[Openmp-commits] [clang] [llvm] [openmp] [Clang][OpenMP] Add reverse directive (PR #92916)

[Michael Kruse via Openmp-commits]

================
@@ -15749,6 +15757,186 @@ StmtResult SemaOpenMP::ActOnOpenMPUnrollDirective(ArrayRef<OMPClause *> Clauses,
                                     buildPreInits(Context, PreInits));
 }
 
+StmtResult SemaOpenMP::ActOnOpenMPReverseDirective(Stmt *AStmt,
+                                                   SourceLocation StartLoc,
+                                                   SourceLocation EndLoc) {
+  ASTContext &Context = getASTContext();
+  Scope *CurScope = SemaRef.getCurScope();
+
+  // Empty statement should only be possible if there already was an error.
+  if (!AStmt)
+    return StmtError();
+
+  constexpr unsigned NumLoops = 1;
+  Stmt *Body = nullptr;
+  SmallVector<OMPLoopBasedDirective::HelperExprs, NumLoops> LoopHelpers(
+      NumLoops);
+  SmallVector<SmallVector<Stmt *, 0>, NumLoops + 1> OriginalInits;
+  if (!checkTransformableLoopNest(OMPD_reverse, AStmt, NumLoops, LoopHelpers,
+                                  Body, OriginalInits))
+    return StmtError();
+
+  // Delay applying the transformation to when template is completely
+  // instantiated.
+  if (SemaRef.CurContext->isDependentContext())
+    return OMPReverseDirective::Create(Context, StartLoc, EndLoc, AStmt,
+                                       nullptr, nullptr);
+
+  assert(LoopHelpers.size() == NumLoops &&
+         "Expecting a single-dimensional loop iteration space");
+  assert(OriginalInits.size() == NumLoops &&
+         "Expecting a single-dimensional loop iteration space");
+  OMPLoopBasedDirective::HelperExprs &LoopHelper = LoopHelpers.front();
+
+  // Find the loop statement.
+  Stmt *LoopStmt = nullptr;
+  collectLoopStmts(AStmt, {LoopStmt});
+
+  // Determine the PreInit declarations.
+  SmallVector<Stmt *> PreInits;
+  addLoopPreInits(Context, LoopHelper, LoopStmt, OriginalInits[0], PreInits);
+
+  auto *IterationVarRef = cast<DeclRefExpr>(LoopHelper.IterationVarRef);
+  QualType IVTy = IterationVarRef->getType();
+  uint64_t IVWidth = Context.getTypeSize(IVTy);
+  auto *OrigVar = cast<DeclRefExpr>(LoopHelper.Counters.front());
+
+  // Iteration variable SourceLocations.
+  SourceLocation OrigVarLoc = OrigVar->getExprLoc();
+  SourceLocation OrigVarLocBegin = OrigVar->getBeginLoc();
+  SourceLocation OrigVarLocEnd = OrigVar->getEndLoc();
+
+  // Locations pointing to the transformation.
+  SourceLocation TransformLoc = StartLoc;
+  SourceLocation TransformLocBegin = StartLoc;
+  SourceLocation TransformLocEnd = EndLoc;
+
+  // Internal variable names.
+  std::string OrigVarName = OrigVar->getNameInfo().getAsString();
+  std::string TripCountName = (Twine(".tripcount.") + OrigVarName).str();
+  std::string ForwardIVName = (Twine(".forward.iv.") + OrigVarName).str();
+  std::string ReversedIVName = (Twine(".reversed.iv.") + OrigVarName).str();
----------------
Meinersbur wrote:

`Twine` stores a non-owning reference to its argument, here `OrigVarName`. If that string goes out of scope or is changed, that becomes a dangling reference. `Twine` was designed to be valid within a statement only; at the end of that statement temporaries may be released. From `Twine.h`:
```
  /// A Twine is not intended for use directly and should not be stored, its
  /// implementation relies on the ability to store pointers to temporary stack
  /// objects which may be deallocated at the end of a statement. Twines should
  /// only be used as const references in arguments, when an API wishes
  /// to accept possibly-concatenated strings.
```

In this case only two strings are concatenated, so there are no temporary strings that `Twine` could help avoiding. I may still have used `Twine` here because either I originally also appended a suffix, or expected `Twine` to benefit from being able to precompute the target string length. If that's the case, `std::string::operator+` should be able to do the same. Let me remove `Twine` here completely.

https://github.com/llvm/llvm-project/pull/92916