[XRay][clang] Add a flag to enable/disable linking XRay deps explicitly

[dberris]

Summary

This change introduces -fxray-link-deps and -fnoxray-link-deps. The
-fnoxray-link-deps allows for directly controlling which specific XRay
runtime to link. The default is for clang to link the XRay runtime that
is shipped with the compiler (if there are any), but users may want to
explicitly add the XRay dependencies from other locations or other
means.

clang/include/clang/Driver/Options.td

def fxray_never_instrument :
   HelpText<"Filename defining the whitelist for imbuing the 'never instrument' XRay attribute.">;
 
 def fxray_always_emit_customevents : Flag<["-"], "fxray-always-emit-customevents">, Group<f_Group>,
   Flags<[CC1Option]>,
   HelpText<"Determine whether to always emit __xray_customevent(...) calls even if the function it appears in is not always instrumented.">;
 def fnoxray_always_emit_customevents : Flag<["-"], "fno-xray-always-emit-customevents">, Group<f_Group>,
   Flags<[CC1Option]>;
 
+def fxray_link_deps : Flag<["-"], "fxray-link-deps">, Group<f_Group>,
+  Flags<[CC1Option]>,
+  HelpText<"Tells clang to add the link dependencies for XRay.">;
+def fnoxray_link_deps : Flag<["-"], "fnoxray-link-deps">, Group<f_Group>,
+  Flags<[CC1Option]>;
+
 def ffine_grained_bitfield_accesses : Flag<["-"],
   "ffine-grained-bitfield-accesses">, Group<f_clang_Group>, Flags<[CC1Option]>,
   HelpText<"Use separate accesses for bitfields with legal widths and alignments.">;
 def fno_fine_grained_bitfield_accesses : Flag<["-"],
   "fno-fine-grained-bitfield-accesses">, Group<f_clang_Group>, Flags<[CC1Option]>,
   HelpText<"Use large-integer access for consecutive bitfield runs.">;
 
 def flat__namespace : Flag<["-"], "flat_namespace">;

clang/include/clang/Driver/XRayArgs.h

 
 class XRayArgs {
   std::vector<std::string> AlwaysInstrumentFiles;
   std::vector<std::string> NeverInstrumentFiles;
   std::vector<std::string> ExtraDeps;
   bool XRayInstrument = false;
   int InstructionThreshold = 200;
   bool XRayAlwaysEmitCustomEvents = false;
+  bool XRayRT = true;
 
 public:
   /// Parses the XRay arguments from an argument list.
   XRayArgs(const ToolChain &TC, const llvm::opt::ArgList &Args);
   void addArgs(const ToolChain &TC, const llvm::opt::ArgList &Args,
                llvm::opt::ArgStringList &CmdArgs, types::ID InputType) const;
 
-  bool needsXRayRt() const { return XRayInstrument; }
+  bool needsXRayRt() const { return XRayInstrument && XRayRT; }
 };
 
 } // namespace driver
 } // namespace clang
 
 #endif // LLVM_CLANG_DRIVER_XRAYARGS_H

clang/lib/Driver/XRayArgs.cpp

if (Args.hasFlag(options::OPT_fxray_instrument,
     // By default, the back-end will not emit the lowering for XRay customevent
     // calls if the function is not instrumented. In the future we will change
     // this default to be the reverse, but in the meantime we're going to
     // introduce the new functionality behind a flag.
     if (Args.hasFlag(options::OPT_fxray_always_emit_customevents,
                      options::OPT_fnoxray_always_emit_customevents, false))
       XRayAlwaysEmitCustomEvents = true;
 
+    if (!Args.hasFlag(options::OPT_fxray_link_deps,
+                      options::OPT_fnoxray_link_deps, true))
+      XRayRT = false;
+
     // Validate the always/never attribute files. We also make sure that they
     // are treated as actual dependencies.
     for (const auto &Filename :
          Args.getAllArgValues(options::OPT_fxray_always_instrument)) {
       if (llvm::sys::fs::exists(Filename)) {
         AlwaysInstrumentFiles.push_back(Filename);
         ExtraDeps.push_back(Filename);
       } else

clang/test/Driver/XRay/xray-nolinkdeps.cpp

+// RUN: %clang -v -o /dev/null -fxray-instrument -fnoxray-link-deps %s -### \
+// RUN:     2>&1 | FileCheck --check-prefix DISABLE %s
+// RUN: %clang -v -o /dev/null -fxray-instrument -fxray-link-deps %s -### \
+// RUN:     2>&1 | FileCheck --check-prefix ENABLE %s
+// ENABLE: clang_rt.xray
+// DISABLE-NOT: clang_rt.xray