[Driver] Use libatomic for 32-bit SPARC atomics support on Linux

[ro]

Summary

This is the Linux/sparc64 equivalent to D118021, necessary to provide an external implementation of atomics on 32-bit SPARC which LLVM cannot inline even with -mcpu=v9 or an equivalent default.

Tested on sparc64-unknown-linux-gnu.

[glaubitz]

This should address the current CI issues on sparc64 together with D130571.

[MaskRay]

clang/lib/Driver/ToolChains/Gnu.cpp

634

// TODO ... and attach a bug link

637

Such --as-needed usage is a bit fragile if clang driver in the future passes --as-needed in the beginning. Better to use --push-state if you have a not-too-old ld.

[MaskRay]

This needs a test. For now you can use linux-ld.c which has some sparc tests.

[ro]

clang/lib/Driver/ToolChains/Gnu.cpp

634

It's split between the original bug report and a patch review: I only noticed later (when compiler-rt started to make use of 64-bit atomics) that those aren't implemented by clang -m32 -mcpu=v9.

637

Good point: I initially fell into that trap in D130571 when I added --as-needed -latomic --no-as-needed to SANITIZER_COMMON_LINK_FLAGS: it got added early to the link line, before the objects using atomics, and had no effect. Only then did Iearn that target_link_libraries accepts not only library names, but also linker flags.

As for --push-state, it was introduced in GNU ld 2.25 back in 2014, to it's save to assume it's present.

[ro]

• Switch to --push-state/--pop-state.
• Add testcase.

clang/lib/Driver/ToolChains/Gnu.cpp

if (!Args.hasArg(options::OPT_nodefaultlibs)) {
                            JA.isHostOffloading(Action::OFK_OpenMP),
                            /* GompNeedsRT= */ true))
         // OpenMP runtimes implies pthreads when using the GNU toolchain.
         // FIXME: Does this really make sense for all GNU toolchains?
         WantPthread = true;
 
       AddRunTimeLibs(ToolChain, D, CmdArgs, Args);
 
+      // LLVM support for atomics on 32-bit SPARC V8+ is incomplete, so
+      // forcibly link with libatomic as a workaround.
+      // TODO: Issue #41880 and D118021.
+      if (getToolChain().getTriple().getArch() == llvm::Triple::sparc) {
+        CmdArgs.push_back("--push-state");
+        CmdArgs.push_back("--as-needed");
+        CmdArgs.push_back("-latomic");
+        CmdArgs.push_back("--pop-state");
+      }
+
       if (WantPthread && !isAndroid)
         CmdArgs.push_back("-lpthread");
 
       if (Args.hasArg(options::OPT_fsplit_stack))
         CmdArgs.push_back("--wrap=pthread_create");
 
       if (!Args.hasArg(options::OPT_nolibc))
         CmdArgs.push_back("-lc");

clang/test/Driver/linux-ld.c

 // CHECK-MIPS64EL-GNUABI64-NOT: "--hash-style={{gnu|both}}"
 //
 // RUN: %clang -### %s -no-pie 2>&1 \
 // RUN:     --target=sparc-unknown-linux-gnu \
 // RUN:   | FileCheck --check-prefix=CHECK-SPARCV8 %s
 // CHECK-SPARCV8: "{{.*}}ld{{(.exe)?}}"
 // CHECK-SPARCV8: "-m" "elf32_sparc"
 // CHECK-SPARCV8: "-dynamic-linker" "{{(/usr/sparc-unknown-linux-gnu)?}}/lib/ld-linux.so.2"
+// CHECK-SPARCV8: "--push-state" "--as-needed" "-latomic" "--pop-state"
 //
 // RUN: %clang -### %s -no-pie 2>&1 \
 // RUN:     --target=sparcel-unknown-linux-gnu \
 // RUN:   | FileCheck --check-prefix=CHECK-SPARCV8EL %s
 // CHECK-SPARCV8EL: "{{.*}}ld{{(.exe)?}}"
 // CHECK-SPARCV8EL: "-m" "elf32_sparc"
 // CHECK-SPARCV8EL: "-dynamic-linker" "{{(/usr/sparcel-unknown-linux-gnu)?}}/lib/ld-linux.so.2"
 //
 // RUN: %clang -### %s -no-pie 2>&1 \
 // RUN:     --target=sparcv9-unknown-linux-gnu \
 // RUN:   | FileCheck --check-prefix=CHECK-SPARCV9 %s
 // CHECK-SPARCV9: "{{.*}}ld{{(.exe)?}}"
 // CHECK-SPARCV9: "-m" "elf64_sparc"
 // CHECK-SPARCV9: "-dynamic-linker" "{{(/usr/sparcv9-unknown-linux-gnu)?}}/lib{{(64)?}}/ld-linux.so.2"
+// CHECK-SPARCV9-NOT: "-latomic"
 
 // Test linker invocation on Android.
 // RUN: %clang -### %s -no-pie 2>&1 \
 // RUN:     --target=arm-linux-androideabi -rtlib=platform --unwindlib=platform \
 // RUN:     --gcc-toolchain="" \
 // RUN:     --sysroot=%S/Inputs/basic_android_tree/sysroot \
 // RUN:   | FileCheck --check-prefix=CHECK-ANDROID %s
 // RUN: %clang -### %s -no-pie 2>&1 \