[Driver] SafeStack does not need a runtime library on Fuchsia

include/clang/Driver/SanitizerArgs.h

class SanitizerArgs {
   bool MsanUseAfterDtor = false;
   bool CfiCrossDso = false;
   int AsanFieldPadding = 0;
   bool AsanSharedRuntime = false;
   bool AsanUseAfterScope = true;
   bool AsanGlobalsDeadStripping = false;
   bool LinkCXXRuntimes = false;
   bool NeedPIE = false;
+  bool SafeStackRuntime = false;
   bool Stats = false;
   bool TsanMemoryAccess = true;
   bool TsanFuncEntryExit = true;
   bool TsanAtomics = true;
 
  public:
   /// Parses the sanitizer arguments from an argument list.
   SanitizerArgs(const ToolChain &TC, const llvm::opt::ArgList &Args);
 
   bool needsAsanRt() const { return Sanitizers.has(SanitizerKind::Address); }
   bool needsSharedAsanRt() const { return AsanSharedRuntime; }
   bool needsTsanRt() const { return Sanitizers.has(SanitizerKind::Thread); }
   bool needsMsanRt() const { return Sanitizers.has(SanitizerKind::Memory); }
   bool needsFuzzer() const { return Sanitizers.has(SanitizerKind::Fuzzer); }
   bool needsLsanRt() const {
     return Sanitizers.has(SanitizerKind::Leak) &&
            !Sanitizers.has(SanitizerKind::Address);
   }
   bool needsUbsanRt() const;
   bool needsDfsanRt() const { return Sanitizers.has(SanitizerKind::DataFlow); }
-  bool needsSafeStackRt() const {
-    return Sanitizers.has(SanitizerKind::SafeStack);
-  }
+  bool needsSafeStackRt() const { return SafeStackRuntime; }
   bool needsCfiRt() const;
   bool needsCfiDiagRt() const;
   bool needsStatsRt() const { return Stats; }
   bool needsEsanRt() const {
     return Sanitizers.hasOneOf(SanitizerKind::Efficiency);
   }
 
   bool requiresPIE() const;

lib/Driver/SanitizerArgs.cpp

if (AllAddedKinds & Address) {
     // See https://sourceware.org/bugzilla/show_bug.cgi?id=19002
     AsanGlobalsDeadStripping =
         !TC.getTriple().isOSBinFormatELF() || TC.getTriple().isOSFuchsia() ||
         Args.hasArg(options::OPT_fsanitize_address_globals_dead_stripping);
   } else {
     AsanUseAfterScope = false;
   }
 
+  if (AllAddedKinds & SafeStack) {
+    // SafeStack runtime is built into the system on Fuchsia.
+    SafeStackRuntime = !TC.getTriple().isOSFuchsia();
+  }
+
   // Parse -link-cxx-sanitizer flag.
   LinkCXXRuntimes =
       Args.hasArg(options::OPT_fsanitize_link_cxx_runtime) || D.CCCIsCXX();
 
   // Finally, initialize the set of available and recoverable sanitizers.
   Sanitizers.Mask |= Kinds;
   RecoverableSanitizers.Mask |= RecoverableKinds;
   TrapSanitizers.Mask |= TrappingKinds;

test/Driver/fuchsia.c

 // CHECK-RELOCATABLE-NOT: "-pie"
 // CHECK-RELOCATABLE-NOT: "--build-id"
 // CHECK-RELOCATABLE: "-r"
 
 // RUN: %clang %s -### --target=x86_64-unknown-fuchsia \
 // RUN:     -fsanitize=safe-stack 2>&1 \
 // RUN:     | FileCheck %s -check-prefix=CHECK-SAFESTACK
 // CHECK-SAFESTACK: "-fsanitize=safe-stack"
+// CHECK-SAFESTACK-NOT: "{{.*[/\\]}}libclang_rt.safestack-x86_64.a"
+// CHECK-SAFESTACK-NOT: "__safestack_init"
 
 // RUN: %clang %s -### --target=x86_64-unknown-fuchsia \
 // RUN:     -fsanitize=address 2>&1 \
 // RUN:     | FileCheck %s -check-prefix=CHECK-ASAN-X86
 // CHECK-ASAN-X86: "-fsanitize=address"
 // CHECK-ASAN-X86: "-fsanitize-address-globals-dead-stripping"
 // CHECK-ASAN-X86: "-dynamic-linker" "asan/ld.so.1"
 // CHECK-ASAN-X86: "{{.*[/\\]}}libclang_rt.asan-x86_64.so"