[Fuchsia] Rely on linker switch rather than dead code ref for profile runtime

Follow the model used on Linux, where the clang driver passes the
linker a -u switch to force the profile runtime to be linked in,
rather than having every TU emit a dead function with a reference.

Differential Revision: https://reviews.llvm.org/D79835

clang/lib/Driver/ToolChains/Fuchsia.cpp

@@ -15,6 +15,7 @@
 #include "clang/Driver/Options.h"
 #include "clang/Driver/SanitizerArgs.h"
 #include "llvm/Option/ArgList.h"
+#include "llvm/ProfileData/InstrProf.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/VirtualFileSystem.h"
@@ -365,3 +366,13 @@ SanitizerMask Fuchsia::getDefaultSanitizers() const {
   }
   return Res;
 }
+
+void Fuchsia::addProfileRTLibs(const llvm::opt::ArgList &Args,
+                               llvm::opt::ArgStringList &CmdArgs) const {
+  // Add linker option -u__llvm_profile_runtime to cause runtime
+  // initialization module to be linked in.
+  if (needsProfileRT(Args))
+    CmdArgs.push_back(Args.MakeArgString(
+        Twine("-u", llvm::getInstrProfRuntimeHookVarName())));
+  ToolChain::addProfileRTLibs(Args, CmdArgs);
+}

clang/lib/Driver/ToolChains/Fuchsia.h

@@ -69,6 +69,9 @@ public:
   SanitizerMask getSupportedSanitizers() const override;
   SanitizerMask getDefaultSanitizers() const override;
 
+  void addProfileRTLibs(const llvm::opt::ArgList &Args,
+                        llvm::opt::ArgStringList &CmdArgs) const override;
+
   RuntimeLibType
   GetRuntimeLibType(const llvm::opt::ArgList &Args) const override;
   CXXStdlibType

clang/test/Driver/fuchsia.c

@@ -242,3 +242,21 @@
 // RUN:     -gsplit-dwarf -c %s 2>&1 \
 // RUN:     | FileCheck %s -check-prefix=CHECK-SPLIT-DWARF
 // CHECK-SPLIT-DWARF: "-split-dwarf-output" "fuchsia.dwo"
+
+// RUN: %clang %s -### --target=aarch64-fuchsia \
+// RUN:     -fprofile-instr-generate -fcoverage-mapping \
+// RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \
+// RUN:     -fuse-ld=lld 2>&1 \
+// RUN:     | FileCheck %s -check-prefix=CHECK-PROFRT-AARCH64
+// CHECK-PROFRT-AARCH64: "-resource-dir" "[[RESOURCE_DIR:[^"]+]]"
+// CHECK-PROFRT-AARCH64: "-u__llvm_profile_runtime"
+// CHECK-PROFRT-AARCH64: "[[RESOURCE_DIR]]{{/|\\\\}}lib{{/|\\\\}}aarch64-fuchsia{{/|\\\\}}libclang_rt.profile.a"
+
+// RUN: %clang %s -### --target=x86_64-fuchsia \
+// RUN:     -fprofile-instr-generate -fcoverage-mapping \
+// RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \
+// RUN:     -fuse-ld=lld 2>&1 \
+// RUN:     | FileCheck %s -check-prefix=CHECK-PROFRT-X86_64
+// CHECK-PROFRT-X86_64: "-resource-dir" "[[RESOURCE_DIR:[^"]+]]"
+// CHECK-PROFRT-X86_64: "-u__llvm_profile_runtime"
+// CHECK-PROFRT-X86_64: "[[RESOURCE_DIR]]{{/|\\\\}}lib{{/|\\\\}}x86_64-fuchsia{{/|\\\\}}libclang_rt.profile.a"

llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp

@@ -1034,9 +1034,9 @@ void InstrProfiling::emitRegistration() {
 }
 
 bool InstrProfiling::emitRuntimeHook() {
-  // We expect the linker to be invoked with -u<hook_var> flag for linux,
+  // We expect the linker to be invoked with -u<hook_var> flag for Linux or
-  // for which case there is no need to emit the user function.
+  // Fuchsia, in which case there is no need to emit the user function.
-  if (TT.isOSLinux())
+  if (TT.isOSLinux() || TT.isOSFuchsia())
     return false;
 
   // If the module's provided its own runtime, we don't need to do anything.

llvm/test/Instrumentation/InstrProfiling/linkage.ll

@@ -2,9 +2,11 @@
 
 ; RUN: opt < %s -mtriple=x86_64-apple-macosx10.10.0 -instrprof -S | FileCheck %s --check-prefixes=POSIX,MACHO
 ; RUN: opt < %s -mtriple=x86_64-unknown-linux -instrprof -S | FileCheck %s --check-prefixes=POSIX,LINUX
+; RUN: opt < %s -mtriple=x86_64-unknown-fuchsia -instrprof -S | FileCheck %s --check-prefixes=POSIX,LINUX
 ; RUN: opt < %s  -mtriple=x86_64-pc-win32-coff -instrprof -S | FileCheck %s --check-prefixes=COFF
 ; RUN: opt < %s -mtriple=x86_64-apple-macosx10.10.0 -passes=instrprof -S | FileCheck %s --check-prefixes=POSIX,MACHO
 ; RUN: opt < %s -mtriple=x86_64-unknown-linux -passes=instrprof -S | FileCheck %s --check-prefixes=POSIX,LINUX
+; RUN: opt < %s -mtriple=x86_64-unknown-fuchsia -passes=instrprof -S | FileCheck %s --check-prefixes=POSIX,LINUX
 ; RUN: opt < %s  -mtriple=x86_64-pc-win32-coff -passes=instrprof -S | FileCheck %s --check-prefixes=COFF
 
 ; MACHO: @__llvm_profile_runtime = external global i32