[flang][cmake] Make CMake copy "omp_lib.h" into the build directory

Any header or module file in the Flang source directory is of no use to
the compiler unless it is copied into the build directory. Indeed, all
compiler search paths are relative to the compiler executable (flang-new
in our case). Hence, "omp_lib.h" should be copied into the build
directory alongside other compiler-provided files that can be "included"
(header files) or "used" (module files).

For now, "omp_lib.h" is copied into "<build-dir>/include/flang/OpenMP".
We may decide to change this in future. For example, Clang copies a
bunch of runtime headers into “<build-dir>/lib/clang/<version-number>”.
We could also consider using a similar header from a different
sub-project.

Flang's driver search path is updated accordingly. A rule for
"installing" the "omp_lib.h" header is _yet to be added_ (we will also
need to determine the suitable location for this).

Differential Revision: https://reviews.llvm.org/D122015

1 files changed, 14 insertions, 0 deletions

diff --git a/flang/lib/Frontend/CompilerInvocation.cpp b/flang/lib/Frontend/CompilerInvocation.cpp
index cf92e71..cb94dac 100644
--- a/flang/lib/Frontend/CompilerInvocation.cpp
+++ b/flang/lib/Frontend/CompilerInvocation.cpp
@@ -381,6 +381,15 @@ static std::string getIntrinsicDir() {
   return std::string(driverPath);
 }
 
+// Generate the path to look for OpenMP headers
+static std::string getOpenMPHeadersDir() {
+  llvm::SmallString<128> includePath;
+  includePath.assign(llvm::sys::fs::getMainExecutable(nullptr, nullptr));
+  llvm::sys::path::remove_filename(includePath);
+  includePath.append("/../include/flang/OpenMP/");
+  return std::string(includePath);
+}
+
 /// Parses all preprocessor input arguments and populates the preprocessor
 /// options accordingly.
 ///
@@ -626,6 +635,11 @@ void CompilerInvocation::SetDefaultFortranOpts() {
 
   std::vector<std::string> searchDirectories{"."s};
   fortranOptions.searchDirectories = searchDirectories;
+
+  // Add the location of omp_lib.h to the search directories. Currently this is
+  // identical to the modules' directory.
+  fortranOptions.searchDirectories.emplace_back(getOpenMPHeadersDir());
+
   fortranOptions.isFixedForm = false;
 }