[Ada] Do not assume a priority value of zero is a valid priority

[Pierre-Marie de Rodat <derodat@adacore.com>]

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While a priority value of zero is typically valid on most systems, there
are some targets where zero may be reserved for OS purposes (for
example: RTEMS where zero is reserved for use by the idle thread and
should not be used by applications). This patch removes one occurrence
in GNARL where a Priority object was initialized to zero instead of
Priority'First, and adjusts the Priority type on RTEMS to prevent the
use of priority level zero. By contrast,
System.Tasking.Unspecified_Priority is hardcoded as -1 since it is used
in init.c, which does not have access to the Priority type.

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

gcc/ada/

	* libgnarl/s-taskin.adb (Initialize_ATCB): Initialize
	T.Common.Current_Priority to Priority'First.
	* libgnarl/s-taskin.ads (Unspecified_Priority): Redefined as -1.
	* libgnat/system-rtems.ads: Start priority range from 1, as 0 is
	reserved by the operating system.

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diff --git a/gcc/ada/libgnarl/s-taskin.adb b/gcc/ada/libgnarl/s-taskin.adb
--- a/gcc/ada/libgnarl/s-taskin.adb
+++ b/gcc/ada/libgnarl/s-taskin.adb
@@ -127,7 +127,7 @@ package body System.Tasking is
       end if;
       pragma Assert (T.Common.Domain /= null);
 
-      T.Common.Current_Priority         := 0;
+      T.Common.Current_Priority         := Priority'First;
       T.Common.Protected_Action_Nesting := 0;
       T.Common.Call                     := null;
       T.Common.Task_Arg                 := Task_Arg;


diff --git a/gcc/ada/libgnarl/s-taskin.ads b/gcc/ada/libgnarl/s-taskin.ads
--- a/gcc/ada/libgnarl/s-taskin.ads
+++ b/gcc/ada/libgnarl/s-taskin.ads
@@ -773,7 +773,10 @@ package System.Tasking is
    -- Priority info --
    -------------------
 
-   Unspecified_Priority : constant Integer := System.Priority'First - 1;
+   Unspecified_Priority : constant Integer := -1;
+   --  Indicates that a task has an unspecified priority. This is hardcoded as
+   --  -1 rather than System.Priority'First - 1 as the value needs to be used
+   --  in init.c to specify that the main task has no specified priority.
 
    Priority_Not_Boosted : constant Integer := System.Priority'First - 1;
    --  Definition of Priority actually has to come from the RTS configuration


diff --git a/gcc/ada/libgnat/system-rtems.ads b/gcc/ada/libgnat/system-rtems.ads
--- a/gcc/ada/libgnat/system-rtems.ads
+++ b/gcc/ada/libgnat/system-rtems.ads
@@ -109,15 +109,13 @@ package System is
    --             hardware priority levels.  Protected Object ceilings can
    --             override these values.
    --  245        is used by the Interrupt_Manager task
-   --  0          is reserved for the RTEMS IDLE task and really should not
-   --             be accessible from Ada but GNAT initializes
-   --             Current_Priority to 0 so it must be valid
+   --  0          is reserved for the RTEMS IDLE task
 
    Max_Priority           : constant Positive := 244;
    Max_Interrupt_Priority : constant Positive := 254;
 
-   subtype Any_Priority       is Integer      range   0 .. 254;
-   subtype Priority           is Any_Priority range   0 .. 244;
+   subtype Any_Priority       is Integer      range   1 .. 254;
+   subtype Priority           is Any_Priority range   1 .. 244;
    subtype Interrupt_Priority is Any_Priority range 245 .. 254;
 
    Default_Priority : constant Priority := 122;