[Driver][Gnu] -r: imply -nostdlib like GCC

[MaskRay]

Summary

See gcc -dumpspecs that -r essentially implies -nostdlib and suppresses
default -l* and crt*.o. The behavior makes sense because otherwise there will be
assuredly conflicting definitions when the relocatable output is linked into the
final executable/shared object.

[thesamesam]

Looks good, although the documentation for -r and other driver options could be better (not your fault though).

From https://gcc.gnu.org/onlinedocs/gcc/Link-Options.html:

-r

    Produce a relocatable object as output. This is also known as partial linking.

[phosek]

LGTM

clang/lib/Driver/ToolChains/Gnu.cpp

if (!Args.hasArg(options::OPT_shared) && !IsStaticPIE) {
       CmdArgs.push_back(Args.MakeArgString(Twine(D.DyldPrefix) +
                                            ToolChain.getDynamicLinker(Args)));
     }
   }
 
   CmdArgs.push_back("-o");
   CmdArgs.push_back(Output.getFilename());
 
-  if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles)) {
+  if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles,
+                   options::OPT_r)) {
     if (!isAndroid && !IsIAMCU) {
       const char *crt1 = nullptr;
       if (!Args.hasArg(options::OPT_shared)) {
         if (Args.hasArg(options::OPT_pg))
           crt1 = "gcrt1.o";
         else if (IsPIE)
           crt1 = "Scrt1.o";
         else if (IsStaticPIE)
void tools::gnutools::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   bool NeedsSanitizerDeps = addSanitizerRuntimes(ToolChain, Args, CmdArgs);
   bool NeedsXRayDeps = addXRayRuntime(ToolChain, Args, CmdArgs);
   addLinkerCompressDebugSectionsOption(ToolChain, Args, CmdArgs);
   AddLinkerInputs(ToolChain, Inputs, Args, CmdArgs, JA);
   // The profile runtime also needs access to system libraries.
   getToolChain().addProfileRTLibs(Args, CmdArgs);
 
   if (D.CCCIsCXX() &&
-      !Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
+      !Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs,
+                   options::OPT_r)) {
     if (ToolChain.ShouldLinkCXXStdlib(Args)) {
       bool OnlyLibstdcxxStatic = Args.hasArg(options::OPT_static_libstdcxx) &&
                                  !Args.hasArg(options::OPT_static);
       if (OnlyLibstdcxxStatic)
         CmdArgs.push_back("-Bstatic");
       ToolChain.AddCXXStdlibLibArgs(Args, CmdArgs);
       if (OnlyLibstdcxxStatic)
         CmdArgs.push_back("-Bdynamic");
     }
     CmdArgs.push_back("-lm");
   }
   // Silence warnings when linking C code with a C++ '-stdlib' argument.
   Args.ClaimAllArgs(options::OPT_stdlib_EQ);
 
-  if (!Args.hasArg(options::OPT_nostdlib)) {
+  if (!Args.hasArg(options::OPT_nostdlib, options::OPT_r)) {
     if (!Args.hasArg(options::OPT_nodefaultlibs)) {
       if (IsStatic || IsStaticPIE)
         CmdArgs.push_back("--start-group");
 
       if (NeedsSanitizerDeps)
         linkSanitizerRuntimeDeps(ToolChain, CmdArgs);
 
       if (NeedsXRayDeps)

clang/test/Driver/linux-cross.cpp

 // RUN:   --stdlib=platform --rtlib=platform 2>&1 | FileCheck %s --check-prefix=MUSL_I686
 // MUSL_I686: "-dynamic-linker" "/lib/ld-musl-i386.so.1"
 
 // RUN: %clang -### %s --target=x86_64-linux-muslx32 --sysroot= \
 // RUN:   --stdlib=platform --rtlib=platform 2>&1 | FileCheck %s --check-prefix=MUSL_X32
 // RUN: %clang -### %s --target=i686-linux-musl -mx32 --sysroot= \
 // RUN:   --stdlib=platform --rtlib=platform 2>&1 | FileCheck %s --check-prefix=MUSL_X32
 // MUSL_X32: "-dynamic-linker" "/lib/ld-musl-x32.so.1"
+
+/// -r suppresses default -l and crt*.o like -nostdlib.
+// RUN: %clang -### %s --target=x86_64-linux-gnu --sysroot=%S/Inputs/debian_multiarch_tree \
+// RUN:   -ccc-install-dir %S/Inputs/basic_linux_tree/usr/bin -resource-dir=%S/Inputs/resource_dir \
+// RUN:   --stdlib=platform --rtlib=platform -r 2>&1 | FileCheck %s --check-prefix=RELOCATABLE
+// RELOCATABLE:      "-internal-isystem"
+// RELOCATABLE-SAME: {{^}} "[[SYSROOT:[^"]+]]/usr/lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/10"
+// RELOCATABLE:      "-L
+// RELOCATABLE-SAME: {{^}}[[SYSROOT]]/usr/lib/gcc/x86_64-linux-gnu/10"
+// RELOCATABLE-NOT:  "-l
+// RELOCATABLE-NOT:  crt{{[^.]+}}.o