[clang] Add a -canonical-prefixes option

[thakis]

Summary

In https://reviews.llvm.org/D47480 I complained that there's no positive
form of this flag, so let's add one :)

https://gcc.gnu.org/PR29931 also has a pending patch to add the positive
form to gcc (but there's admittedly not a lot of movement on that bug).

This doesn't change any defaults.

[rnk]

lgtm

+@jansvoboda11 for lack of a better alternative owner for Driver patches.

[jansvoboda11]

LGTM

clang/include/clang/Driver/Options.td

def module_file_info : Flag<["-"], "module-file-info">, Flags<[NoXarchOption,CC1Option]>, Group<Action_Group>,
   HelpText<"Provide information about a particular module file">;
 def mthumb : Flag<["-"], "mthumb">, Group<m_Group>;
 def mtune_EQ : Joined<["-"], "mtune=">, Group<m_Group>,
   HelpText<"Only supported on X86 and RISC-V. Otherwise accepted for compatibility with GCC.">;
 def multi__module : Flag<["-"], "multi_module">;
 def multiply__defined__unused : Separate<["-"], "multiply_defined_unused">;
 def multiply__defined : Separate<["-"], "multiply_defined">;
 def mwarn_nonportable_cfstrings : Flag<["-"], "mwarn-nonportable-cfstrings">, Group<m_Group>;
+def canonical_prefixes : Flag<["-"], "canonical-prefixes">, Flags<[HelpHidden, CoreOption]>,
+  HelpText<"Use absolute paths for invoking subcommands (default)">;
 def no_canonical_prefixes : Flag<["-"], "no-canonical-prefixes">, Flags<[HelpHidden, CoreOption]>,
-  HelpText<"Use relative instead of canonical paths">;
+  HelpText<"Use relative paths for invoking subcommands">;
 def no_cpp_precomp : Flag<["-"], "no-cpp-precomp">, Group<clang_ignored_f_Group>;
 def no_integrated_cpp : Flag<["-", "--"], "no-integrated-cpp">, Flags<[NoXarchOption]>;
 def no_pedantic : Flag<["-", "--"], "no-pedantic">, Group<pedantic_Group>;
 def no__dead__strip__inits__and__terms : Flag<["-"], "no_dead_strip_inits_and_terms">;
 def nobuiltininc : Flag<["-"], "nobuiltininc">, Flags<[CC1Option, CoreOption]>,
   HelpText<"Disable builtin #include directories">,
   MarshallingInfoNegativeFlag<HeaderSearchOpts<"UseBuiltinIncludes">>;
 def nogpuinc : Flag<["-"], "nogpuinc">, HelpText<"Do not add include paths for CUDA/HIP and"

clang/lib/Driver/Driver.cpp

if (VFS->setCurrentWorkingDirectory(WD->getValue()))
       Diag(diag::err_drv_unable_to_set_working_directory) << WD->getValue();
 
   // FIXME: This stuff needs to go into the Compilation, not the driver.
   bool CCCPrintPhases;
 
   // Silence driver warnings if requested
   Diags.setIgnoreAllWarnings(Args.hasArg(options::OPT_w));
 
-  // -no-canonical-prefixes is used very early in main.
+  // -canonical-prefixes, -no-canonical-prefixes are used very early in main.
+  Args.ClaimAllArgs(options::OPT_canonical_prefixes);
   Args.ClaimAllArgs(options::OPT_no_canonical_prefixes);
 
   // f(no-)integated-cc1 is also used very early in main.
   Args.ClaimAllArgs(options::OPT_fintegrated_cc1);
   Args.ClaimAllArgs(options::OPT_fno_integrated_cc1);
 
   // Ignore -pipe.
   Args.ClaimAllArgs(options::OPT_pipe);

clang/test/Driver/no-canonical-prefixes.c

 // Due to ln -sf:
 // REQUIRES: shell
 // RUN: mkdir -p %t.real
 // RUN: cd %t.real
 // RUN: ln -sf %clang test-clang
 // RUN: cd ..
 // Important to remove %t.fake: If it already is a symlink to %t.real when
 // `ln -sf %t.real %t.fake` runs, then that would symlink %t.real to itself,
 // forming a cycle.
 // RUN: rm -f %t.fake
 // RUN: ln -sf %t.real %t.fake
 // RUN: cd %t.fake
-// RUN: ./test-clang -v -S %s 2>&1 | FileCheck --check-prefix=CANONICAL %s
-// RUN: ./test-clang -v -S %s -no-canonical-prefixes 2>&1 | FileCheck --check-prefix=NON-CANONICAL %s
+// RUN: ./test-clang -v -S %s 2>&1 \
+// RUN:     | FileCheck --check-prefix=CANONICAL %s
+// RUN: ./test-clang -v -S %s 2>&1 \
+// RUN:     -no-canonical-prefixes \
+// RUN:     | FileCheck --check-prefix=NON-CANONICAL %s
+// RUN: ./test-clang -v -S %s 2>&1 \
+// RUN:     -no-canonical-prefixes \
+// RUN:     -canonical-prefixes \
+// RUN:     | FileCheck --check-prefix=CANONICAL %s
+// RUN: ./test-clang -v -S %s 2>&1 \
+// RUN:     -no-canonical-prefixes \
+// RUN:     -canonical-prefixes \
+// RUN:     -no-canonical-prefixes \
+// RUN:     | FileCheck --check-prefix=NON-CANONICAL %s
 //
 // FIXME: This should really be '.real'.
 // CANONICAL: InstalledDir: {{.*}}.fake
 // CANONICAL: {{[/|\\]*}}clang{{.*}}" -cc1
 //
 // NON-CANONICAL: InstalledDir: .{{$}}
 // NON-CANONICAL: test-clang" -cc1

clang/tools/driver/driver.cpp

int main(int Argc, const char **Argv) {
 
   // Handle options that need handling before the real command line parsing in
   // Driver::BuildCompilation()
   bool CanonicalPrefixes = true;
   for (int i = 1, size = Args.size(); i < size; ++i) {
     // Skip end-of-line response file markers
     if (Args[i] == nullptr)
       continue;
-    if (StringRef(Args[i]) == "-no-canonical-prefixes") {
+    if (StringRef(Args[i]) == "-canonical-prefixes")
+      CanonicalPrefixes = true;
+    else if (StringRef(Args[i]) == "-no-canonical-prefixes")
       CanonicalPrefixes = false;
-      break;
-    }
   }
 
   // Handle CL and _CL_ which permits additional command line options to be
   // prepended or appended.
   if (ClangCLMode) {
     // Arguments in "CL" are prepended.
     llvm::Optional<std::string> OptCL = llvm::sys::Process::GetEnv("CL");
     if (OptCL.hasValue()) {