2018-08-01  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* doc/generic.texi (STRING_CST): Update.
	* varasm.c (check_string_literal): New checking function.
	(output_constant): Use it.

diff -pur gcc/varasm.c gcc/varasm.c
--- gcc/varasm.c	2018-07-17 11:19:27.000000000 +0200
+++ gcc/varasm.c	2018-07-31 10:16:12.058827505 +0200
@@ -4774,6 +4774,29 @@ initializer_constant_valid_for_bitfield_
   return false;
 }
 
+/* Check if a STRING_CST fits into the field.
+   Tolerate only the case when the NUL termination
+   does not fit into the field.   */
+
+static bool
+check_string_literal (tree string, unsigned HOST_WIDE_INT size)
+{
+  tree eltype = TREE_TYPE (TREE_TYPE (string));
+  unsigned HOST_WIDE_INT elts = tree_to_uhwi (TYPE_SIZE_UNIT (eltype));
+  const char *p = TREE_STRING_POINTER (string);
+  int len = TREE_STRING_LENGTH (string);
+
+  if (elts != 1 && elts != 2 && elts != 4)
+    return false;
+  if (len <= 0 || len % elts != 0)
+    return false;
+  if (size < (unsigned)len && size != len - elts)
+    return false;
+  if (memcmp (p + len - elts, "\0\0\0\0", elts) != 0)
+    return false;
+  return true;
+}
+
 /* output_constructor outer state of relevance in recursive calls, typically
    for nested aggregate bitfields.  */
 
@@ -4942,6 +4965,7 @@ output_constant (tree exp, unsigned HOST
 	case STRING_CST:
 	  thissize
 	    = MIN ((unsigned HOST_WIDE_INT)TREE_STRING_LENGTH (exp), size);
+	  gcc_checking_assert (check_string_literal (exp, size));
 	  assemble_string (TREE_STRING_POINTER (exp), thissize);
 	  break;
 	case VECTOR_CST:
Index: gcc/doc/generic.texi
===================================================================
--- gcc/doc/generic.texi	(revision 263072)
+++ gcc/doc/generic.texi	(working copy)
@@ -1162,9 +1162,13 @@ These nodes represent string-constants.  The @code
 returns the length of the string, as an @code{int}.  The
 @code{TREE_STRING_POINTER} is a @code{char*} containing the string
 itself.  The string may not be @code{NUL}-terminated, and it may contain
-embedded @code{NUL} characters.  Therefore, the
-@code{TREE_STRING_LENGTH} includes the trailing @code{NUL} if it is
-present.
+embedded @code{NUL} characters.  However, the
+@code{TREE_STRING_LENGTH} always includes a trailing @code{NUL} that
+is not part of the language string literal but appended by the front end.
+If the string shall not be @code{NUL}-terminated the @code{TREE_TYPE}
+is one character shorter than @code{TREE_STRING_LENGTH}.
+Excess caracters other than one trailing @code{NUL} character are not
+permitted.
 
 For wide string constants, the @code{TREE_STRING_LENGTH} is the number
 of bytes in the string, and the @code{TREE_STRING_POINTER}
@@ -1173,6 +1177,11 @@ target system (that is, as integers in the target
 non-wide string constants are distinguished only by the @code{TREE_TYPE}
 of the @code{STRING_CST}.
 
+String constants may also be used for other purpose like assember
+constraints or attributes.  These have no @code{TREE_TYPE} and
+need no explicit @code{NUL}-termination.  Note there is always
+another @code{NUL}-byte beyond @code{TREE_STRING_LENGTH}.
+
 FIXME: The formats of string constants are not well-defined when the
 target system bytes are not the same width as host system bytes.