xref: /freebsd/contrib/llvm-project/clang/lib/Driver/ToolChains/WebAssembly.cpp (revision e6bfd18d21b225af6a0ed67ceeaf1293b7b9eba5)
1 //===--- WebAssembly.cpp - WebAssembly ToolChain Implementation -*- C++ -*-===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 
9 #include "WebAssembly.h"
10 #include "CommonArgs.h"
11 #include "Gnu.h"
12 #include "clang/Basic/Version.h"
13 #include "clang/Config/config.h"
14 #include "clang/Driver/Compilation.h"
15 #include "clang/Driver/Driver.h"
16 #include "clang/Driver/DriverDiagnostic.h"
17 #include "clang/Driver/Options.h"
18 #include "llvm/Option/ArgList.h"
19 #include "llvm/Support/FileSystem.h"
20 #include "llvm/Support/Path.h"
21 #include "llvm/Support/VirtualFileSystem.h"
22 
23 using namespace clang::driver;
24 using namespace clang::driver::tools;
25 using namespace clang::driver::toolchains;
26 using namespace clang;
27 using namespace llvm::opt;
28 
29 /// Following the conventions in https://wiki.debian.org/Multiarch/Tuples,
30 /// we remove the vendor field to form the multiarch triple.
31 std::string WebAssembly::getMultiarchTriple(const Driver &D,
32                                             const llvm::Triple &TargetTriple,
33                                             StringRef SysRoot) const {
34     return (TargetTriple.getArchName() + "-" +
35             TargetTriple.getOSAndEnvironmentName()).str();
36 }
37 
38 std::string wasm::Linker::getLinkerPath(const ArgList &Args) const {
39   const ToolChain &ToolChain = getToolChain();
40   if (const Arg* A = Args.getLastArg(options::OPT_fuse_ld_EQ)) {
41     StringRef UseLinker = A->getValue();
42     if (!UseLinker.empty()) {
43       if (llvm::sys::path::is_absolute(UseLinker) &&
44           llvm::sys::fs::can_execute(UseLinker))
45         return std::string(UseLinker);
46 
47       // Accept 'lld', and 'ld' as aliases for the default linker
48       if (UseLinker != "lld" && UseLinker != "ld")
49         ToolChain.getDriver().Diag(diag::err_drv_invalid_linker_name)
50             << A->getAsString(Args);
51     }
52   }
53 
54   return ToolChain.GetProgramPath(ToolChain.getDefaultLinker());
55 }
56 
57 void wasm::Linker::ConstructJob(Compilation &C, const JobAction &JA,
58                                 const InputInfo &Output,
59                                 const InputInfoList &Inputs,
60                                 const ArgList &Args,
61                                 const char *LinkingOutput) const {
62 
63   const ToolChain &ToolChain = getToolChain();
64   const char *Linker = Args.MakeArgString(getLinkerPath(Args));
65   ArgStringList CmdArgs;
66 
67   CmdArgs.push_back("-m");
68   if (ToolChain.getTriple().isArch64Bit())
69     CmdArgs.push_back("wasm64");
70   else
71     CmdArgs.push_back("wasm32");
72 
73   if (Args.hasArg(options::OPT_s))
74     CmdArgs.push_back("--strip-all");
75 
76   Args.AddAllArgs(CmdArgs, options::OPT_L);
77   Args.AddAllArgs(CmdArgs, options::OPT_u);
78   ToolChain.AddFilePathLibArgs(Args, CmdArgs);
79 
80   const char *Crt1 = "crt1.o";
81   const char *Entry = nullptr;
82 
83   // If crt1-command.o exists, it supports new-style commands, so use it.
84   // Otherwise, use the old crt1.o. This is a temporary transition measure.
85   // Once WASI libc no longer needs to support LLVM versions which lack
86   // support for new-style command, it can make crt1.o the same as
87   // crt1-command.o. And once LLVM no longer needs to support WASI libc
88   // versions before that, it can switch to using crt1-command.o.
89   if (ToolChain.GetFilePath("crt1-command.o") != "crt1-command.o")
90     Crt1 = "crt1-command.o";
91 
92   if (const Arg *A = Args.getLastArg(options::OPT_mexec_model_EQ)) {
93     StringRef CM = A->getValue();
94     if (CM == "command") {
95       // Use default values.
96     } else if (CM == "reactor") {
97       Crt1 = "crt1-reactor.o";
98       Entry = "_initialize";
99     } else {
100       ToolChain.getDriver().Diag(diag::err_drv_invalid_argument_to_option)
101           << CM << A->getOption().getName();
102     }
103   }
104   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles))
105     CmdArgs.push_back(Args.MakeArgString(ToolChain.GetFilePath(Crt1)));
106   if (Entry) {
107     CmdArgs.push_back(Args.MakeArgString("--entry"));
108     CmdArgs.push_back(Args.MakeArgString(Entry));
109   }
110 
111   AddLinkerInputs(ToolChain, Inputs, Args, CmdArgs, JA);
112 
113   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
114     if (ToolChain.ShouldLinkCXXStdlib(Args))
115       ToolChain.AddCXXStdlibLibArgs(Args, CmdArgs);
116 
117     if (Args.hasArg(options::OPT_pthread)) {
118       CmdArgs.push_back("-lpthread");
119       CmdArgs.push_back("--shared-memory");
120     }
121 
122     CmdArgs.push_back("-lc");
123     AddRunTimeLibs(ToolChain, ToolChain.getDriver(), CmdArgs, Args);
124   }
125 
126   CmdArgs.push_back("-o");
127   CmdArgs.push_back(Output.getFilename());
128 
129   C.addCommand(std::make_unique<Command>(JA, *this,
130                                          ResponseFileSupport::AtFileCurCP(),
131                                          Linker, CmdArgs, Inputs, Output));
132 
133   // When optimizing, if wasm-opt is available, run it.
134   if (Arg *A = Args.getLastArg(options::OPT_O_Group)) {
135     auto WasmOptPath = ToolChain.GetProgramPath("wasm-opt");
136     if (WasmOptPath != "wasm-opt") {
137       StringRef OOpt = "s";
138       if (A->getOption().matches(options::OPT_O4) ||
139           A->getOption().matches(options::OPT_Ofast))
140         OOpt = "4";
141       else if (A->getOption().matches(options::OPT_O0))
142         OOpt = "0";
143       else if (A->getOption().matches(options::OPT_O))
144         OOpt = A->getValue();
145 
146       if (OOpt != "0") {
147         const char *WasmOpt = Args.MakeArgString(WasmOptPath);
148         ArgStringList CmdArgs;
149         CmdArgs.push_back(Output.getFilename());
150         CmdArgs.push_back(Args.MakeArgString(llvm::Twine("-O") + OOpt));
151         CmdArgs.push_back("-o");
152         CmdArgs.push_back(Output.getFilename());
153         C.addCommand(std::make_unique<Command>(
154             JA, *this, ResponseFileSupport::AtFileCurCP(), WasmOpt, CmdArgs,
155             Inputs, Output));
156       }
157     }
158   }
159 }
160 
161 /// Given a base library directory, append path components to form the
162 /// LTO directory.
163 static std::string AppendLTOLibDir(const std::string &Dir) {
164     // The version allows the path to be keyed to the specific version of
165     // LLVM in used, as the bitcode format is not stable.
166     return Dir + "/llvm-lto/" LLVM_VERSION_STRING;
167 }
168 
169 WebAssembly::WebAssembly(const Driver &D, const llvm::Triple &Triple,
170                          const llvm::opt::ArgList &Args)
171     : ToolChain(D, Triple, Args) {
172 
173   assert(Triple.isArch32Bit() != Triple.isArch64Bit());
174 
175   getProgramPaths().push_back(getDriver().getInstalledDir());
176 
177   auto SysRoot = getDriver().SysRoot;
178   if (getTriple().getOS() == llvm::Triple::UnknownOS) {
179     // Theoretically an "unknown" OS should mean no standard libraries, however
180     // it could also mean that a custom set of libraries is in use, so just add
181     // /lib to the search path. Disable multiarch in this case, to discourage
182     // paths containing "unknown" from acquiring meanings.
183     getFilePaths().push_back(SysRoot + "/lib");
184   } else {
185     const std::string MultiarchTriple =
186         getMultiarchTriple(getDriver(), Triple, SysRoot);
187     if (D.isUsingLTO()) {
188       // For LTO, enable use of lto-enabled sysroot libraries too, if available.
189       // Note that the directory is keyed to the LLVM revision, as LLVM's
190       // bitcode format is not stable.
191       auto Dir = AppendLTOLibDir(SysRoot + "/lib/" + MultiarchTriple);
192       getFilePaths().push_back(Dir);
193     }
194     getFilePaths().push_back(SysRoot + "/lib/" + MultiarchTriple);
195   }
196 }
197 
198 bool WebAssembly::IsMathErrnoDefault() const { return false; }
199 
200 bool WebAssembly::IsObjCNonFragileABIDefault() const { return true; }
201 
202 bool WebAssembly::UseObjCMixedDispatch() const { return true; }
203 
204 bool WebAssembly::isPICDefault() const { return false; }
205 
206 bool WebAssembly::isPIEDefault(const llvm::opt::ArgList &Args) const {
207   return false;
208 }
209 
210 bool WebAssembly::isPICDefaultForced() const { return false; }
211 
212 bool WebAssembly::IsIntegratedAssemblerDefault() const { return true; }
213 
214 bool WebAssembly::hasBlocksRuntime() const { return false; }
215 
216 // TODO: Support profiling.
217 bool WebAssembly::SupportsProfiling() const { return false; }
218 
219 bool WebAssembly::HasNativeLLVMSupport() const { return true; }
220 
221 void WebAssembly::addClangTargetOptions(const ArgList &DriverArgs,
222                                         ArgStringList &CC1Args,
223                                         Action::OffloadKind) const {
224   if (!DriverArgs.hasFlag(clang::driver::options::OPT_fuse_init_array,
225                           options::OPT_fno_use_init_array, true))
226     CC1Args.push_back("-fno-use-init-array");
227 
228   // '-pthread' implies atomics, bulk-memory, mutable-globals, and sign-ext
229   if (DriverArgs.hasFlag(options::OPT_pthread, options::OPT_no_pthread,
230                          false)) {
231     if (DriverArgs.hasFlag(options::OPT_mno_atomics, options::OPT_matomics,
232                            false))
233       getDriver().Diag(diag::err_drv_argument_not_allowed_with)
234           << "-pthread"
235           << "-mno-atomics";
236     if (DriverArgs.hasFlag(options::OPT_mno_bulk_memory,
237                            options::OPT_mbulk_memory, false))
238       getDriver().Diag(diag::err_drv_argument_not_allowed_with)
239           << "-pthread"
240           << "-mno-bulk-memory";
241     if (DriverArgs.hasFlag(options::OPT_mno_mutable_globals,
242                            options::OPT_mmutable_globals, false))
243       getDriver().Diag(diag::err_drv_argument_not_allowed_with)
244           << "-pthread"
245           << "-mno-mutable-globals";
246     if (DriverArgs.hasFlag(options::OPT_mno_sign_ext, options::OPT_msign_ext,
247                            false))
248       getDriver().Diag(diag::err_drv_argument_not_allowed_with)
249           << "-pthread"
250           << "-mno-sign-ext";
251     CC1Args.push_back("-target-feature");
252     CC1Args.push_back("+atomics");
253     CC1Args.push_back("-target-feature");
254     CC1Args.push_back("+bulk-memory");
255     CC1Args.push_back("-target-feature");
256     CC1Args.push_back("+mutable-globals");
257     CC1Args.push_back("-target-feature");
258     CC1Args.push_back("+sign-ext");
259   }
260 
261   if (!DriverArgs.hasFlag(options::OPT_mmutable_globals,
262                           options::OPT_mno_mutable_globals, false)) {
263     // -fPIC implies +mutable-globals because the PIC ABI used by the linker
264     // depends on importing and exporting mutable globals.
265     llvm::Reloc::Model RelocationModel;
266     unsigned PICLevel;
267     bool IsPIE;
268     std::tie(RelocationModel, PICLevel, IsPIE) =
269         ParsePICArgs(*this, DriverArgs);
270     if (RelocationModel == llvm::Reloc::PIC_) {
271       if (DriverArgs.hasFlag(options::OPT_mno_mutable_globals,
272                              options::OPT_mmutable_globals, false)) {
273         getDriver().Diag(diag::err_drv_argument_not_allowed_with)
274             << "-fPIC"
275             << "-mno-mutable-globals";
276       }
277       CC1Args.push_back("-target-feature");
278       CC1Args.push_back("+mutable-globals");
279     }
280   }
281 
282   if (DriverArgs.getLastArg(options::OPT_fwasm_exceptions)) {
283     // '-fwasm-exceptions' is not compatible with '-mno-exception-handling'
284     if (DriverArgs.hasFlag(options::OPT_mno_exception_handing,
285                            options::OPT_mexception_handing, false))
286       getDriver().Diag(diag::err_drv_argument_not_allowed_with)
287           << "-fwasm-exceptions"
288           << "-mno-exception-handling";
289     // '-fwasm-exceptions' is not compatible with
290     // '-mllvm -enable-emscripten-cxx-exceptions'
291     for (const Arg *A : DriverArgs.filtered(options::OPT_mllvm)) {
292       if (StringRef(A->getValue(0)) == "-enable-emscripten-cxx-exceptions")
293         getDriver().Diag(diag::err_drv_argument_not_allowed_with)
294             << "-fwasm-exceptions"
295             << "-mllvm -enable-emscripten-cxx-exceptions";
296     }
297     // '-fwasm-exceptions' implies exception-handling feature
298     CC1Args.push_back("-target-feature");
299     CC1Args.push_back("+exception-handling");
300     // Backend needs -wasm-enable-eh to enable Wasm EH
301     CC1Args.push_back("-mllvm");
302     CC1Args.push_back("-wasm-enable-eh");
303   }
304 
305   for (const Arg *A : DriverArgs.filtered(options::OPT_mllvm)) {
306     StringRef Opt = A->getValue(0);
307     if (Opt.startswith("-emscripten-cxx-exceptions-allowed")) {
308       // '-mllvm -emscripten-cxx-exceptions-allowed' should be used with
309       // '-mllvm -enable-emscripten-cxx-exceptions'
310       bool EmEHArgExists = false;
311       for (const Arg *A : DriverArgs.filtered(options::OPT_mllvm)) {
312         if (StringRef(A->getValue(0)) == "-enable-emscripten-cxx-exceptions") {
313           EmEHArgExists = true;
314           break;
315         }
316       }
317       if (!EmEHArgExists)
318         getDriver().Diag(diag::err_drv_argument_only_allowed_with)
319             << "-mllvm -emscripten-cxx-exceptions-allowed"
320             << "-mllvm -enable-emscripten-cxx-exceptions";
321 
322       // Prevent functions specified in -emscripten-cxx-exceptions-allowed list
323       // from being inlined before reaching the wasm backend.
324       StringRef FuncNamesStr = Opt.split('=').second;
325       SmallVector<StringRef, 4> FuncNames;
326       FuncNamesStr.split(FuncNames, ',');
327       for (auto Name : FuncNames) {
328         CC1Args.push_back("-mllvm");
329         CC1Args.push_back(DriverArgs.MakeArgString("--force-attribute=" + Name +
330                                                    ":noinline"));
331       }
332     }
333 
334     if (Opt.startswith("-wasm-enable-sjlj")) {
335       // '-mllvm -wasm-enable-sjlj' is not compatible with
336       // '-mno-exception-handling'
337       if (DriverArgs.hasFlag(options::OPT_mno_exception_handing,
338                              options::OPT_mexception_handing, false))
339         getDriver().Diag(diag::err_drv_argument_not_allowed_with)
340             << "-mllvm -wasm-enable-sjlj"
341             << "-mno-exception-handling";
342       // '-mllvm -wasm-enable-sjlj' is not compatible with
343       // '-mllvm -enable-emscripten-cxx-exceptions'
344       // because we don't allow Emscripten EH + Wasm SjLj
345       for (const Arg *A : DriverArgs.filtered(options::OPT_mllvm)) {
346         if (StringRef(A->getValue(0)) == "-enable-emscripten-cxx-exceptions")
347           getDriver().Diag(diag::err_drv_argument_not_allowed_with)
348               << "-mllvm -wasm-enable-sjlj"
349               << "-mllvm -enable-emscripten-cxx-exceptions";
350       }
351       // '-mllvm -wasm-enable-sjlj' is not compatible with
352       // '-mllvm -enable-emscripten-sjlj'
353       for (const Arg *A : DriverArgs.filtered(options::OPT_mllvm)) {
354         if (StringRef(A->getValue(0)) == "-enable-emscripten-sjlj")
355           getDriver().Diag(diag::err_drv_argument_not_allowed_with)
356               << "-mllvm -wasm-enable-sjlj"
357               << "-mllvm -enable-emscripten-sjlj";
358       }
359       // '-mllvm -wasm-enable-sjlj' implies exception-handling feature
360       CC1Args.push_back("-target-feature");
361       CC1Args.push_back("+exception-handling");
362       // Backend needs '-exception-model=wasm' to use Wasm EH instructions
363       CC1Args.push_back("-exception-model=wasm");
364     }
365   }
366 }
367 
368 ToolChain::RuntimeLibType WebAssembly::GetDefaultRuntimeLibType() const {
369   return ToolChain::RLT_CompilerRT;
370 }
371 
372 ToolChain::CXXStdlibType
373 WebAssembly::GetCXXStdlibType(const ArgList &Args) const {
374   if (Arg *A = Args.getLastArg(options::OPT_stdlib_EQ)) {
375     StringRef Value = A->getValue();
376     if (Value == "libc++")
377       return ToolChain::CST_Libcxx;
378     else if (Value == "libstdc++")
379       return ToolChain::CST_Libstdcxx;
380     else
381       getDriver().Diag(diag::err_drv_invalid_stdlib_name)
382           << A->getAsString(Args);
383   }
384   return ToolChain::CST_Libcxx;
385 }
386 
387 void WebAssembly::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
388                                             ArgStringList &CC1Args) const {
389   if (DriverArgs.hasArg(clang::driver::options::OPT_nostdinc))
390     return;
391 
392   const Driver &D = getDriver();
393 
394   if (!DriverArgs.hasArg(options::OPT_nobuiltininc)) {
395     SmallString<128> P(D.ResourceDir);
396     llvm::sys::path::append(P, "include");
397     addSystemInclude(DriverArgs, CC1Args, P);
398   }
399 
400   if (DriverArgs.hasArg(options::OPT_nostdlibinc))
401     return;
402 
403   // Check for configure-time C include directories.
404   StringRef CIncludeDirs(C_INCLUDE_DIRS);
405   if (CIncludeDirs != "") {
406     SmallVector<StringRef, 5> dirs;
407     CIncludeDirs.split(dirs, ":");
408     for (StringRef dir : dirs) {
409       StringRef Prefix =
410           llvm::sys::path::is_absolute(dir) ? "" : StringRef(D.SysRoot);
411       addExternCSystemInclude(DriverArgs, CC1Args, Prefix + dir);
412     }
413     return;
414   }
415 
416   if (getTriple().getOS() != llvm::Triple::UnknownOS) {
417     const std::string MultiarchTriple =
418         getMultiarchTriple(D, getTriple(), D.SysRoot);
419     addSystemInclude(DriverArgs, CC1Args, D.SysRoot + "/include/" + MultiarchTriple);
420   }
421   addSystemInclude(DriverArgs, CC1Args, D.SysRoot + "/include");
422 }
423 
424 void WebAssembly::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,
425                                                ArgStringList &CC1Args) const {
426 
427   if (DriverArgs.hasArg(options::OPT_nostdlibinc) ||
428       DriverArgs.hasArg(options::OPT_nostdincxx))
429     return;
430 
431   switch (GetCXXStdlibType(DriverArgs)) {
432   case ToolChain::CST_Libcxx:
433     addLibCxxIncludePaths(DriverArgs, CC1Args);
434     break;
435   case ToolChain::CST_Libstdcxx:
436     addLibStdCXXIncludePaths(DriverArgs, CC1Args);
437     break;
438   }
439 }
440 
441 void WebAssembly::AddCXXStdlibLibArgs(const llvm::opt::ArgList &Args,
442                                       llvm::opt::ArgStringList &CmdArgs) const {
443 
444   switch (GetCXXStdlibType(Args)) {
445   case ToolChain::CST_Libcxx:
446     CmdArgs.push_back("-lc++");
447     if (Args.hasArg(options::OPT_fexperimental_library))
448       CmdArgs.push_back("-lc++experimental");
449     CmdArgs.push_back("-lc++abi");
450     break;
451   case ToolChain::CST_Libstdcxx:
452     CmdArgs.push_back("-lstdc++");
453     break;
454   }
455 }
456 
457 SanitizerMask WebAssembly::getSupportedSanitizers() const {
458   SanitizerMask Res = ToolChain::getSupportedSanitizers();
459   if (getTriple().isOSEmscripten()) {
460     Res |= SanitizerKind::Vptr | SanitizerKind::Leak | SanitizerKind::Address;
461   }
462   return Res;
463 }
464 
465 Tool *WebAssembly::buildLinker() const {
466   return new tools::wasm::Linker(*this);
467 }
468 
469 void WebAssembly::addLibCxxIncludePaths(
470     const llvm::opt::ArgList &DriverArgs,
471     llvm::opt::ArgStringList &CC1Args) const {
472   const Driver &D = getDriver();
473   std::string SysRoot = computeSysRoot();
474   std::string LibPath = SysRoot + "/include";
475   const std::string MultiarchTriple =
476       getMultiarchTriple(D, getTriple(), SysRoot);
477   bool IsKnownOs = (getTriple().getOS() != llvm::Triple::UnknownOS);
478 
479   std::string Version = detectLibcxxVersion(LibPath);
480   if (Version.empty())
481     return;
482 
483   // First add the per-target include path if the OS is known.
484   if (IsKnownOs) {
485     std::string TargetDir = LibPath + "/" + MultiarchTriple + "/c++/" + Version;
486     addSystemInclude(DriverArgs, CC1Args, TargetDir);
487   }
488 
489   // Second add the generic one.
490   addSystemInclude(DriverArgs, CC1Args, LibPath + "/c++/" + Version);
491 }
492 
493 void WebAssembly::addLibStdCXXIncludePaths(
494     const llvm::opt::ArgList &DriverArgs,
495     llvm::opt::ArgStringList &CC1Args) const {
496   // We cannot use GCCInstallationDetector here as the sysroot usually does
497   // not contain a full GCC installation.
498   // Instead, we search the given sysroot for /usr/include/xx, similar
499   // to how we do it for libc++.
500   const Driver &D = getDriver();
501   std::string SysRoot = computeSysRoot();
502   std::string LibPath = SysRoot + "/include";
503   const std::string MultiarchTriple =
504       getMultiarchTriple(D, getTriple(), SysRoot);
505   bool IsKnownOs = (getTriple().getOS() != llvm::Triple::UnknownOS);
506 
507   // This is similar to detectLibcxxVersion()
508   std::string Version;
509   {
510     std::error_code EC;
511     Generic_GCC::GCCVersion MaxVersion =
512         Generic_GCC::GCCVersion::Parse("0.0.0");
513     SmallString<128> Path(LibPath);
514     llvm::sys::path::append(Path, "c++");
515     for (llvm::vfs::directory_iterator LI = getVFS().dir_begin(Path, EC), LE;
516          !EC && LI != LE; LI = LI.increment(EC)) {
517       StringRef VersionText = llvm::sys::path::filename(LI->path());
518       if (VersionText[0] != 'v') {
519         auto Version = Generic_GCC::GCCVersion::Parse(VersionText);
520         if (Version > MaxVersion)
521           MaxVersion = Version;
522       }
523     }
524     if (MaxVersion.Major > 0)
525       Version = MaxVersion.Text;
526   }
527 
528   if (Version.empty())
529     return;
530 
531   // First add the per-target include path if the OS is known.
532   if (IsKnownOs) {
533     std::string TargetDir = LibPath + "/c++/" + Version + "/" + MultiarchTriple;
534     addSystemInclude(DriverArgs, CC1Args, TargetDir);
535   }
536 
537   // Second add the generic one.
538   addSystemInclude(DriverArgs, CC1Args, LibPath + "/c++/" + Version);
539   // Third the backward one.
540   addSystemInclude(DriverArgs, CC1Args, LibPath + "/c++/" + Version + "/backward");
541 }
542