[Driver] Use -push-/-pop-state and -as-needed for libc++ on Fuchsia

[phosek]

Summary

This avoids introducing unnecessary DT_NEEDED entries when using
C++ driver for linking C code or C++ code that doesn't use C++
standard library.

[mcgrathr]

clang/lib/Driver/ToolChains/Fuchsia.cpp

125 ↗

(On Diff #171617)

I'd use the -- version of all these GNU-compatible options since that's the GNU-compatible syntax.
(But -static is still single-dash.)

132 ↗

(On Diff #171617)

-lm belongs inside the --as-needed umbrella too.

[MaskRay]

lib/Driver/ToolChains/Fuchsia.cpp

128

If Fuchsia doesn't use gold, it is fine. gold diverges from ld.bfd (lld) in that -static switches the whole link to its special static mode. (as usually while you link libstdc++/libc++ statically, you can still link other libraries normally)

In ld.bfd/lld, -Bstatic is synonym with -static.

[mcgrathr]

lib/Driver/ToolChains/Fuchsia.cpp

128

I think this part of the change was unintentional and should be undone.
Using --pop-state obviates the need for -Bdynamic to undo -Bstatic, but -Bstatic is still the right switch here.

[mcgrathr]

lib/Driver/ToolChains/Fuchsia.cpp

128

Actually, it's wrong two ways: the --pop-state should come before -lm. Neither -static nor -Bstatic should apply to -lm (or to -lc that comes later, which -static might). -static vs -Bstatic is only a latent bug given lld, but the -lm issue breaks the Zircon build today.

[phosek]

lib/Driver/ToolChains/Fuchsia.cpp

128

Addressed in D54082.

lib/Driver/ToolChains/Fuchsia.cpp

void fuchsia::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   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");
+          CmdArgs.push_back("-static");
         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");
 

test/Driver/fuchsia.cpp

 // CHECK: "--sysroot=[[SYSROOT]]"
 // CHECK: "-pie"
 // CHECK: "--build-id"
 // CHECK: "-dynamic-linker" "ld.so.1"
 // CHECK: Scrt1.o
 // CHECK-NOT: crti.o
 // CHECK-NOT: crtbegin.o
 // CHECK: "-L[[SYSROOT]]{{/|\\\\}}lib"
-// CHECK: "-lc++" "-lm"
+// CHECK: "--push-state"
+// CHECK: "--as-needed"
+// CHECK: "-lc++"
+// CHECK: "-lm"
+// CHECK: "--pop-state"
 // CHECK: "{{.*[/\\]}}libclang_rt.builtins-x86_64.a"
 // CHECK: "-lc"
 // CHECK-NOT: crtend.o
 // CHECK-NOT: crtn.o
 
 // RUN: %clangxx %s -### --target=x86_64-unknown-fuchsia -stdlib=libstdc++ \
 // RUN:     -fuse-ld=lld 2>&1 \
 // RUN:     | FileCheck %s -check-prefix=CHECK-STDLIB
 // CHECK-STDLIB: error: invalid library name in argument '-stdlib=libstdc++'
 
 // RUN: %clangxx %s -### --target=x86_64-unknown-fuchsia -static-libstdc++ \
 // RUN:     -fuse-ld=lld 2>&1 \
 // RUN:     | FileCheck %s -check-prefix=CHECK-STATIC
-// CHECK-STATIC: "-Bstatic"
+// CHECK-STATIC: "--push-state"
+// CHECK-STATIC: "--as-needed"
+// CHECK-STATIC: "-static"
 // CHECK-STATIC: "-lc++"
-// CHECK-STATIC: "-Bdynamic"
 // CHECK-STATIC: "-lm"
+// CHECK-STATIC: "--pop-state"
 // CHECK-STATIC: "-lc"