[Darwin] Introduce a new flag, -fapple-link-rtlib that forces linking…

… of the builtins library.

This driver flag is useful when users want to link against the compiler's
builtins, but nothing else, and so use flags like -nostdlib.

Darwin can't use -nolibc & nostdlib++ like other platforms on because we
disable all runtime lib linking with -static, which we still want to have
an option to link with the builtins.

Differential Revision: https://reviews.llvm.org/D58320

llvm-svn: 360483

clang/include/clang/Driver/Options.td

@@ -1253,6 +1253,8 @@ def fno_fine_grained_bitfield_accesses : Flag<["-"],
 def flat__namespace : Flag<["-"], "flat_namespace">;
 def flax_vector_conversions : Flag<["-"], "flax-vector-conversions">, Group<f_Group>;
 def flimited_precision_EQ : Joined<["-"], "flimited-precision=">, Group<f_Group>;
+def fapple_link_rtlib : Flag<["-"], "fapple-link-rtlib">, Group<f_Group>,
+  HelpText<"Force linking the clang builtins runtime library">;
 def flto_EQ : Joined<["-"], "flto=">, Flags<[CoreOption, CC1Option]>, Group<f_Group>,
   HelpText<"Set LTO mode to either 'full' or 'thin'">, Values<"thin,full">;
 def flto : Flag<["-"], "flto">, Flags<[CoreOption, CC1Option]>, Group<f_Group>,

clang/lib/Driver/ToolChains/Darwin.cpp

@@ -593,15 +593,26 @@ void darwin::Linker::ConstructJob(Compilation &C, const JobAction &JA,
 
   if (getToolChain().ShouldLinkCXXStdlib(Args))
     getToolChain().AddCXXStdlibLibArgs(Args, CmdArgs);
-  if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
+
+  bool NoStdOrDefaultLibs =
+      Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs);
+  bool ForceLinkBuiltins = Args.hasArg(options::OPT_fapple_link_rtlib);
+  if (!NoStdOrDefaultLibs || ForceLinkBuiltins) {
     // link_ssp spec is empty.
 
-    // Let the tool chain choose which runtime library to link.
-    getMachOToolChain().AddLinkRuntimeLibArgs(Args, CmdArgs);
+    // If we have both -nostdlib/nodefaultlibs and -fapple-link-rtlib then
+    // we just want to link the builtins, not the other libs like libSystem.
+    if (NoStdOrDefaultLibs && ForceLinkBuiltins) {
+      getMachOToolChain().AddLinkRuntimeLib(Args, CmdArgs, "builtins");
+    } else {
+      // Let the tool chain choose which runtime library to link.
+      getMachOToolChain().AddLinkRuntimeLibArgs(Args, CmdArgs,
+                                                ForceLinkBuiltins);
 
-    // No need to do anything for pthreads. Claim argument to avoid warning.
-    Args.ClaimAllArgs(options::OPT_pthread);
-    Args.ClaimAllArgs(options::OPT_pthreads);
+      // No need to do anything for pthreads. Claim argument to avoid warning.
+      Args.ClaimAllArgs(options::OPT_pthread);
+      Args.ClaimAllArgs(options::OPT_pthreads);
+    }
   }
 
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles)) {
@@ -1128,16 +1139,20 @@ ToolChain::RuntimeLibType DarwinClang::GetRuntimeLibType(
 }
 
 void DarwinClang::AddLinkRuntimeLibArgs(const ArgList &Args,
-                                        ArgStringList &CmdArgs) const {
+                                        ArgStringList &CmdArgs,
+                                        bool ForceLinkBuiltinRT) const {
   // Call once to ensure diagnostic is printed if wrong value was specified
   GetRuntimeLibType(Args);
 
   // Darwin doesn't support real static executables, don't link any runtime
   // libraries with -static.
   if (Args.hasArg(options::OPT_static) ||
       Args.hasArg(options::OPT_fapple_kext) ||
-      Args.hasArg(options::OPT_mkernel))
+      Args.hasArg(options::OPT_mkernel)) {
+    if (ForceLinkBuiltinRT)
+      AddLinkRuntimeLib(Args, CmdArgs, "builtins");
     return;
+  }
 
   // Reject -static-libgcc for now, we can deal with this when and if someone
   // cares. This is useful in situations where someone wants to statically link
@@ -2106,7 +2121,8 @@ DerivedArgList *MachO::TranslateArgs(const DerivedArgList &Args,
 }
 
 void MachO::AddLinkRuntimeLibArgs(const ArgList &Args,
-                                  ArgStringList &CmdArgs) const {
+                                  ArgStringList &CmdArgs,
+                                  bool ForceLinkBuiltinRT) const {
   // Embedded targets are simple at the moment, not supporting sanitizers and
   // with different libraries for each member of the product { static, PIC } x
   // { hard-float, soft-float }

clang/lib/Driver/ToolChains/Darwin.h

@@ -157,7 +157,8 @@ class LLVM_LIBRARY_VISIBILITY MachO : public ToolChain {
   /// FIXME: This API is intended for use with embedded libraries only, and is
   /// misleadingly named.
   virtual void AddLinkRuntimeLibArgs(const llvm::opt::ArgList &Args,
-                                     llvm::opt::ArgStringList &CmdArgs) const;
+                                     llvm::opt::ArgStringList &CmdArgs,
+                                     bool ForceLinkBuiltinRT = false) const;
 
   virtual void addStartObjectFileArgs(const llvm::opt::ArgList &Args,
                                       llvm::opt::ArgStringList &CmdArgs) const {
@@ -495,7 +496,8 @@ class LLVM_LIBRARY_VISIBILITY DarwinClang : public Darwin {
   RuntimeLibType GetRuntimeLibType(const llvm::opt::ArgList &Args) const override;
 
   void AddLinkRuntimeLibArgs(const llvm::opt::ArgList &Args,
-                             llvm::opt::ArgStringList &CmdArgs) const override;
+                             llvm::opt::ArgStringList &CmdArgs,
+                             bool ForceLinkBuiltinRT = false) const override;
 
   void AddClangCXXStdlibIncludeArgs(
       const llvm::opt::ArgList &DriverArgs,

clang/test/Driver/darwin-fapple-link-rtlib.c

@@ -0,0 +1,6 @@
+// RUN: %clang -target arm64-apple-ios12.0 %s -nostdlib -fapple-link-rtlib -resource-dir=%S/Inputs/resource_dir -### 2>&1 | FileCheck %s
+// RUN: %clang -target arm64-apple-ios12.0 %s -static -fapple-link-rtlib -resource-dir=%S/Inputs/resource_dir -### 2>&1 | FileCheck %s
+// RUN: %clang -target arm64-apple-ios12.0 %s -fapple-link-rtlib -resource-dir=%S/Inputs/resource_dir -### 2>&1 | FileCheck %s --check-prefix=DEFAULT
+// CHECK-NOT: "-lSystem"
+// DEFAULT: "-lSystem"
+// CHECK: libclang_rt.ios.a