[cfe-dev] Using Libtooling to find for loop break statements

[David Rector via cfe-dev]

Here is how a RecursiveASTVisitor could do it (not tested):

```
#include "clang/AST/RecursiveASTVisitor.h"

/// Usage: 
///   std::map<ForStmt *, std::vector<BreakStmt *>> for2breaks;
///   ForBreakMapper(for2breaks).TraverseDecl(
///       /*Context.getTranslationUnitDecl(), some function decl, etc…);
///
class ForBreakMapper : public RecursiveASTVisitor<ForBreakMapper> {
  using BaseType = RecursiveASTVisitor<ForBreakMapper>;

  ForStmt *ActiveFor = nullptr;
  std::map<ForStmt *, std::vector<BreakStmt *>> &forbreaks_mapref;
  
public:
  ForBreakMapper(decltype(for2breaks_mapref) forbreaks_mapref)
      : for2breaks_mapref(for2breaks_mapref) {}

  // Need this override to disable "data recursion" and thus
  // guarantee VisitBreakStmt happens within its parent's 
  // Traverse*Stmt call:
  bool TraverseStmt(Stmt *S) { return BaseType::TraverseStmt(S); }
  
  bool TraverseForStmt(ForStmt *S) {
    SaveAndRestore<ForStmt *> RAII(ActiveFor, S);
    return BaseType::TraverseForStmt(S);
  }
  
  // Nullify ActiveFor for SwitchStmts (any others?)
  bool TraverseSwitchStmt(SwitchStmt *S) {
    SaveAndRestore<ForStmt *> RAII(ActiveFor, nullptr);
    return BaseType::TraverseSwitchStmt(S);
  }
  
  bool VisitBreakStmt(BreakStmt *S) {
    if (ActiveFor && S)
      forbreaks_mapref[ActiveFor].push_back(S);
    return true;
  }
};
```

I’m a big fan of this general technique, i.e. using llvm::SaveAndRestore in Traverse* overrides to keep track of traversal state data, which you then use in your Visit* overrides.  

You do need that seemingly-trivial TraverseStmt override whenever you do this sort of thing with Stmts though, otherwise the Visit* statements will not be called within the ancestor’s Traverse* statements (due to the "data recursion" queueing performed by default in the base RecursiveASTVisitor.)

Good luck,

- Dave

> On Aug 12, 2020, at 11:16 AM, Phil King via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 
> Hi,
> 
> I am trying to find all of the break statements that can cause termination of for loops. The following gives me all the break statements under a “for” node, but I cannot see how to filter out those that cannot terminate the current loop (e.g., they may terminate a case statement, be enclosed within a nested loop, etc.):
> 
> forStmt( forEachDescentant( breakStmt().bind( “myBreaks” ) ) ).bind( “myFors” )
> 
> How should I go about filtering out the unwanted breaks?
> 
> Sent from my iPhone
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> cfe-dev at lists.llvm.org
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