[flang-commits] [flang] [flang][OpenMP] Lower DO and SIMD variants in metadirectives (PR #210810)

[via flang-commits]

================
@@ -30,6 +29,52 @@ subroutine test_vendor_no_match()
 #endif
 end subroutine
 
+! An inapplicable variant must not have its clauses lowered.
+! CHECK-LABEL: func.func @_QPtest_inapplicable_assume()
+! CHECK:         fir.call @_FortranAioOutputInteger32
+! CHECK-NOT:     fir.call @_FortranAioOutputInteger32
----------------
MattPD wrote:

I said the excluded `assume` variant lowers to `llvm.intr.assume`, but it doesn't.

I locally flipped the vendor in `test_inapplicable_assume` to `llvm`, so `assume` wins selection. Lowering aborts:

```
Unimplemented: holds
UNREACHABLE executed at flang/lib/Lower/OpenMP/Clauses.cpp:1033!
```

Variant lowering routes every clause through that `Clauses.cpp` path, where `Holds` is an `llvm_unreachable`. Only the direct `!$omp assume` path calls `AssumeOp::create`. So no variant emits `llvm.intr.assume`, selected or not, and these `CHECK-NOT` lines cannot fail.

That leaves `test_unselected_fallback_clause` and `test_unselected_ranked_clause` testing nothing.

Can you anchor on a clause operand that variant lowering emits? This works on your branch at 5.1 and 5.2:

```fortran
! CHECK-LABEL: func.func @_QPtest_selected_num_threads(
! CHECK:         fir.call @_QPf
! CHECK:         omp.parallel num_threads(
subroutine test_selected_num_threads(x)
  integer :: x
  integer :: f
  external :: f
  !$omp begin metadirective &
  !$omp & when(implementation={vendor(llvm)}: parallel num_threads(f())) &
  !$omp & otherwise(nothing)
  x = 1
  !$omp end metadirective
end subroutine

! CHECK-LABEL: func.func @_QPtest_unselected_num_threads(
! CHECK-NOT:     fir.call @_QPf
! CHECK-NOT:     omp.parallel
! CHECK:         return
subroutine test_unselected_num_threads(x)
  integer :: x
  integer :: f
  external :: f
  !$omp begin metadirective &
  !$omp & when(implementation={vendor("unknown")}: parallel num_threads(f())) &
  !$omp & otherwise(nothing)
  x = 1
  !$omp end metadirective
end subroutine
```

The first is the missing positive control. Both need the `#ifdef OMP_52` split.

The abort predates this PR. Filed as https://github.com/llvm/llvm-project/issues/212437.


https://github.com/llvm/llvm-project/pull/210810