64, i64, i64, ptr, ptr, i32, [2 x i16] } { i64 -8510055422695886042, i64 742261418966908927, i64 sub (i64 ptrtoint (ptr @__profc_available_externally.742261418966908927 to i64), i64 ptrtoint (ptr @__profd_available_externally.742261418966908927 to i64)), ptr null, ptr null, i32 1, [2 x i16] zeroinitializer }, section "__llvm_prf_data", comdat($__profc_available_externally.742261418966908927), align 8
+
+;; Ensure when not instrumenting a non-comdat function, then if we generate an
+;; alias, then it is private. We check comdat versions in comdat.ll
+; CHECK: @foo.local = private alias i32 (i32), ptr @foo
+; CHECK: @[[WEAK_2:[a-zA-Z0-9_$"\\.-]+]].local = private alias void (), ptr @weak
+; CHECK: @[[LINKONCE_3:[a-zA-Z0-9_$"\\.-]+]].local = private alias void (), ptr @linkonce
+; CHECK: @[[WEAKODR_4:[a-zA-Z0-9_$"\\.-]+]].local = private alias void (), ptr @weakodr
+; CHECK: @[[LINKONCEODR_5:[a-zA-Z0-9_$"\\.-]+]].local = private alias void (), ptr @linkonceodr
+
+;; We should never generate an alias for available_externally functions
+; CHECK-NOT: @[[AVAILABLE_EXTERNALLY_6:[a-zA-Z0-9_$"\\.-]+]] = private alias void (), ptr @available_externally
+
+define i32 @foo(i32 %0) {
+; CHECK-LABEL: @foo(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[PGOCOUNT:%.*]] = load i64, ptr @__profc_foo, align 8
+; CHECK-NEXT:    [[TMP1:%.*]] = add i64 [[PGOCOUNT]], 1
+; CHECK-NEXT:    store i64 [[TMP1]], ptr @__profc_foo, align 8
+; CHECK-NEXT:    ret i32 0
+entry:
+  ret i32 0
+}
+
+define weak void @weak() {
+; CHECK-LABEL: @weak(
+; CHECK-NEXT:    [[PGOCOUNT:%.*]] = load i64, ptr @__profc_weak, align 8
+; CHECK-NEXT:    [[TMP1:%.*]] = add i64 [[PGOCOUNT]], 1
+; CHECK-NEXT:    store i64 [[TMP1]], ptr @__profc_weak, align 8
+; CHECK-NEXT:    ret void
+  ret void
+}
+
+define linkonce void @linkonce() {
+; CHECK-LABEL: @linkonce(
+; CHECK-NEXT:    [[PGOCOUNT:%.*]] = load i64, ptr @__profc_linkonce, align 8
+; CHECK-NEXT:    [[TMP1:%.*]] = add i64 [[PGOCOUNT]], 1
+; CHECK-NEXT:    store i64 [[TMP1]], ptr @__profc_linkonce, align 8
+; CHECK-NEXT:    ret void
+  ret void
+}
+
+define weak_odr void @weakodr() {
+; CHECK-LABEL: @weakodr(
+; CHECK-NEXT:    [[PGOCOUNT:%.*]] = load i64, ptr @__profc_weakodr, align 8
+; CHECK-NEXT:    [[TMP1:%.*]] = add i64 [[PGOCOUNT]], 1
+; CHECK-NEXT:    store i64 [[TMP1]], ptr @__profc_weakodr, align 8
+; CHECK-NEXT:    ret void
+  ret void
+}
+
+define linkonce_odr void @linkonceodr() {
+; CHECK-LABEL: @linkonceodr(
+; CHECK-NEXT:    [[PGOCOUNT:%.*]] = load i64, ptr @__profc_linkonceodr, align 8
+; CHECK-NEXT:    [[TMP1:%.*]] = add i64 [[PGOCOUNT]], 1
+; CHECK-NEXT:    store i64 [[TMP1]], ptr @__profc_linkonceodr, align 8
+; CHECK-NEXT:    ret void
+  ret void
+}
+
+define available_externally void @available_externally(){
+; CHECK-LABEL: @available_externally(
+; CHECK-NEXT:    [[PGOCOUNT:%.*]] = load i64, ptr @__profc_available_externally.742261418966908927, align 8
+; CHECK-NEXT:    [[TMP1:%.*]] = add i64 [[PGOCOUNT]], 1
+; CHECK-NEXT:    store i64 [[TMP1]], ptr @__profc_available_externally.742261418966908927, align 8
+; CHECK-NEXT:    ret void
+  ret void
+}