[flang] Detect circularly defined interfaces of procedures

[PeteSteinfeld]

Summary

It's possible to define a procedure whose interface depends on a procedure
which has an interface that depends on the original procedure. Such a circular
definition was causing the compiler to fall into an infinite loop when
resolving the name of the second procedure. It's also possible to create
circular dependency chains of more than two procedures.

I fixed this by adding the function HasCycle() to the class DeclarationVisitor
and calling it from DeclareProcEntity() to detect procedures with such
circularly defined interfaces. I marked the associated symbols of such
procedures by calling SetError() on them. When processing subsequent
procedures, I called HasError() before attempting to analyze their interfaces.
Unfortunately, this did not work.

With help from Tim, we determined that the SymbolSet used to track the
erroneous symbols was instantiated using a "<" operator which was defined using
the location of the name of the procedure. But the location of the procedure
name was being changed by a call to ReplaceName() between the times that the
calls to SetError() and HasError() were made. This caused HasError() to
incorrectly report that a symbol was not in the set of erroneous symbols.

I fixed this by changing SymbolSet to be an unordered set that uses the
contents of the name of the symbol as the basis for its hash function. This
works because the contents of the name of the symbol is preserved by
ReplaceName() even though its location changes.

I also fixed the error message used when reporting recursively defined dummy
procedure arguments.

I also added tests that will crash the compiler without this change.

Note that the "<" operator is used in other contexts, for example, in the map
of characterized procedures, maps of items in equivalence sets, maps of
structure constructor values, ... All of these situations happen after name
resolution has been completed and all calls to ReplaceName() have already
happened and thus are not subject to the problem I ran into when ReplaceName()
was called when processing procedure entities.

Note also that the implementation of the "<" operator uses the relative
location in the cooked character stream as the basis of its implementation.
This is potentially problematic when symbols from diffent compilation units
(for example symbols originating in .mod files) are put into the same map since
their names will appear in two different source streams which may not be
allocated in the same relative positions in memory. But I was unable to create
a test that caused a problem. Using a direct comparison of the content of the
name of the symbol in the "<" operator has problems. Symbols in enclosing or
parallel scopes can have the same name. Also using the location of the symbol
in the cooked character stream has the advantage that it preserves the the
order of the symbols in a structure constructor constant, which makes matching
the values with the symbols relatively easy.

This change supersedes D97201.

[klausler]

flang/include/flang/Semantics/symbol.h

775

We should think about renaming this typedef. It's not really a set of arbitrary Symbol references any more, since two Symbols with the same name strings will clash. SymbolSet seems to me like a name for what would be an unordered_set that hashed the addresses of the Symbols instead.

We don't have to fix this in this patch; I'll try to clean things up more afterwards.

[awarzynski]

@PeteSteinfeld, thank you for resubmitting https://reviews.llvm.org/D97201, but this is still failing on 6 out 8 our public Flang buildbots:

• http://lab.llvm.org:8011/#/builders/21/builds/9625
• http://lab.llvm.org:8011/#/builders/134/builds/2395
• http://lab.llvm.org:8011/#/builders/79/builds/6298
• http://lab.llvm.org:8011/#/builders/66/builds/2084
• http://lab.llvm.org:8011/#/builders/135/builds/2485
• http://lab.llvm.org:8011/#/builders/32/builds/3551

If I understand correctly, std::hash that you use here is implementation-dependent and hence the the expected result in resolve102.f90 is not 100% deterministic and hence it is not matched on certain platforms.

In D97201 I proposed a solution that to the best of my knowledge will work on every platform (I tested it on AArch64). If you prefer a different approach, I'm happy to test it on an AArch64 machine before this is merged. Please could you add me as a reviewer for that? I am very keen to help with this and to make it work on every of the supported platforms.

flang/include/flang/Semantics/semantics.h

private:
   parser::Messages messages_;
   evaluate::FoldingContext foldingContext_;
   ConstructStack constructStack_;
   struct IndexVarInfo {
     parser::CharBlock location;
     IndexVarKind kind;
   };
   std::map<SymbolRef, const IndexVarInfo> activeIndexVars_;
-  std::set<SymbolRef> errorSymbols_;
+  SymbolSet errorSymbols_;
   std::set<std::string> tempNames_;
 };
 
 class Semantics {
 public:
   explicit Semantics(SemanticsContext &context, parser::Program &program,
       parser::CharBlock charBlock, bool debugModuleWriter = false)
       : context_{context}, program_{program} {

flang/include/flang/Semantics/symbol.h

 #define FORTRAN_SEMANTICS_SYMBOL_H_
 
 #include "type.h"
 #include "flang/Common/Fortran.h"
 #include "flang/Common/enum-set.h"
 #include "flang/Common/reference.h"
 #include "llvm/ADT/DenseMapInfo.h"
 #include <array>
+#include <functional>
 #include <list>
 #include <optional>
-#include <set>
+#include <unordered_set>
 #include <vector>
 
 namespace llvm {
 class raw_ostream;
 }
 
 namespace Fortran::semantics {
 
return std::visit(common::visitors{
                           [](const auto &) { return false; },
                       },
         details_);
   }
 
   bool operator==(const Symbol &that) const { return this == &that; }
   bool operator!=(const Symbol &that) const { return !(*this == that); }
   bool operator<(const Symbol &that) const {
-    // For sets of symbols: collate them by source location
+    // For maps of symbols: collate them by source location
     return name_.begin() < that.name_.begin();
   }
 
   int Rank() const {
     return std::visit(
         common::visitors{
             [](const SubprogramDetails &sd) {
               return sd.isFunction() ? sd.result().Rank() : 0;
return std::visit(
       },
       details_);
 }
 
 inline bool operator<(SymbolRef x, SymbolRef y) { return *x < *y; }
 inline bool operator<(MutableSymbolRef x, MutableSymbolRef y) {
   return *x < *y;
 }
-using SymbolSet = std::set<SymbolRef>;
+struct SymbolHash {
+  std::size_t operator()(SymbolRef symRef) const {
+    std::hash<std::string> hasher;
+    return hasher(symRef->name().ToString());
+  }
+};
+using SymbolSet = std::unordered_set<SymbolRef, SymbolHash>;
 
 } // namespace Fortran::semantics
 
 // Define required  info so that SymbolRef can be used inside llvm::DenseMap.
 namespace llvm {
 template <> struct DenseMapInfo<Fortran::semantics::SymbolRef> {
   static inline Fortran::semantics::SymbolRef getEmptyKey() {
     auto ptr = DenseMapInfo<const Fortran::semantics::Symbol *>::getEmptyKey();

flang/lib/Evaluate/characteristics.cpp

static std::optional<Procedure> CharacterizeProcedure(
     const semantics::Symbol &original, FoldingContext &context,
     semantics::SymbolSet &seenProcs) {
   Procedure result;
   const auto &symbol{original.GetUltimate()};
   if (seenProcs.find(symbol) != seenProcs.end()) {
     std::string procsList{GetSeenProcs(seenProcs)};
     context.messages().Say(symbol.name(),
         "Procedure '%s' is recursively defined.  Procedures in the cycle:"
-        " '%s'"_err_en_US,
+        " %s"_err_en_US,
         symbol.name(), procsList);
     return std::nullopt;
   }
   seenProcs.insert(symbol);
   CopyAttrs<Procedure, Procedure::Attr>(symbol, result,
       {
           {semantics::Attr::PURE, Procedure::Attr::Pure},
           {semantics::Attr::ELEMENTAL, Procedure::Attr::Elemental},

flang/lib/Semantics/resolve-names.cpp

if (context().HasError(symbol) || symbol.has<T>()) {
             name, symbol, "'%s' is already declared as an object"_err_en_US);
       }
     } else if (!CheckPossibleBadForwardRef(symbol)) {
       SayAlreadyDeclared(name, symbol);
     }
     context().SetError(symbol);
     return symbol;
   }
+  bool HasCycle(const Symbol &, const ProcInterface &);
 };
 
 // Resolve construct entities and statement entities.
 // Check that construct names don't conflict with other names.
 class ConstructVisitor : public virtual DeclarationVisitor {
 public:
   bool Pre(const parser::ConcurrentHeader &);
   bool Pre(const parser::LocalitySpec::Local &);
return !symbol.GetType() &&
                      },
                      [](const auto &) { return false; },
                  },
           symbol.details());
 }
 
 void ScopeHandler::ApplyImplicitRules(
     Symbol &symbol, bool allowForwardReference) {
-  if (!NeedsType(symbol)) {
+  if (context().HasError(symbol) || !NeedsType(symbol)) {
     return;
   }
   if (const DeclTypeSpec * type{GetImplicitType(symbol)}) {
     symbol.set(Symbol::Flag::Implicit);
     symbol.SetType(*type);
     return;
   }
   if (symbol.has<ProcEntityDetails>() && !symbol.attrs().test(Attr::EXTERNAL)) {
if (!arraySpec().empty() || !coarraySpec().empty()) {
     SetBindNameOn(symbol);
     if (symbol.attrs().test(Attr::EXTERNAL)) {
       ConvertToProcEntity(symbol);
     }
     return symbol;
   }
 }
 
+bool DeclarationVisitor::HasCycle(
+    const Symbol &procSymbol, const ProcInterface &interface) {
+  SymbolSet procsInCycle;
+  procsInCycle.insert(procSymbol);
+  const ProcInterface *thisInterface{&interface};
+  bool haveInterface{true};
+  while (haveInterface) {
+    haveInterface = false;
+    if (const Symbol * interfaceSymbol{thisInterface->symbol()}) {
+      if (procsInCycle.count(*interfaceSymbol) > 0) {
+        for (const auto procInCycle : procsInCycle) {
+          Say(procInCycle->name(),
+              "The interface for procedure '%s' is recursively "
+              "defined"_err_en_US,
+              procInCycle->name());
+          context().SetError(*procInCycle);
+        }
+        return true;
+      } else if (const auto *procDetails{
+                     interfaceSymbol->detailsIf<ProcEntityDetails>()}) {
+        haveInterface = true;
+        thisInterface = &procDetails->interface();
+        procsInCycle.insert(*interfaceSymbol);
+      }
+    }
+  }
+  return false;
+}
+
 Symbol &DeclarationVisitor::DeclareProcEntity(
     const parser::Name &name, Attrs attrs, const ProcInterface &interface) {
   Symbol &symbol{DeclareEntity<ProcEntityDetails>(name, attrs)};
   if (auto *details{symbol.detailsIf<ProcEntityDetails>()}) {
     if (details->IsInterfaceSet()) {
       SayWithDecl(name, symbol,
           "The interface for procedure '%s' has already been "
           "declared"_err_en_US);
       context().SetError(symbol);
-    } else {
-      if (interface.type()) {
-        symbol.set(Symbol::Flag::Function);
-      } else if (interface.symbol()) {
-        if (interface.symbol()->test(Symbol::Flag::Function)) {
-          symbol.set(Symbol::Flag::Function);
-        } else if (interface.symbol()->test(Symbol::Flag::Subroutine)) {
-          symbol.set(Symbol::Flag::Subroutine);
-        }
-      }
-      details->set_interface(interface);
-      SetBindNameOn(symbol);
-      SetPassNameOn(symbol);
-    }
-  }
+    } else if (HasCycle(symbol, interface)) {
+      return symbol;
+    } else if (interface.type()) {
   return symbol;
 }
 
 Symbol &DeclarationVisitor::DeclareObjectEntity(
     const parser::Name &name, Attrs attrs) {
   Symbol &symbol{DeclareEntity<ObjectEntityDetails>(name, attrs)};
   if (auto *details{symbol.detailsIf<ObjectEntityDetails>()}) {
     if (auto *type{GetDeclTypeSpec()}) {
if (!NameIsKnownOrIntrinsic(name)) {
     Symbol &symbol{MakeSymbol(InclusiveScope(), name.source, Attrs{})};
     Resolve(name, symbol);
   }
   return name.symbol;
 }
 
 void DeclarationVisitor::CheckExplicitInterface(const parser::Name &name) {
   if (const Symbol * symbol{name.symbol}) {
-    if (!symbol->HasExplicitInterface()) {
+    if (!context().HasError(*symbol) && !symbol->HasExplicitInterface()) {
       Say(name,
           "'%s' must be an abstract interface or a procedure with "
           "an explicit interface"_err_en_US,
           symbol->name());
     }
   }
 }
 

flang/test/Semantics/resolve102.f90

 ! RUN: %S/test_errors.sh %s %t %f18
 
 ! Tests for circularly defined procedures
-!ERROR: Procedure 'sub' is recursively defined.  Procedures in the cycle: ''sub', 'p2''
+!ERROR: Procedure 'sub' is recursively defined.  Procedures in the cycle: 'p2', 'sub'
 subroutine sub(p2)
   PROCEDURE(sub) :: p2
 
   call sub()
 end subroutine
 
 subroutine circular
-  !ERROR: Procedure 'p' is recursively defined.  Procedures in the cycle: ''p', 'sub', 'p2''
+  !ERROR: Procedure 'p' is recursively defined.  Procedures in the cycle: 'p2', 'p', 'sub'
   procedure(sub) :: p
 
   call p(sub)
 
   contains
     subroutine sub(p2)
       procedure(p) :: p2
     end subroutine
 end subroutine circular
 
 program iface
-  !ERROR: Procedure 'p' is recursively defined.  Procedures in the cycle: ''p', 'sub', 'p2''
+  !ERROR: Procedure 'p' is recursively defined.  Procedures in the cycle: 'p2', 'p', 'sub'
   procedure(sub) :: p
   interface
     subroutine sub(p2)
       import p
       procedure(p) :: p2
     end subroutine
   end interface
   call p(sub)
 end program
 
 Program mutual
   Procedure(sub1) :: p
 
   Call p(sub)
 
   contains
-    !ERROR: Procedure 'sub1' is recursively defined.  Procedures in the cycle: ''p', 'sub1', 'arg''
+    !ERROR: Procedure 'sub1' is recursively defined.  Procedures in the cycle: 'p', 'arg', 'sub1'
     Subroutine sub1(arg)
       procedure(sub1) :: arg
     End Subroutine
 
     Subroutine sub(p2)
       Procedure(sub1) :: p2
     End Subroutine
 End Program
 
 Program mutual1
   Procedure(sub1) :: p
 
   Call p(sub)
 
   contains
-    !ERROR: Procedure 'sub1' is recursively defined.  Procedures in the cycle: ''p', 'sub1', 'arg', 'sub', 'p2''
+    !ERROR: Procedure 'sub1' is recursively defined.  Procedures in the cycle: 'p2', 'sub', 'p', 'arg', 'sub1'
     Subroutine sub1(arg)
       procedure(sub) :: arg
     End Subroutine
 
     Subroutine sub(p2)
       Procedure(sub1) :: p2
     End Subroutine
 End Program
+
+program twoCycle
+  !ERROR: The interface for procedure 'p1' is recursively defined
+  !ERROR: The interface for procedure 'p2' is recursively defined
+  procedure(p1) p2
+  procedure(p2) p1
+  call p1
+  call p2
+end program
+
+program threeCycle
+  !ERROR: The interface for procedure 'p1' is recursively defined
+  !ERROR: The interface for procedure 'p2' is recursively defined
+  procedure(p1) p2
+  !ERROR: The interface for procedure 'p3' is recursively defined
+  procedure(p2) p3
+  procedure(p3) p1
+  call p1
+  call p2
+  call p3
+end program