[Ada] Lock_Free aspect takes precedence over Ceiling_Locking locking policy

["Marc Poulhiès" <poulhies@adacore.com>]

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Allow the Lock_Free aspect of a protected type to be True (but only by
explicit specification) if Ceiling_Locking locking policy has been specified.
Clarify that the Lock_Free aspect takes precedence over the Ceiling_Locking
policy in that case.

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

gcc/ada/

	* sem_ch9.adb
	(Satisfies_Lock_Free_Requirements): If Ceiling_Locking locking
	policy has been specified, then either return False (if Lock_Free
	was not explicitly specified) or generate a warning that ceiling
	locking will not be implemented for this protected unit (if
	Lock_Free was explicitly specified). Generate an error message (in
	addition to returning False) if an explicit Lock_Free aspect
	specification is rejected because atomic primitives are not
	supported on the given target.
	* doc/gnat_rm/implementation_defined_pragmas.rst: Clarify that the
	Lock_Free aspect for a protected unit takes precedence over the
	Ceiling_Locking locking policy in the case where both apply.
	* gnat_rm.texi: Regenerate.

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diff --git a/gcc/ada/doc/gnat_rm/implementation_defined_pragmas.rst b/gcc/ada/doc/gnat_rm/implementation_defined_pragmas.rst
--- a/gcc/ada/doc/gnat_rm/implementation_defined_pragmas.rst
+++ b/gcc/ada/doc/gnat_rm/implementation_defined_pragmas.rst
@@ -3752,7 +3752,12 @@ In addition, each protected subprogram body must satisfy:
 * May not dereferenced access values
 * Function calls and attribute references must be static
 
-
+If the Lock_Free aspect is specified to be True for a protected unit
+and the Ceiling_Locking locking policy is in effect, then the run-time
+actions associated with the Ceiling_Locking locking policy (described in
+Ada RM D.3) are not performed when a protected operation of the protected
+unit is executed.
+  
 Pragma Loop_Invariant
 =====================
 


diff --git a/gcc/ada/gnat_rm.texi b/gcc/ada/gnat_rm.texi
--- a/gcc/ada/gnat_rm.texi
+++ b/gcc/ada/gnat_rm.texi
@@ -19,7 +19,7 @@
 
 @copying
 @quotation
-GNAT Reference Manual , Aug 25, 2022
+GNAT Reference Manual , Sep 09, 2022
 
 AdaCore
 
@@ -5273,6 +5273,12 @@ May not dereferenced access values
 Function calls and attribute references must be static
 @end itemize
 
+If the Lock_Free aspect is specified to be True for a protected unit
+and the Ceiling_Locking locking policy is in effect, then the run-time
+actions associated with the Ceiling_Locking locking policy (described in
+Ada RM D.3) are not performed when a protected operation of the protected
+unit is executed.
+
 @node Pragma Loop_Invariant,Pragma Loop_Optimize,Pragma Lock_Free,Implementation Defined Pragmas
 @anchor{gnat_rm/implementation_defined_pragmas pragma-loop-invariant}@anchor{95}
 @section Pragma Loop_Invariant


diff --git a/gcc/ada/sem_ch9.adb b/gcc/ada/sem_ch9.adb
--- a/gcc/ada/sem_ch9.adb
+++ b/gcc/ada/sem_ch9.adb
@@ -651,9 +651,34 @@ package body Sem_Ch9 is
 
             begin
                if not Support_Atomic_Primitives_On_Target then
+                  if Lock_Free_Given then
+                     Error_Msg_N
+                       ("Lock_Free aspect requires target support for "
+                          & "atomic primitives", N);
+                  end if;
                   return False;
                end if;
 
+               --  Deal with case where Ceiling_Locking locking policy is
+               --  in effect.
+
+               if Locking_Policy = 'C' then
+                  if Lock_Free_Given then
+                     --  Explicit Lock_Free aspect spec overrides
+                     --  Ceiling_Locking so we generate a warning.
+
+                     Error_Msg_N
+                       ("Lock_Free aspect specification overrides "
+                          & "Ceiling_Locking locking policy??", N);
+                  else
+                     --  If Ceiling_Locking locking policy is in effect, then
+                     --  Lock_Free can be explicitly specified but it is
+                     --  never the default.
+
+                     return False;
+                  end if;
+               end if;
+
                --  Get the number of errors detected by the compiler so far
 
                if Lock_Free_Given then