[Ada] Alignment may be specified as zero

An Alignment clause or an aspect_specification for Alignment may be
specified as 0, which is treated the same as 1.

2019-08-14  Bob Duff  <duff@adacore.com>

gcc/ada/

	* sem_ch13.adb (Get_Alignment_Value): Return 1 for Alignment 0,
	and do not give an error.
	* doc/gnat_rm/representation_clauses_and_pragmas.rst: Update the
	corresponding documentation.
	* gnat_rm.texi: Regenerate.

gcc/testsuite/

	* gnat.dg/alignment15.adb: New testcase.

From-SVN: r274473

gcc/ada/ChangeLog

+2019-08-14  Bob Duff  <duff@adacore.com>
+
+	* sem_ch13.adb (Get_Alignment_Value): Return 1 for Alignment 0,
+	and do not give an error.
+	* doc/gnat_rm/representation_clauses_and_pragmas.rst: Update the
+	corresponding documentation.
+	* gnat_rm.texi: Regenerate.
+
 2019-08-14  Eric Botcazou  <ebotcazou@adacore.com>
 
 	* inline.adb (Add_Pending_Instantiation): Fix off-by-one error

gcc/ada/doc/gnat_rm/representation_clauses_and_pragmas.rst

@@ -30,9 +30,11 @@ Alignment Clauses
 
 .. index:: Alignment Clause
 
-GNAT requires that all alignment clauses specify a power of 2, and all
-default alignments are always a power of 2.  The default alignment
-values are as follows:
+GNAT requires that all alignment clauses specify 0 or a power of 2, and
+all default alignments are always a power of 2. Specifying 0 is the
+same as specifying 1.
+
+The default alignment values are as follows:
 
 * *Elementary Types*.
 

gcc/ada/gnat_rm.texi

@@ -21,7 +21,7 @@
 
 @copying
 @quotation
-GNAT Reference Manual , Jul 31, 2019
+GNAT Reference Manual , Aug 01, 2019
 
 AdaCore
 
@@ -18369,9 +18369,11 @@ and this section describes the additional capabilities provided.
 
 @geindex Alignment Clause
 
-GNAT requires that all alignment clauses specify a power of 2, and all
-default alignments are always a power of 2.  The default alignment
-values are as follows:
+GNAT requires that all alignment clauses specify 0 or a power of 2, and
+all default alignments are always a power of 2. Specifying 0 is the
+same as specifying 1.
+
+The default alignment values are as follows:
 
 
 @itemize *

gcc/ada/sem_ch13.adb

@@ -11509,7 +11509,7 @@ package body Sem_Ch13 is
       if Align = No_Uint then
          return No_Uint;
 
-      elsif Align <= 0 then
+      elsif Align < 0 then
 
          --  This error is suppressed in ASIS mode to allow for different ASIS
          --  back ends or ASIS-based tools to query the illegal clause.
@@ -11520,6 +11520,11 @@ package body Sem_Ch13 is
 
          return No_Uint;
 
+      --  If Alignment is specified to be 0, we treat it the same as 1
+
+      elsif Align = 0 then
+         return Uint_1;
+
       else
          for J in Int range 0 .. 64 loop
             declare

gcc/testsuite/ChangeLog

 2019-08-14  Bob Duff  <duff@adacore.com>
 
+	* gnat.dg/alignment15.adb: New testcase.
+
+2019-08-14  Bob Duff  <duff@adacore.com>
+
 	* gnat.dg/warn27.adb: New testcase.
 
 2019-08-14  Bob Duff  <duff@adacore.com>

gcc/testsuite/gnat.dg/alignment15.adb

+--  { dg-compile }
+
+procedure Alignment15 is
+   type T0 is record
+      X : Integer;
+   end record;
+   for T0'Alignment use 0;
+
+   type T00 is record
+      X : Integer;
+   end record with Alignment => 0;
+
+   Dummy0  : T0;
+   Dummy00 : T00;
+begin
+   null;
+end;