[Driver] Avoid including -lm on the link line with -nostdlib++

[phosek]

Summary

Drivers include -lm because standard C++ libraries such as libc++
and libstdc++ depend on libmath and when statically linking C++
library, this dependency is needed. However, when -nostdlib++ is
used, there's no reason to include -lm anymore.

[thakis]

-lm is kind of the reason -nostdlib++ exists at all -- if you don't want _any_ stdlibs, you can just -nostdlib, no?

[phosek]

In D54681#1329240, @thakis wrote:

-lm is kind of the reason -nostdlib++ exists at all -- if you don't want _any_ stdlibs, you can just -nostdlib, no?

We still want standard libraries like -lc and compiler builtins, -nostdlib would remove those. Keeping -lm around is assuming that user is going to link against C++ library that needs -lm which is a lot of assumptions that IMHO driver shouldn't be doing.

clang/lib/Driver/ToolChains/DragonFly.cpp

void dragonfly::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
     CmdArgs.push_back("-L/usr/lib/gcc50");
 
     if (!Args.hasArg(options::OPT_static)) {
       CmdArgs.push_back("-rpath");
       CmdArgs.push_back("/usr/lib/gcc50");
     }
 
-    if (D.CCCIsCXX()) {
-      if (getToolChain().ShouldLinkCXXStdlib(Args))
-        getToolChain().AddCXXStdlibLibArgs(Args, CmdArgs);
+    if (getToolChain().ShouldLinkCXXStdlib(Args)) {
       CmdArgs.push_back("-lm");
     }
 
     if (Args.hasArg(options::OPT_pthread))
       CmdArgs.push_back("-lpthread");
 
     if (!Args.hasArg(options::OPT_nolibc)) {
       CmdArgs.push_back("-lc");

clang/lib/Driver/ToolChains/Fuchsia.cpp

void fuchsia::Linker::ConstructJob(Compilation &C, const JobAction &JA,
 
   AddLinkerInputs(ToolChain, Inputs, Args, CmdArgs, JA);
   ToolChain.addProfileRTLibs(Args, CmdArgs);
 
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
     if (Args.hasArg(options::OPT_static))
       CmdArgs.push_back("-Bdynamic");
 
-    if (D.CCCIsCXX()) {
-      if (ToolChain.ShouldLinkCXXStdlib(Args)) {
-        bool OnlyLibstdcxxStatic = Args.hasArg(options::OPT_static_libstdcxx) &&
-                                   !Args.hasArg(options::OPT_static);
-        CmdArgs.push_back("--push-state");
-        CmdArgs.push_back("--as-needed");
-        if (OnlyLibstdcxxStatic)
-          CmdArgs.push_back("-Bstatic");
-        ToolChain.AddCXXStdlibLibArgs(Args, CmdArgs);
-        if (OnlyLibstdcxxStatic)
-          CmdArgs.push_back("-Bdynamic");
-        CmdArgs.push_back("-lm");
-        CmdArgs.push_back("--pop-state");
-      }
-    }
 
     AddRunTimeLibs(ToolChain, D, CmdArgs, Args);
 
     if (Args.hasArg(options::OPT_pthread) ||
         Args.hasArg(options::OPT_pthreads))
       CmdArgs.push_back("-lpthread");
 
     if (Args.hasArg(options::OPT_fsplit_stack))

clang/lib/Driver/ToolChains/Gnu.cpp

if (Args.hasArg(options::OPT_Z_Xlinker__no_demangle))
     CmdArgs.push_back("--no-demangle");
 
   bool NeedsSanitizerDeps = addSanitizerRuntimes(ToolChain, Args, CmdArgs);
   bool NeedsXRayDeps = addXRayRuntime(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)) {
-    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_nodefaultlibs)) {
       if (Args.hasArg(options::OPT_static))

clang/lib/Driver/ToolChains/Hexagon.cpp

Args.AddAllArgs(CmdArgs,
                    options::OPT_t, options::OPT_u_Group});
 
   AddLinkerInputs(HTC, Inputs, Args, CmdArgs, JA);
 
   //----------------------------------------------------------------------------
   // Libraries
   //----------------------------------------------------------------------------
   if (IncStdLib && IncDefLibs) {
-    if (D.CCCIsCXX()) {
-      if (HTC.ShouldLinkCXXStdlib(Args))
-        HTC.AddCXXStdlibLibArgs(Args, CmdArgs);
+    if (HTC.ShouldLinkCXXStdlib(Args)) {
       CmdArgs.push_back("-lm");
     }
 
     CmdArgs.push_back("--start-group");
 
     if (!IsShared) {
       for (StringRef Lib : OsLibs)
         CmdArgs.push_back(Args.MakeArgString("-l" + Lib));

clang/lib/Driver/ToolChains/Minix.cpp

void tools::minix::Assembler::ConstructJob(Compilation &C, const JobAction &JA,
   C.addCommand(llvm::make_unique<Command>(JA, *this, Exec, CmdArgs, Inputs));
 }
 
 void tools::minix::Linker::ConstructJob(Compilation &C, const JobAction &JA,
                                  const InputInfo &Output,
                                  const InputInfoList &Inputs,
                                  const ArgList &Args,
                                  const char *LinkingOutput) const {
-  const Driver &D = getToolChain().getDriver();
   ArgStringList CmdArgs;
 
   if (Output.isFilename()) {
     CmdArgs.push_back("-o");
     CmdArgs.push_back(Output.getFilename());
   } else {
     assert(Output.isNothing() && "Invalid output.");
   }
void tools::minix::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   Args.AddAllArgs(CmdArgs,
                   {options::OPT_L, options::OPT_T_Group, options::OPT_e});
 
   AddLinkerInputs(getToolChain(), Inputs, Args, CmdArgs, JA);
 
   getToolChain().addProfileRTLibs(Args, CmdArgs);
 
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
-    if (D.CCCIsCXX()) {
-      if (getToolChain().ShouldLinkCXXStdlib(Args))
-        getToolChain().AddCXXStdlibLibArgs(Args, CmdArgs);
+    if (getToolChain().ShouldLinkCXXStdlib(Args)) {
       CmdArgs.push_back("-lm");
     }
   }
 
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles)) {
     if (Args.hasArg(options::OPT_pthread))
       CmdArgs.push_back("-lpthread");
     CmdArgs.push_back("-lc");

clang/lib/Driver/ToolChains/NaCl.cpp

void nacltools::Linker::ConstructJob(Compilation &C, const JobAction &JA,
 
   ToolChain.AddFilePathLibArgs(Args, CmdArgs);
 
   if (Args.hasArg(options::OPT_Z_Xlinker__no_demangle))
     CmdArgs.push_back("--no-demangle");
 
   AddLinkerInputs(ToolChain, Inputs, Args, CmdArgs, JA);
 
-  if (D.CCCIsCXX() &&
-      !Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
-    if (ToolChain.ShouldLinkCXXStdlib(Args)) {
-      bool OnlyLibstdcxxStatic =
-          Args.hasArg(options::OPT_static_libstdcxx) && !IsStatic;
-      if (OnlyLibstdcxxStatic)
-        CmdArgs.push_back("-Bstatic");
-      ToolChain.AddCXXStdlibLibArgs(Args, CmdArgs);
-      if (OnlyLibstdcxxStatic)
-        CmdArgs.push_back("-Bdynamic");
-    }
     CmdArgs.push_back("-lm");
   }
 
   if (!Args.hasArg(options::OPT_nostdlib)) {
     if (!Args.hasArg(options::OPT_nodefaultlibs)) {
       // Always use groups, since it has no effect on dynamic libraries.
       CmdArgs.push_back("--start-group");
       CmdArgs.push_back("-lc");

clang/lib/Driver/ToolChains/NetBSD.cpp

case llvm::Triple::x86_64:
       break;
     default:
       break;
     }
   }
 
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
     addOpenMPRuntime(CmdArgs, getToolChain(), Args);
-    if (D.CCCIsCXX()) {
-      if (ToolChain.ShouldLinkCXXStdlib(Args))
-        ToolChain.AddCXXStdlibLibArgs(Args, CmdArgs);
+    if (ToolChain.ShouldLinkCXXStdlib(Args)) {
       CmdArgs.push_back("-lm");
     }
     if (NeedsSanitizerDeps)
       linkSanitizerRuntimeDeps(getToolChain(), CmdArgs);
     if (NeedsXRayDeps)
       linkXRayRuntimeDeps(ToolChain, CmdArgs);
     if (Args.hasArg(options::OPT_pthread))
       CmdArgs.push_back("-lpthread");

clang/lib/Driver/ToolChains/OpenBSD.cpp

 
 void openbsd::Linker::ConstructJob(Compilation &C, const JobAction &JA,
                                    const InputInfo &Output,
                                    const InputInfoList &Inputs,
                                    const ArgList &Args,
                                    const char *LinkingOutput) const {
   const toolchains::OpenBSD &ToolChain =
       static_cast<const toolchains::OpenBSD &>(getToolChain());
-  const Driver &D = getToolChain().getDriver();
   ArgStringList CmdArgs;
 
   // Silence warning for "clang -g foo.o -o foo"
   Args.ClaimAllArgs(options::OPT_g_Group);
   // and "clang -emit-llvm foo.o -o foo"
   Args.ClaimAllArgs(options::OPT_emit_llvm);
   // and for "clang -w foo.o -o foo". Other warning options are already
   // handled somewhere else.
Args.AddAllArgs(CmdArgs, {options::OPT_T_Group, options::OPT_e,
                             options::OPT_s, options::OPT_t,
                             options::OPT_Z_Flag, options::OPT_r});
 
   bool NeedsSanitizerDeps = addSanitizerRuntimes(ToolChain, Args, CmdArgs);
   bool NeedsXRayDeps = addXRayRuntime(ToolChain, Args, CmdArgs);
   AddLinkerInputs(ToolChain, Inputs, Args, CmdArgs, JA);
 
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
-    if (D.CCCIsCXX()) {
-      if (ToolChain.ShouldLinkCXXStdlib(Args))
-        ToolChain.AddCXXStdlibLibArgs(Args, CmdArgs);
+    if (ToolChain.ShouldLinkCXXStdlib(Args)) {
       if (Args.hasArg(options::OPT_pg))
         CmdArgs.push_back("-lm_p");
       else
         CmdArgs.push_back("-lm");
     }
     if (NeedsSanitizerDeps) {
       CmdArgs.push_back(ToolChain.getCompilerRTArgString(Args, "builtins", false));
       linkSanitizerRuntimeDeps(ToolChain, CmdArgs);

clang/lib/Driver/ToolChains/PS4CPU.cpp

if (Args.hasArg(options::OPT_Z_Xlinker__no_demangle))
     CmdArgs.push_back("--no-demangle");
 
   AddLinkerInputs(ToolChain, Inputs, Args, CmdArgs, JA);
 
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
     // For PS4, we always want to pass libm, libstdc++ and libkernel
     // libraries for both C and C++ compilations.
     CmdArgs.push_back("-lkernel");
-    if (D.CCCIsCXX()) {
-      if (ToolChain.ShouldLinkCXXStdlib(Args))
-        ToolChain.AddCXXStdlibLibArgs(Args, CmdArgs);
+    if (ToolChain.ShouldLinkCXXStdlib(Args)) {
       if (Args.hasArg(options::OPT_pg))
         CmdArgs.push_back("-lm_p");
       else
         CmdArgs.push_back("-lm");
     }
     // FIXME: For some reason GCC passes -lgcc and -lgcc_s before adding
     // the default system libraries. Just mimic this for now.
     if (Args.hasArg(options::OPT_pg))

clang/test/Driver/nostdlibxx.cpp

 // RUN: %clangxx -target i686-pc-linux-gnu -### -nostdlib++ %s 2> %t
 // RUN: FileCheck < %t %s
 
-// We should still have -lm and the C standard libraries, but not -lstdc++.
+// We should still have C standard libraries, but not -lstdc++.
 
 // CHECK-NOT: -lstdc++
 // CHECK-NOT: -lc++
-// CHECK: -lm