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[Ada] Improve checking for invalid index values when accessing array elements

Message ID 20211004084727.GA1536092@adacore.com
State New
Headers show
Series [Ada] Improve checking for invalid index values when accessing array elements | expand

Commit Message

Pierre-Marie de Rodat Oct. 4, 2021, 8:47 a.m. UTC 
Two improvements to the previous change on this topic:

1) Add a guard to prevent a call to Number_Of_Dimensions that would pass
   in a non-array type. This is needed in error cases (see ACATS test
   B95094C).

2) Do not generate the new validity checks in the case where the
   index type in question has a specified Default_Initial_Value aspect
   (which rules out the possibility that an object is invalid because it
   is uninitialized).

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* exp_ch4.adb (Expand_N_Indexed_Component): The two improvements
	described above.
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Patch

diff --git a/gcc/ada/exp_ch4.adb b/gcc/ada/exp_ch4.adb
--- a/gcc/ada/exp_ch4.adb
+++ b/gcc/ada/exp_ch4.adb
@@ -7255,11 +7255,15 @@  package body Exp_Ch4 is
       --  Generate index and validity checks
 
       declare
-         Dims_Checked : Dimension_Set (Dimensions => Number_Dimensions (T));
+         Dims_Checked : Dimension_Set (Dimensions =>
+                                         (if Is_Array_Type (T)
+                                          then Number_Dimensions (T)
+                                          else 1));
          --  Dims_Checked is used to avoid generating two checks (one in
          --  Generate_Index_Checks, one in Apply_Subscript_Validity_Checks)
          --  for the same index value in cases where the index check eliminates
-         --  the need for the validity check.
+         --  the need for the validity check. The Is_Array_Type test avoids
+         --  cascading errors.
 
       begin
          Generate_Index_Checks (N, Checks_Generated => Dims_Checked);
@@ -7284,6 +7288,27 @@  package body Exp_Ch4 is
                --  If Validity_Check_Subscripts is True then we need to
                --  ensure validity, so we adjust Dims_Checked accordingly.
                Dims_Checked.Elements := (others => False);
+
+            elsif Is_Array_Type (T) then
+               --  We are only adding extra validity checks here to
+               --  deal with uninitialized variables (but this includes
+               --  assigning one uninitialized variable to another). Other
+               --  ways of producing invalid objects imply erroneousness, so
+               --  the compiler can do whatever it wants for those cases.
+               --  If an index type has the Default_Value aspect specified,
+               --  then we don't have to worry about the possibility of an
+               --  uninitialized variable, so no need for these extra
+               --  validity checks.
+
+               declare
+                  Idx : Node_Id := First_Index (T);
+               begin
+                  for No_Check_Needed of Dims_Checked.Elements loop
+                     No_Check_Needed := No_Check_Needed
+                       or else Has_Aspect (Etype (Idx), Aspect_Default_Value);
+                     Next_Index (Idx);
+                  end loop;
+               end;
             end if;
 
             Apply_Subscript_Validity_Checks