[clang] Allow -DDEFAULT_SYSROOT to be a relative path

[sbc100]

Summary

In this case we interpret the path as relative the clang binary.

This allows SDKs to be build that include clang along with a custom
sysroot without requiring users to specify --sysroot to point to the
directory where they install the SDK.

See https://github.com/WebAssembly/wasi-sdk/issues/58

[sbc100]

I have no idea how to write a test for this, but I tested it locally with wasi SDK.

Hopefully fucia will find this useful too as a generic version of https://reviews.llvm.org/D42019

[phosek]

clang/lib/Driver/Driver.cpp

140

I don't think this is an intended use for GetResourcesPath since this is not a resource path. Since you set custom resource dir, this is equivalent to:

llvm::sys::path::append(llvm::sys::path::parent_path(ClangExecutable), SysRoot)

On line 144 we already set Dir = llvm::sys::path::parent_path(ClangExecutable), so if you move this below you can just use SysRoot = llvm::sys::path::append(Dir, SysRoot) which should be sufficient.

[phosek]

Is this patch up to date?

[sbc100]

feedback

[sbc100]

squash

[sbc100]

Hmm.. arc diff was behaving strangely for me and failed to update this PR as I expected. Should fixed now.

[phosek]

clang/lib/Driver/Driver.cpp

143

Does this return true or false when sysroot is empty?

145

Nit: according to LLVM conventions, this should FullPath (or just P which is also very common temporary path variables).

[sbc100]

feedback

[sbc100]

clang/lib/Driver/Driver.cpp

143

Good questions. As it happens it looks like it returns true, which is not the result we want here. Added explicit check.

[phosek]

LGTM

clang/CMakeLists.txt

 
 set(CLANG_RESOURCE_DIR "" CACHE STRING
   "Relative directory from the Clang binary to its resource files.")
 
 set(C_INCLUDE_DIRS "" CACHE STRING
   "Colon separated list of directories clang will search for headers.")
 
 set(GCC_INSTALL_PREFIX "" CACHE PATH "Directory where gcc is installed." )
-set(DEFAULT_SYSROOT "" CACHE PATH
+set(DEFAULT_SYSROOT "" CACHE STRING
   "Default <path> to all compiler invocations for --sysroot=<path>." )
 
 set(ENABLE_LINKER_BUILD_ID OFF CACHE BOOL "pass --build-id to ld")
 
 set(ENABLE_X86_RELAX_RELOCATIONS OFF CACHE BOOL
     "enable x86 relax relocations by default")
 
 set(ENABLE_EXPERIMENTAL_NEW_PASS_MANAGER FALSE CACHE BOOL

clang/lib/Driver/Driver.cpp

: Diags(Diags), VFS(std::move(VFS)), Mode(GCCMode),
       TargetTriple(TargetTriple), CCCGenericGCCName(""), Saver(Alloc),
       CheckInputsExist(true), GenReproducer(false),
       SuppressMissingInputWarning(false) {
   // Provide a sane fallback if no VFS is specified.
   if (!this->VFS)
     this->VFS = llvm::vfs::getRealFileSystem();
 
   Name = std::string(llvm::sys::path::filename(ClangExecutable));
   Dir = std::string(llvm::sys::path::parent_path(ClangExecutable));
   InstalledDir = Dir; // Provide a sensible default installed dir.
 
+  if ((!SysRoot.empty()) && llvm::sys::path::is_relative(SysRoot)) {
+    // Prepend InstalledDir if SysRoot is relative
+    SmallString<128> P(InstalledDir);
+    llvm::sys::path::append(P, SysRoot);
+    SysRoot = std::string(P);
+  }
+
 #if defined(CLANG_CONFIG_FILE_SYSTEM_DIR)
   SystemConfigDir = CLANG_CONFIG_FILE_SYSTEM_DIR;
 #endif
 #if defined(CLANG_CONFIG_FILE_USER_DIR)
   UserConfigDir = CLANG_CONFIG_FILE_USER_DIR;
 #endif
 
   // Compute the path to the resource directory.