[clang][Fuchsia] Introduce compat multilibs

[leonardchan]

Summary

These are GCC-compatible multilibs that use the generic Itanium C++ ABI instead of the Fuchsia C++ ABI.

[phosek]

clang/lib/Driver/ToolChains/Fuchsia.cpp

249–250

We should probably build the compat multilib with exceptions to maximize compatibility.

[leonardchan]

clang/lib/Driver/ToolChains/Fuchsia.cpp

249–250

Can do. So just have the compat multilib only then?

[phosek]

LGTM

clang/lib/Driver/ToolChains/Fuchsia.cpp

245

clang/lib/Driver/ToolChains/Fuchsia.cpp

Fuchsia::Fuchsia(const Driver &D, const llvm::Triple &Triple,
   Multilibs.push_back(Multilib("relative-vtables+hwasan", {}, {}, 10)
                           .flag("+fexperimental-relative-c++-abi-vtables")
                           .flag("+fsanitize=hwaddress"));
   Multilibs.push_back(Multilib("relative-vtables+hwasan+noexcept", {}, {}, 11)
                           .flag("+fexperimental-relative-c++-abi-vtables")
                           .flag("+fsanitize=hwaddress")
                           .flag("-fexceptions")
                           .flag("+fno-exceptions"));
+  // Use Itanium C++ ABI for the compat multilib.
+  Multilibs.push_back(Multilib("compat", {}, {}, 12).flag("+fc++-abi=itanium"));
 
   Multilibs.FilterOut([&](const Multilib &M) {
     std::vector<std::string> RD = FilePaths(M);
     return std::all_of(RD.begin(), RD.end(), [&](std::string P) {
       return !getVFS().exists(P);
     });
   });
 
   Multilib::flags_list Flags;
   addMultilibFlag(
       Args.hasFlag(options::OPT_fexceptions, options::OPT_fno_exceptions, true),
       "fexceptions", Flags);
   addMultilibFlag(getSanitizerArgs().needsAsanRt(), "fsanitize=address", Flags);
   addMultilibFlag(getSanitizerArgs().needsHwasanRt(), "fsanitize=hwaddress",
                   Flags);
 
   addMultilibFlag(
       Args.hasFlag(options::OPT_fexperimental_relative_cxx_abi_vtables,
                    options::OPT_fno_experimental_relative_cxx_abi_vtables,
                    /*default=*/false),
       "fexperimental-relative-c++-abi-vtables", Flags);
+  addMultilibFlag(Args.getLastArgValue(options::OPT_fcxx_abi_EQ) == "itanium",
+                  "fc++-abi=itanium", Flags);
 
   Multilibs.setFilePathsCallback(FilePaths);
 
   if (Multilibs.select(Flags, SelectedMultilib))
     if (!SelectedMultilib.isDefault())
       if (const auto &PathsCallback = Multilibs.filePathsCallback())
         for (const auto &Path : PathsCallback(SelectedMultilib))
           // Prepend the multilib path to ensure it takes the precedence.

clang/test/Driver/fuchsia.cpp

 // RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \
 // RUN:     -fuse-ld=lld 2>&1\
 // RUN:     | FileCheck %s -check-prefixes=CHECK-MULTILIB-X86,CHECK-MULTILIB-RELATIVE-VTABLES-HWASAN-X86
 // RUN: %clangxx %s -### --target=x86_64-unknown-fuchsia -fexperimental-relative-c++-abi-vtables -fno-exceptions -fsanitize=hwaddress \
 // RUN:     -ccc-install-dir %S/Inputs/basic_fuchsia_tree/bin \
 // RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \
 // RUN:     -fuse-ld=lld 2>&1\
 // RUN:     | FileCheck %s -check-prefixes=CHECK-MULTILIB-X86,CHECK-MULTILIB-RELATIVE-VTABLES-HWASAN-NOEXCEPT-X86
+
+// Test compat multilibs.
+// RUN: %clangxx %s -### --target=x86_64-unknown-fuchsia -fc++-abi=itanium \
+// RUN:     -ccc-install-dir %S/Inputs/basic_fuchsia_tree/bin \
+// RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \
+// RUN:     -fuse-ld=lld 2>&1\
+// RUN:     | FileCheck %s -check-prefixes=CHECK-MULTILIB-X86,CHECK-MULTILIB-COMPAT-X86
+// RUN: %clangxx %s -### --target=x86_64-unknown-fuchsia -fc++-abi=itanium -fc++-abi=fuchsia \
+// RUN:     -ccc-install-dir %S/Inputs/basic_fuchsia_tree/bin \
+// RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \
+// RUN:     -fuse-ld=lld 2>&1\
+// RUN:     | FileCheck %s -check-prefixes=CHECK-MULTILIB-X86
+// RUN: %clangxx %s -### --target=x86_64-unknown-fuchsia -fc++-abi=fuchsia -fc++-abi=itanium \
+// RUN:     -ccc-install-dir %S/Inputs/basic_fuchsia_tree/bin \
+// RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \
+// RUN:     -fuse-ld=lld 2>&1\
+// RUN:     | FileCheck %s -check-prefixes=CHECK-MULTILIB-X86,CHECK-MULTILIB-COMPAT-X86
 // CHECK-MULTILIB-X86: "-resource-dir" "[[RESOURCE_DIR:[^"]+]]"
 // CHECK-MULTILIB-ASAN-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}asan"
 // CHECK-MULTILIB-NOEXCEPT-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}noexcept"
 // CHECK-MULTILIB-ASAN-NOEXCEPT-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}asan+noexcept"
 // CHECK-MULTILIB-RELATIVE-VTABLES-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}relative-vtables"
 // CHECK-MULTILIB-RELATIVE-VTABLES-NOEXCEPT-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}relative-vtables+noexcept"
 // CHECK-MULTILIB-RELATIVE-VTABLES-ASAN-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}relative-vtables+asan"
 // CHECK-MULTILIB-RELATIVE-VTABLES-ASAN-NOEXCEPT-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}relative-vtables+asan+noexcept"
 // CHECK-MULTILIB-HWASAN-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}hwasan"
 // CHECK-MULTILIB-HWASAN-NOEXCEPT-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}hwasan+noexcept"
 // CHECK-MULTILIB-RELATIVE-VTABLES-HWASAN-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}relative-vtables+hwasan"
 // CHECK-MULTILIB-RELATIVE-VTABLES-HWASAN-NOEXCEPT-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}relative-vtables+hwasan+noexcept"
+// CHECK-MULTILIB-COMPAT-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia{{/|\\\\}}compat"
 // CHECK-MULTILIB-X86: "-L{{.*}}{{/|\\\\}}..{{/|\\\\}}lib{{/|\\\\}}x86_64-unknown-fuchsia"