[clang] [flang] [FLANG] allow -fopenmp= (PR #86816)

[Mats Petersson via cfe-commits]

================
@@ -764,6 +762,32 @@ void Flang::ConstructJob(Compilation &C, const JobAction &JA,
   // Add other compile options
   addOtherOptions(Args, CmdArgs);
 
+  // Forward flags for OpenMP. We don't do this if the current action is an
+  // device offloading action other than OpenMP.
+  if (Args.hasFlag(options::OPT_fopenmp, options::OPT_fopenmp_EQ,
+                   options::OPT_fno_openmp, false) &&
+      (JA.isDeviceOffloading(Action::OFK_None) ||
+       JA.isDeviceOffloading(Action::OFK_OpenMP))) {
+    switch (D.getOpenMPRuntime(Args)) {
+    case Driver::OMPRT_OMP:
+    case Driver::OMPRT_IOMP5:
+      // Clang can generate useful OpenMP code for these two runtime libraries.
+      CmdArgs.push_back("-fopenmp");
+      Args.AddAllArgs(CmdArgs, options::OPT_fopenmp_version_EQ);
+
+      // FIXME: Clang supports a whole bunch more flags here.
+      break;
+    default:
----------------
Leporacanthicus wrote:

So, it definitely is done this way on purpose - see clang/test/Driver/fopenmp.c (only "interesting" lines)
``` // RUN: %clang -target x86_64-linux-gnu -fopenmp=libomp -c %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-CC1-OPENMP
// RUN: %clang -target x86_64-linux-gnu -fopenmp=libgomp -c %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-CC1-NO-OPENMP
...
// CHECK-CC1-OPENMP: "-cc1"
// CHECK-CC1-OPENMP: "-fopenmp"
//
// CHECK-CC1-NO-OPENMP: "-cc1"
// CHECK-CC1-NO-OPENMP-NOT: "-fopenmp"
```
We can certainly add a warning or error, but Clang doesn't... :)


https://github.com/llvm/llvm-project/pull/86816