[Ada] Accept Structural in aspect Subprogram_Variant and pragma Loop_Variant

Add a new form of variants to ensure termination of loops or recursive
subprograms. Structural variants correspond to objects which designate a
part of the data-structure they used to designate in the previous loop
iteration or recursive call. They only imply termination if the
data-structure is acyclic, which is the case in SPARK but not in Ada in
general. The fact that these variants are correct is only verified
formally by the proof tool and not by the compiler or dynamically at
execution like other forms of variants.

gcc/ada/

	* snames.ads-tmpl: Add "Structural" as a name.
	* sem_prag.adb: (Analyze_Pragma): Accept modifier "Structural"
	in pragmas Loop_Variant and Subprogram_Variant. Check that items
	associated to Structural occur alone in the pragma associations.
	(Analyze_Subprogram_Variant_In_Decl_Part): Idem.
	* exp_prag.adb (Expand_Pragma_Loop_Variant): Discard structural
	variants.
	(Expand_Pragma_Subprogram_Variant): Idem.

gcc/testsuite/

	* gnat.dg/loopvar.adb: Update expected error message.

4 files changed, 64 insertions, 25 deletions

diff --git a/gcc/ada/exp_prag.adb b/gcc/ada/exp_prag.adb
index 0fe8bff..bf2d297 100644
--- a/gcc/ada/exp_prag.adb
+++ b/gcc/ada/exp_prag.adb
@@ -2694,10 +2694,15 @@ package body Exp_Prag is
       --  If pragma is not enabled, rewrite as Null statement. If pragma is
       --  disabled, it has already been rewritten as a Null statement.
       --
-      --  Likewise, do this in CodePeer mode, because the expanded code is too
+      --  Likewise, ignore structural variants for execution.
+      --
+      --  Also do this in CodePeer mode, because the expanded code is too
       --  complicated for CodePeer to analyse.
 
-      if Is_Ignored (N) or else CodePeer_Mode then
+      if Is_Ignored (N)
+        or else Chars (Last_Var) = Name_Structural
+        or else CodePeer_Mode
+      then
          Rewrite (N, Make_Null_Statement (Loc));
          Analyze (N);
          return;
@@ -3058,10 +3063,12 @@ package body Exp_Prag is
 
       Loc : constant Source_Ptr := Sloc (Prag);
 
-      Aggr         : Node_Id;
+      Aggr         : constant Node_Id :=
+        Expression (First (Pragma_Argument_Associations (Prag)));
       Formal_Map   : Elist_Id;
       Last         : Node_Id;
-      Last_Variant : Node_Id;
+      Last_Variant : constant Node_Id :=
+        Nlists.Last (Component_Associations (Aggr));
       Proc_Bod     : Node_Id;
       Proc_Decl    : Node_Id;
       Proc_Id      : Entity_Id;
@@ -3069,14 +3076,15 @@ package body Exp_Prag is
       Variant      : Node_Id;
 
    begin
-      --  Do nothing if pragma is not present or is disabled
+      --  Do nothing if pragma is not present or is disabled.
+      --  Also ignore structural variants for execution.
 
-      if Is_Ignored (Prag) then
+      if Is_Ignored (Prag)
+        or else Chars (Nlists.Last (Choices (Last_Variant))) = Name_Structural
+      then
          return;
       end if;
 
-      Aggr := Expression (First (Pragma_Argument_Associations (Prag)));
-
       --  The expansion of Subprogram Variant is quite distributed as it
       --  produces various statements to capture and compare the arguments.
       --  To preserve the original context, set the Is_Assertion_Expr flag.
@@ -3115,7 +3123,6 @@ package body Exp_Prag is
 
       Last         := Proc_Decl;
       Curr_Decls   := New_List;
-      Last_Variant := Nlists.Last (Component_Associations (Aggr));
 
       Variant := First (Component_Associations (Aggr));
       while Present (Variant) loop
diff --git a/gcc/ada/sem_prag.adb b/gcc/ada/sem_prag.adb
index 9ef3a06..57a74db 100644
--- a/gcc/ada/sem_prag.adb
+++ b/gcc/ada/sem_prag.adb
@@ -19423,7 +19423,8 @@ package body Sem_Prag is
                if Chars (Variant) = No_Name then
                   Error_Pragma_Arg_Ident ("expect name `Increases`", Variant);
 
-               elsif Chars (Variant) not in Name_Decreases | Name_Increases
+               elsif Chars (Variant) not in
+                    Name_Decreases | Name_Increases | Name_Structural
                then
                   declare
                      Name : String := Get_Name_String (Chars (Variant));
@@ -19448,11 +19449,24 @@ package body Sem_Prag is
                         Error_Pragma_Arg_Ident
                           ("expect name `Decreases`", Variant);
 
+                     elsif Name'Length >= 4
+                       and then Name (1 .. 4) = "stru"
+                     then
+                        Error_Pragma_Arg_Ident
+                          ("expect name `Structural`", Variant);
+
                      else
                         Error_Pragma_Arg_Ident
-                          ("expect name `Increases` or `Decreases`", Variant);
+                          ("expect name `Increases`, `Decreases`,"
+                           & " or `Structural`", Variant);
                      end if;
                   end;
+
+               elsif Chars (Variant) = Name_Structural
+                 and then List_Length (Pragma_Argument_Associations (N)) > 1
+               then
+                  Error_Pragma_Arg_Ident
+                    ("Structural variant shall be the only variant", Variant);
                end if;
 
                --  Preanalyze_Assert_Expression, but without enforcing any of
@@ -19460,9 +19474,12 @@ package body Sem_Prag is
 
                Preanalyze_Assert_Expression (Expression (Variant));
 
-               --  Expression of a discrete type is allowed
+               --  Expression of a discrete type is allowed. Nothing to
+               --  check for structural variants.
 
-               if Is_Discrete_Type (Etype (Expression (Variant))) then
+               if Chars (Variant) = Name_Structural
+                 or else Is_Discrete_Type (Etype (Expression (Variant)))
+               then
                   null;
 
                --  Expression of a Big_Integer type (or its ghost variant) is
@@ -24227,13 +24244,16 @@ package body Sem_Prag is
          -- Subprogram_Variant --
          ------------------------
 
-         --  pragma Subprogram_Variant ( SUBPROGRAM_VARIANT_ITEM
-         --                           {, SUBPROGRAM_VARIANT_ITEM } );
-
-         --  SUBPROGRAM_VARIANT_ITEM ::=
-         --    CHANGE_DIRECTION => discrete_EXPRESSION
+         --  pragma Subprogram_Variant ( SUBPROGRAM_VARIANT_LIST );
 
-         --  CHANGE_DIRECTION ::= Increases | Decreases
+         --  SUBPROGRAM_VARIANT_LIST ::= STRUCTURAL_SUBPROGRAM_VARIANT_ITEM
+         --                            | NUMERIC_SUBPROGRAM_VARIANT_ITEMS
+         --  NUMERIC_SUBPROGRAM_VARIANT_ITEMS ::=
+         --    NUMERIC_SUBPROGRAM_VARIANT_ITEM
+         --      {, NUMERIC_SUBPROGRAM_VARIANT_ITEM}
+         --  NUMERIC_SUBPROGRAM_VARIANT_ITEM ::= CHANGE_DIRECTION => EXPRESSION
+         --  STRUCTURAL_SUBPROGRAM_VARIANT_ITEM ::= Structural => EXPRESSION
+         --  CHANGE_DIRECTION        ::= Increases | Decreases
 
          --  Characteristics:
 
@@ -29435,9 +29455,9 @@ package body Sem_Prag is
          --  Check placement of OTHERS if available (SPARK RM 6.1.3(1))
 
          if Nkind (Direction) = N_Identifier then
-            if Chars (Direction) /= Name_Decreases
-                 and then
-               Chars (Direction) /= Name_Increases
+            if Chars (Direction) not in Name_Decreases
+                                      | Name_Increases
+                                      | Name_Structural
             then
                Error_Msg_N ("wrong direction", Direction);
             end if;
@@ -29452,9 +29472,12 @@ package body Sem_Prag is
 
          Preanalyze_Assert_Expression (Expr);
 
-         --  Expression of a discrete type is allowed
+         --  Expression of a discrete type is allowed. Nothing more to check
+         --  for structural variants.
 
-         if Is_Discrete_Type (Etype (Expr)) then
+         if Is_Discrete_Type (Etype (Expr))
+              or else Chars (Direction) = Name_Structural
+         then
             null;
 
          --  Expression of a Big_Integer type (or its ghost variant) is only
@@ -29561,6 +29584,14 @@ package body Sem_Prag is
          Variant := First (Component_Associations (Variants));
          while Present (Variant) loop
             Analyze_Variant (Variant);
+
+            if Chars (First (Choices (Variant))) = Name_Structural
+              and then List_Length (Component_Associations (Variants)) > 1
+            then
+               Error_Msg_N
+                 ("Structural variant shall be the only variant", Variant);
+            end if;
+
             Next (Variant);
          end loop;
 
diff --git a/gcc/ada/snames.ads-tmpl b/gcc/ada/snames.ads-tmpl
index fa7ac8a..5f7d6b7 100644
--- a/gcc/ada/snames.ads-tmpl
+++ b/gcc/ada/snames.ads-tmpl
@@ -875,6 +875,7 @@ package Snames is
    Name_Static                         : constant Name_Id := N + $;
    Name_Stack_Size                     : constant Name_Id := N + $;
    Name_Strict                         : constant Name_Id := N + $;
+   Name_Structural                     : constant Name_Id := N + $;
    Name_Subunit_File_Name              : constant Name_Id := N + $;
    Name_Suppressible                   : constant Name_Id := N + $;
    Name_Synchronous                    : constant Name_Id := N + $;
diff --git a/gcc/testsuite/gnat.dg/loopvar.adb b/gcc/testsuite/gnat.dg/loopvar.adb
index f85402a..e98d20d 100644
--- a/gcc/testsuite/gnat.dg/loopvar.adb
+++ b/gcc/testsuite/gnat.dg/loopvar.adb
@@ -9,7 +9,7 @@ begin
       pragma Loop_Variant (J + 1);           -- { dg-error "expect name \"Increases\"" }
       pragma Loop_Variant (incr => -J + 1);  -- { dg-error "expect name \"Increases\"" }
       pragma Loop_Variant (decr => -J + 1);  -- { dg-error "expect name \"Decreases\"" }
-      pragma Loop_Variant (foof => -J + 1);  -- { dg-error "expect name \"Increases\" or \"Decreases\"" }
+      pragma Loop_Variant (foof => -J + 1);  -- { dg-error "expect name \"Increases\", \"Decreases\", or \"Structural\"" }
       J := J + 2;
    end loop;
 end Loopvar;