[flang][OpenMP] Add -f[no]-openmp-simd (#150269)

Both clang and gfortran support the -fopenmp-simd flag, which enables
OpenMP support only for simd constructs, while disabling the rest of
OpenMP.

Implement the appropriate parse tree rewriting to remove non-SIMD OpenMP
constructs at the parsing stage.

Add a new SimdOnly flang OpenMP IR pass which rewrites generated OpenMP
FIR to handle untangling composite simd constructs, and clean up OpenMP
operations leftover after the parse tree rewriting stage.
With this approach, the two parts of the logic required to make the flag
work can be self-contained within the parse tree rewriter and the MLIR
pass, respectively. It does not need to be implemented within the core
lowering logic itself.

The flag is expected to have no effect if -fopenmp is passed explicitly,
and is only expected to remove OpenMP constructs, not things like OpenMP
library functions calls. This matches the behaviour of other compilers.

---------

Signed-off-by: Kajetan Puchalski <kajetan.puchalski@arm.com>

1 files changed, 9 insertions, 2 deletions

diff --git a/flang/lib/Frontend/CompilerInvocation.cpp b/flang/lib/Frontend/CompilerInvocation.cpp
index 111c5aa4..708fb7f 100644
--- a/flang/lib/Frontend/CompilerInvocation.cpp
+++ b/flang/lib/Frontend/CompilerInvocation.cpp
@@ -1162,8 +1162,15 @@ static bool parseOpenMPArgs(CompilerInvocation &res, llvm::opt::ArgList &args,
                             clang::DiagnosticsEngine &diags) {
   llvm::opt::Arg *arg = args.getLastArg(clang::driver::options::OPT_fopenmp,
                                         clang::driver::options::OPT_fno_openmp);
-  if (!arg || arg->getOption().matches(clang::driver::options::OPT_fno_openmp))
-    return true;
+  if (!arg ||
+      arg->getOption().matches(clang::driver::options::OPT_fno_openmp)) {
+    bool isSimdSpecified = args.hasFlag(
+        clang::driver::options::OPT_fopenmp_simd,
+        clang::driver::options::OPT_fno_openmp_simd, /*Default=*/false);
+    if (!isSimdSpecified)
+      return true;
+    res.getLangOpts().OpenMPSimd = 1;
+  }
 
   unsigned numErrorsBefore = diags.getNumErrors();
   llvm::Triple t(res.getTargetOpts().triple);