[flang-commits] [flang] e925643 - [flang][OpenMP] Warn that declare simd in an interface body has no effect (#199248)

[via flang-commits]

Author: Carlos Seo
Date: 2026-05-26T14:48:14-03:00
New Revision: e925643b1e7fc936955c0cc01c2c3ce1abd2f4ce

URL: https://github.com/llvm/llvm-project/commit/e925643b1e7fc936955c0cc01c2c3ce1abd2f4ce
DIFF: https://github.com/llvm/llvm-project/commit/e925643b1e7fc936955c0cc01c2c3ce1abd2f4ce.diff

LOG: [flang][OpenMP] Warn that declare simd in an interface body has no effect (#199248)

Follow-up to the crash fix for #192581. A DECLARE SIMD directive may
legally appear in the specification part of an interface body, but it
applies to the external procedure being declared rather than to any
definition in this compilation. Flang does not propagate the directive
to callers, so it has no effect and is silently dropped during lowering.

Detect this in semantics by checking whether the program unit containing
the directive is an interface body (SubprogramDetails::isInterface())
and emit an -Wopenmp-usage warning so the user knows the directive is
ignored.

Updates #192581

Added: 
    flang/test/Semantics/OpenMP/declare-simd-interface-body.f90

Modified: 
    flang/lib/Semantics/check-omp-structure.cpp
    flang/test/Semantics/OpenMP/linear-clause03.f90

Removed: 
    


################################################################################
diff  --git a/flang/lib/Semantics/check-omp-structure.cpp b/flang/lib/Semantics/check-omp-structure.cpp
index 2e6c0720c77ea..111c2e1b42c0c 100644
--- a/flang/lib/Semantics/check-omp-structure.cpp
+++ b/flang/lib/Semantics/check-omp-structure.cpp
@@ -1643,6 +1643,19 @@ void OmpStructureChecker::Enter(const parser::OmpDeclareSimdDirective &x) {
   const Scope &containingScope = context_.FindScope(dirName.source);
   const Scope &progUnitScope = GetProgramUnitContaining(containingScope);
 
+  // A DECLARE SIMD directive may legally appear in an interface body, but it
+  // applies to the external procedure being declared rather than to any
+  // definition in this compilation. Flang does not propagate the directive to
+  // callers, so it has no effect; warn the user instead of silently ignoring
+  // it. See https://github.com/llvm/llvm-project/issues/192581.
+  if (const Symbol *progUnitSym{progUnitScope.symbol()}) {
+    if (const auto *subpDetails{progUnitSym->detailsIf<SubprogramDetails>()};
+        subpDetails && subpDetails->isInterface()) {
+      context_.Warn(common::UsageWarning::OpenMPUsage, dirName.source,
+          "'DECLARE SIMD' directive in an interface body has no effect"_warn_en_US);
+    }
+  }
+
   for (const parser::OmpClause &clause : x.v.Clauses().v) {
     const auto *u = std::get_if<parser::OmpClause::Uniform>(&clause.u);
     if (!u) {

diff  --git a/flang/test/Semantics/OpenMP/declare-simd-interface-body.f90 b/flang/test/Semantics/OpenMP/declare-simd-interface-body.f90
new file mode 100644
index 0000000000000..624afa8852f2e
--- /dev/null
+++ b/flang/test/Semantics/OpenMP/declare-simd-interface-body.f90
@@ -0,0 +1,29 @@
+! RUN: %python %S/../test_errors.py %s %flang -fopenmp -Werror
+! A DECLARE SIMD directive in an interface body applies to the external
+! procedure being declared, not to any definition in this compilation, so
+! Flang ignores it. Warn that it has no effect.
+! See https://github.com/llvm/llvm-project/issues/192581.
+
+interface
+  subroutine add2(i)
+  !WARNING: 'DECLARE SIMD' directive in an interface body has no effect [-Wopenmp-usage]
+  !$omp declare simd(add2) linear(i:1)
+    integer :: i
+  end subroutine
+
+  subroutine add3(i)
+  !WARNING: 'DECLARE SIMD' directive in an interface body has no effect [-Wopenmp-usage]
+  !$omp declare simd
+    integer :: i
+  end subroutine
+end interface
+
+contains
+
+! A DECLARE SIMD in an actual definition is fine and must not warn.
+subroutine sub(i)
+!$omp declare simd(sub) linear(i:1)
+  integer :: i
+  i = i + 1
+end subroutine
+end

diff  --git a/flang/test/Semantics/OpenMP/linear-clause03.f90 b/flang/test/Semantics/OpenMP/linear-clause03.f90
index 7143db5927d46..3c89f2e93b4ac 100644
--- a/flang/test/Semantics/OpenMP/linear-clause03.f90
+++ b/flang/test/Semantics/OpenMP/linear-clause03.f90
@@ -6,6 +6,7 @@ module m
   subroutine f(x, y)
     integer, allocatable :: x
     integer :: y
+    !WARNING: 'DECLARE SIMD' directive in an interface body has no effect [-Wopenmp-usage]
     !$omp declare simd(f) linear(ref(x) : 1) linear(uval(y))
   end
 end interface