[flang][acc] Allow orphaned acc cache directive (#184448)

While the spec allows the cache directive at the top of a loop body, the
directive has also been utilized at the top of an acc routine. This PR
removes the semantic check that rejects the cache directive outside of a
loop, allowing orphaned `!$acc cache` similar to CIR.

The OpenACC.md deviation document is updated to note this extension.

flang/docs/OpenACC.md

@@ -25,6 +25,7 @@ local:
   logical expression.
 * `!$acc routine` directive can be placed at the top level.
 * `!$acc cache` directive accepts scalar variable.
+* `!$acc cache` directive is accepted outside of a loop construct.
 * The `!$acc declare` directive accepts assumed size array arguments for
   `deviceptr` and `present` clauses.
 * The OpenACC specification disallows a variable appearing multiple times in

flang/lib/Semantics/check-acc-structure.cpp

@@ -422,7 +422,6 @@ void AccStructureChecker::Enter(const parser::OpenACCRoutineConstruct &x) {
           "part of a subroutine or function definition, or within an interface "
           "body for a subroutine or function in an interface block"_err_en_US);
     }
-    hasAccRoutineDirective = true;
   }
 }
 void AccStructureChecker::Leave(const parser::OpenACCRoutineConstruct &) {
@@ -669,11 +668,6 @@ void AccStructureChecker::Enter(const parser::OpenACCCacheConstruct &x) {
   const auto &verbatim = std::get<parser::Verbatim>(x.t);
   PushContextAndClauseSets(verbatim.source, llvm::acc::Directive::ACCD_cache);
   SetContextDirectiveSource(verbatim.source);
-  if (loopNestLevel == 0 && !hasAccRoutineDirective) {
-    context_.Say(verbatim.source,
-        "The CACHE directive must be inside a loop or an ACC ROUTINE subprogram"_err_en_US);
-  }
-
   // Check cache directive array section constraints
   const auto &objectListWithModifier =
       std::get<parser::AccObjectListWithModifier>(x.t);
@@ -1156,22 +1150,18 @@ void AccStructureChecker::Enter(const parser::OpenACCEndConstruct &x) {
 
 void AccStructureChecker::Enter(const parser::Module &) {
   declareSymbols.clear();
-  hasAccRoutineDirective = false;
 }
 
 void AccStructureChecker::Enter(const parser::FunctionSubprogram &x) {
   declareSymbols.clear();
-  hasAccRoutineDirective = false;
 }
 
 void AccStructureChecker::Enter(const parser::SubroutineSubprogram &) {
   declareSymbols.clear();
-  hasAccRoutineDirective = false;
 }
 
 void AccStructureChecker::Enter(const parser::SeparateModuleSubprogram &) {
   declareSymbols.clear();
-  hasAccRoutineDirective = false;
 }
 
 void AccStructureChecker::Enter(const parser::DoConstruct &) {

flang/lib/Semantics/check-acc-structure.h

@@ -115,7 +115,6 @@ private:
 
   llvm::SmallDenseMap<Symbol *, llvm::acc::Clause> declareSymbols;
   unsigned loopNestLevel = 0;
-  bool hasAccRoutineDirective = false;
 };
 
 } // namespace Fortran::semantics

flang/test/Semantics/OpenACC/acc-cache-validity.f90

@@ -52,7 +52,6 @@ program openacc_cache_validity
 
   end do
 
-  !ERROR: The CACHE directive must be inside a loop or an ACC ROUTINE subprogram
   !$acc cache(a)
 
   call routine_with_cache()