Provide default location of sysroot for Baremetal toolchain.

[abidh]

Summary

Currently, Baremetal toolchain requires user to pass a sysroot location
using a --sysroot flag. This is not very convenient for the user. It also
creates problem for toolchain vendors who don't have a fixed location to
put the sysroot bits.

Clang does provide 'DEFAULT_SYSROOT' which can be used by the toolchain
builder to provide the default location. But it does not work if toolchain
is targeting multiple targets e.g. arm-none-eabi/riscv64-unknown-elf which
clang is capable of doing.

This patch tries to solve this problem by providing a default location of
the toolchain if user does not explicitly provides --sysroot. The exact
location and name can be different but it should fulfill these conditions:

1. The sysroot path should have a target triple element so that multi-target

toolchain problem (as I described above) could be addressed.

2. The location should not be $TOP/$Triple as this is used by gcc generally

and will be a problem for installing both gcc and clang based toolchain at
the same location.

[jroelofs]

LGTM

clang/lib/Driver/ToolChains/BareMetal.cpp

105

Why not always add it, even if it doesn't exist? That's an easier failure mode to diagnose as a user, since you'll see the compiler appending paths that aren't there, rather than having magic behavior.

108

Small string has an implicit conversion to std::string, so you can just return SysRootDir; here.

[abidh]

Handled review comments.

[abidh]

clang/lib/Driver/ToolChains/BareMetal.cpp

108

I was getting a compile error on that. Looking in SmallString.h, it seems that conversion to std::string is explicit.

[jroelofs]

clang/lib/Driver/ToolChains/BareMetal.cpp

105

did return SysRootDir; not work? Why do you need the explicit std::string ctor call?

[jroelofs]

clang/lib/Driver/ToolChains/BareMetal.cpp

108

ohh, I see what's going on. doxygen doesn't list the explicit keyword in the html page, so I thought it was implicit. sorry.

[phosek]

clang/lib/Driver/ToolChains/BareMetal.cpp

102

I apologize for raising a comment on an older change but I ran into this recently, GCC uses ../sys-root as the default sysroot location and I think it'd be useful for Clang to be consistent.

[abidh]

If gcc and clang based toolchains are installed in the same prefix then having sysroot at same location can cause one installation to overwrite parts of other.

clang/lib/Driver/ToolChains/BareMetal.h

void addClangTargetOptions(const llvm::opt::ArgList &DriverArgs,
                              Action::OffloadKind DeviceOffloadKind) const override;
   void AddClangCXXStdlibIncludeArgs(
       const llvm::opt::ArgList &DriverArgs,
       llvm::opt::ArgStringList &CC1Args) const override;
   void AddCXXStdlibLibArgs(const llvm::opt::ArgList &Args,
                            llvm::opt::ArgStringList &CmdArgs) const override;
   void AddLinkRuntimeLib(const llvm::opt::ArgList &Args,
                          llvm::opt::ArgStringList &CmdArgs) const;
+  std::string computeSysRoot() const override;
 };
 
 } // namespace toolchains
 
 namespace tools {
 namespace baremetal {
 
 class LLVM_LIBRARY_VISIBILITY Linker : public Tool {

clang/lib/Driver/ToolChains/BareMetal.cpp

 using namespace clang::driver::toolchains;
 
 BareMetal::BareMetal(const Driver &D, const llvm::Triple &Triple,
                            const ArgList &Args)
     : ToolChain(D, Triple, Args) {
   getProgramPaths().push_back(getDriver().getInstalledDir());
   if (getDriver().getInstalledDir() != getDriver().Dir)
     getProgramPaths().push_back(getDriver().Dir);
-  SmallString<128> SysRoot(getDriver().SysRoot);
+  SmallString<128> SysRoot(computeSysRoot());
   if (!SysRoot.empty()) {
     llvm::sys::path::append(SysRoot, "lib");
     getFilePaths().push_back(std::string(SysRoot));
   }
 }
 
 /// Is the triple {arm,thumb}-none-none-{eabi,eabihf} ?
 static bool isARMBareMetal(const llvm::Triple &Triple) {

 }
 
 std::string BareMetal::getRuntimesDir() const {
   SmallString<128> Dir(getDriver().ResourceDir);
   llvm::sys::path::append(Dir, "lib", "baremetal");
   return std::string(Dir.str());
 }
 
+std::string BareMetal::computeSysRoot() const {
+  if (!getDriver().SysRoot.empty())
+    return getDriver().SysRoot;
+
+  SmallString<128> SysRootDir;
+  llvm::sys::path::append(SysRootDir, getDriver().Dir, "../lib/clang-runtimes",
+                          getDriver().getTargetTriple());
+
+  return std::string(SysRootDir);
+}
+
 void BareMetal::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
                                           ArgStringList &CC1Args) const {
   if (DriverArgs.hasArg(options::OPT_nostdinc))
     return;
 
   if (!DriverArgs.hasArg(options::OPT_nobuiltininc)) {
     SmallString<128> Dir(getDriver().ResourceDir);
     llvm::sys::path::append(Dir, "include");
     addSystemInclude(DriverArgs, CC1Args, Dir.str());
   }
 
   if (!DriverArgs.hasArg(options::OPT_nostdlibinc)) {
-    SmallString<128> Dir(getDriver().SysRoot);
+    SmallString<128> Dir(computeSysRoot());
     if (!Dir.empty()) {
       llvm::sys::path::append(Dir, "include");
       addSystemInclude(DriverArgs, CC1Args, Dir.str());
     }
   }
 }
 
 void BareMetal::addClangTargetOptions(const ArgList &DriverArgs,
                                       ArgStringList &CC1Args,
                                       Action::OffloadKind) const {
   CC1Args.push_back("-nostdsysteminc");
 }
 
 void BareMetal::AddClangCXXStdlibIncludeArgs(
     const ArgList &DriverArgs, ArgStringList &CC1Args) const {
   if (DriverArgs.hasArg(options::OPT_nostdinc) ||
       DriverArgs.hasArg(options::OPT_nostdlibinc) ||
       DriverArgs.hasArg(options::OPT_nostdincxx))
     return;
 
-  StringRef SysRoot = getDriver().SysRoot;
+  std::string SysRoot(computeSysRoot());
   if (SysRoot.empty())
     return;
 
   switch (GetCXXStdlibType(DriverArgs)) {
   case ToolChain::CST_Libcxx: {
     SmallString<128> Dir(SysRoot);
     llvm::sys::path::append(Dir, "include", "c++", "v1");
     addSystemInclude(DriverArgs, CC1Args, Dir.str());

clang/test/Driver/baremetal-sysroot.cpp

+// REQUIRES: shell
+// UNSUPPORTED: system-windows
+
+// Test that when a --sysroot is not provided, driver picks the default
+// location correctly if available.
+
+// RUN: rm -rf %T/baremetal_default_sysroot
+// RUN: mkdir -p %T/baremetal_default_sysroot/bin
+// RUN: mkdir -p %T/baremetal_default_sysroot/lib/clang-runtimes/armv6m-none-eabi
+// RUN: ln -s %clang %T/baremetal_default_sysroot/bin/clang
+
+// RUN: %T/baremetal_default_sysroot/bin/clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
+// RUN:     -target armv6m-none-eabi \
+// RUN:   | FileCheck --check-prefix=CHECK-V6M-C %s
+// CHECK-V6M-C: "{{.*}}clang{{.*}}" "-cc1" "-triple" "thumbv6m-none-unknown-eabi"
+// CHECK-V6M-C-SAME: "-internal-isystem" "{{.*}}/baremetal_default_sysroot{{[/\\]+}}bin{{[/\\]+}}..{{[/\\]+}}lib{{[/\\]+}}clang-runtimes{{[/\\]+}}armv6m-none-eabi{{[/\\]+}}include{{[/\\]+}}c++{{[/\\]+}}v1"
+// CHECk-V6M-C-SAME: "-internal-isystem" "{{.*}}/baremetal_default_sysroot{{[/\\]+}}bin{{[/\\]+}}..{{[/\\]+}}lib{{[/\\]+}}clang-runtimes{{[/\\]+}}armv6m-none-eabi{{[/\\]+}}include"
+// CHECK-V6M-C-SAME: "-x" "c++" "{{.*}}baremetal-sysroot.cpp"
+// CHECK-V6M-C-NEXT: "{{[^"]*}}ld{{(\.(lld|bfd|gold))?}}{{(\.exe)?}}" "{{.*}}.o" "-Bstatic"
+// CHECK-V6M-C-SAME: "-L{{.*}}/baremetal_default_sysroot{{[/\\]+}}bin{{[/\\]+}}..{{[/\\]+}}lib{{[/\\]+}}clang-runtimes{{[/\\]+}}armv6m-none-eabi{{[/\\]+}}lib"
+// CHECK-V6M-C-SAME: "-lc" "-lm" "-lclang_rt.builtins-armv6m"
+// CHECK-V6M-C-SAME: "-o" "{{.*}}.o"