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

[Tom Eccles via cfe-commits]

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@@ -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:
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tblah wrote:

Clang might work differently internally. If we don't pass -fopenmp to the flang frontend driver, the parser will treat the openmp directives as comments. Which in many programs won't lead to any compilation errors but the program could produce different results (e.g. reductions won't happen)

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