[PGO] Eliminate emission of runtime hook user (Linux)

[davidxl]

Summary

This is patch part-2 following: http://reviews.llvm.org/D14030

[davidxl]

Addressed review comments from Justin.

llvm/trunk/lib/Transforms/Instrumentation/InstrProfiling.cpp

void InstrProfiling::emitRegistration() {
   IRBuilder<> IRB(BasicBlock::Create(M->getContext(), "", RegisterF));
   for (Value *Data : UsedVars)
     IRB.CreateCall(RuntimeRegisterF, IRB.CreateBitCast(Data, VoidPtrTy));
   IRB.CreateRetVoid();
 }
 
 void InstrProfiling::emitRuntimeHook() {
 
+  // We expect the linker to be invoked with -u<hook_var> flag for linux,
+  // for which case there is no need to emit the user function.
+  if (Triple(M->getTargetTriple()).isOSLinux())
+    return;
+
   // If the module's provided its own runtime, we don't need to do anything.
   if (M->getGlobalVariable(getInstrProfRuntimeHookVarName())) return;
 
   // Declare an external variable that will pull in the runtime initialization.
   auto *Int32Ty = Type::getInt32Ty(M->getContext());
   auto *Var =
       new GlobalVariable(*M, Int32Ty, false, GlobalValue::ExternalLinkage,
                          nullptr, getInstrProfRuntimeHookVarName());

llvm/trunk/test/Instrumentation/InstrProfiling/linkage.ll

 ;; Check that runtime symbols get appropriate linkage.
 
-; RUN: opt < %s -mtriple=x86_64-apple-macosx10.10.0 -instrprof -S | FileCheck %s
-; RUN: opt < %s -mtriple=x86_64-unknown-linux -instrprof -S | FileCheck %s
+; RUN: opt < %s -mtriple=x86_64-apple-macosx10.10.0 -instrprof -S | FileCheck %s --check-prefix=OTHER --check-prefix=COMMON
+; RUN: opt < %s -mtriple=x86_64-unknown-linux -instrprof -S | FileCheck %s --check-prefix=LINUX --check-prefix=COMMON
 
 @__llvm_profile_name_foo = hidden constant [3 x i8] c"foo"
 @__llvm_profile_name_foo_weak = weak hidden constant [8 x i8] c"foo_weak"
 @"__llvm_profile_name_linkage.ll:foo_internal" = internal constant [23 x i8] c"linkage.ll:foo_internal"
 @__llvm_profile_name_foo_inline = linkonce_odr hidden constant [10 x i8] c"foo_inline"
 
-; CHECK: @__llvm_profile_counters_foo = hidden global
-; CHECK: @__llvm_profile_data_foo = hidden constant
+; COMMON: @__llvm_profile_counters_foo = hidden global
+; COMMON: @__llvm_profile_data_foo = hidden constant
 define void @foo() {
   call void @llvm.instrprof.increment(i8* getelementptr inbounds ([3 x i8], [3 x i8]* @__llvm_profile_name_foo, i32 0, i32 0), i64 0, i32 1, i32 0)
   ret void
 }
 
-; CHECK: @__llvm_profile_counters_foo_weak = weak hidden global
-; CHECK: @__llvm_profile_data_foo_weak = weak hidden constant
+; COMMON: @__llvm_profile_counters_foo_weak = weak hidden global
+; COMMON: @__llvm_profile_data_foo_weak = weak hidden constant
 define weak void @foo_weak() {
   call void @llvm.instrprof.increment(i8* getelementptr inbounds ([8 x i8], [8 x i8]* @__llvm_profile_name_foo_weak, i32 0, i32 0), i64 0, i32 1, i32 0)
   ret void
 }
 
-; CHECK: @"__llvm_profile_counters_linkage.ll:foo_internal" = internal global
-; CHECK: @"__llvm_profile_data_linkage.ll:foo_internal" = internal constant
+; COMMON: @"__llvm_profile_counters_linkage.ll:foo_internal" = internal global
+; COMMON: @"__llvm_profile_data_linkage.ll:foo_internal" = internal constant
 define internal void @foo_internal() {
   call void @llvm.instrprof.increment(i8* getelementptr inbounds ([23 x i8], [23 x i8]* @"__llvm_profile_name_linkage.ll:foo_internal", i32 0, i32 0), i64 0, i32 1, i32 0)
   ret void
 }
 
-; CHECK: @__llvm_profile_counters_foo_inline = linkonce_odr hidden global
-; CHECK: @__llvm_profile_data_foo_inline = linkonce_odr hidden constant
+; COMMON: @__llvm_profile_counters_foo_inline = linkonce_odr hidden global
+; COMMON: @__llvm_profile_data_foo_inline = linkonce_odr hidden constant
 define linkonce_odr void @foo_inline() {
   call void @llvm.instrprof.increment(i8* getelementptr inbounds ([10 x i8], [10 x i8]* @__llvm_profile_name_foo_inline, i32 0, i32 0), i64 0, i32 1, i32 0)
   ret void
 }
 
 declare void @llvm.instrprof.increment(i8*, i64, i32, i32)
 
-; CHECK: @__llvm_profile_runtime = external global i32
+; OTHER: @__llvm_profile_runtime = external global i32
+; LINUX-NOT: @__llvm_profile_runtime = external global i32
 
-; CHECK: define linkonce_odr hidden i32 @__llvm_profile_runtime_user() {{.*}} {
-; CHECK:   %[[REG:.*]] = load i32, i32* @__llvm_profile_runtime
-; CHECK:   ret i32 %[[REG]]
-; CHECK: }
+; OTHER: define linkonce_odr hidden i32 @__llvm_profile_runtime_user() {{.*}} {
+; OTHER:   %[[REG:.*]] = load i32, i32* @__llvm_profile_runtime
+; OTHER:   ret i32 %[[REG]]
+; OTHER: }
+; LINUX-NOT: define linkonce_odr hidden i32 @__llvm_profile_runtime_user() {{.*}} {
+; LINUX-NOT:   %[[REG:.*]] = load i32, i32* @__llvm_profile_runtime