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[Ada] Do not expand code inside ignored ghost bodies


While ignored ghost code is not compiled into the executable, it may
lead to compilation errors when it makes use of language features
requiring runtime support that is not available in the available runtime
library.  These errors are spurious, as the executable will never call
in these runtime units.

This patch deactivates the expansion of code inside ignored ghost bodies
of subprograms and packages, so that this code is still checked for
possible semantic errors, but it does not force the presence of useless
runtime units.

There is no impact on the executable produced.

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

2018-12-11  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_ch6.adb (Analyze_Subprogram_Body_Helper): Deactivate
	expansion in ignored ghost subprogram body.
	* sem_ch7.adb (Analyze_Package_Body_Helper): Deactivate
	expansion in ignored ghost package body.

gcc/testsuite/

	* gnat.dg/ghost4.adb: New testcase.
--- gcc/ada/sem_ch6.adb
+++ gcc/ada/sem_ch6.adb
@@ -3370,6 +3370,7 @@ package body Sem_Ch6 is
 
       Saved_GM   : constant Ghost_Mode_Type := Ghost_Mode;
       Saved_IGR  : constant Node_Id         := Ignored_Ghost_Region;
+      Saved_EA   : constant Boolean         := Expander_Active;
       Saved_ISMP : constant Boolean         :=
                      Ignore_SPARK_Mode_Pragmas_In_Instance;
       --  Save the Ghost and SPARK mode-related data to restore on exit
@@ -3610,6 +3611,18 @@ package body Sem_Ch6 is
          end if;
       end if;
 
+      --  Deactivate expansion inside the body of ignored Ghost entities,
+      --  as this code will ultimately be ignored. This avoids requiring the
+      --  presence of run-time units which are not needed. Only do this for
+      --  user entities, as internally generated entitities might still need
+      --  to be expanded (e.g. those generated for types).
+
+      if Present (Ignored_Ghost_Region)
+        and then Comes_From_Source (Body_Id)
+      then
+         Expander_Active := False;
+      end if;
+
       --  Previously we scanned the body to look for nested subprograms, and
       --  rejected an inline directive if nested subprograms were present,
       --  because the back-end would generate conflicting symbols for the
@@ -4588,6 +4601,10 @@ package body Sem_Ch6 is
       end if;
 
    <<Leave>>
+      if Present (Ignored_Ghost_Region) then
+         Expander_Active := Saved_EA;
+      end if;
+
       Ignore_SPARK_Mode_Pragmas_In_Instance := Saved_ISMP;
       Restore_Ghost_Region (Saved_GM, Saved_IGR);
    end Analyze_Subprogram_Body_Helper;

--- gcc/ada/sem_ch7.adb
+++ gcc/ada/sem_ch7.adb
@@ -669,6 +669,7 @@ package body Sem_Ch7 is
 
       Saved_GM   : constant Ghost_Mode_Type := Ghost_Mode;
       Saved_IGR  : constant Node_Id         := Ignored_Ghost_Region;
+      Saved_EA   : constant Boolean         := Expander_Active;
       Saved_ISMP : constant Boolean         :=
                      Ignore_SPARK_Mode_Pragmas_In_Instance;
       --  Save the Ghost and SPARK mode-related data to restore on exit
@@ -780,6 +781,18 @@ package body Sem_Ch7 is
 
       Mark_And_Set_Ghost_Body (N, Spec_Id);
 
+      --  Deactivate expansion inside the body of ignored Ghost entities,
+      --  as this code will ultimately be ignored. This avoids requiring the
+      --  presence of run-time units which are not needed. Only do this for
+      --  user entities, as internally generated entitities might still need
+      --  to be expanded (e.g. those generated for types).
+
+      if Present (Ignored_Ghost_Region)
+        and then Comes_From_Source (Body_Id)
+      then
+         Expander_Active := False;
+      end if;
+
       --  If the body completes the initial declaration of a compilation unit
       --  which is subject to pragma Elaboration_Checks, set the model of the
       --  pragma because it applies to all parts of the unit.
@@ -1075,6 +1088,10 @@ package body Sem_Ch7 is
          end if;
       end if;
 
+      if Present (Ignored_Ghost_Region) then
+         Expander_Active := Saved_EA;
+      end if;
+
       Ignore_SPARK_Mode_Pragmas_In_Instance := Saved_ISMP;
       Restore_Ghost_Region (Saved_GM, Saved_IGR);
    end Analyze_Package_Body_Helper;

--- /dev/null
new file mode 100644
+++ gcc/testsuite/gnat.dg/ghost4.adb
@@ -0,0 +1,15 @@
+pragma Restrictions (No_Secondary_Stack);
+
+procedure Ghost4 is
+
+   procedure Dummy with Ghost is
+      function Slice (S : String) return String is
+         (S (S'First .. S'First + 3));
+
+      X : String := Slice ("hello");
+   begin
+         null;
+   end Dummy;
+begin
+   Dummy;
+end Ghost4;


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