C backend: fix #858 - internal structure size mismatch due to padding differences

A structure which decreases its alignment (using FIELD = n) is annotated with both the "packed" and the "alignment(n)" attributes. That leads to a problem if such a structure is nested inside another structure that uses a greater alignment than the child structure. In that case the field representing the nested structure is aligned with the same properties as the nested structure.

So the alignment has to be increased explicitly if the nested structure is packed with a smaller alignment than the parent structure. That is accomplished by adding the "alignment(n)" attribute to those fields.

Author: swurzinger

src/compiler/ir-hlc.bas

@@ -1038,6 +1038,23 @@ private sub hEmitStructWithFields( byval s as FBSYMBOL ptr )
 					ln += " __attribute__((packed, aligned(" + str( align ) + ")))"
 				end if
 			end if
+			
+			'' The alignment of nested structures which are packed with a 
+			'' smaller alignment than the natural or specified alignment of the
+			'' parent structure has to be specified explicitly,
+			'' otherwise the field will be packed too.
+			if ( align <= 0 orelse skip ) then
+				if( typeGet( dtype ) = FB_DATATYPE_STRUCT ) then
+					var effectivealign = typeCalcNaturalAlign( dtype, subtype )
+					if ( align > 0 andalso align < effectivealign ) then
+						effectivealign = align
+					end if
+					var fieldudtalign = symbGetUDTAlign( subtype )
+					if( fieldudtalign > 0 andalso effectivealign > fieldudtalign ) then
+						ln += " __attribute__((aligned(" + str( effectivealign ) + ")))"
+					end if
+				end if
+			end if
 
 			ln += ";"
 			hWriteLine( ln, TRUE )