making INT_STRLEN_BOUND tight for unsigned types

[Jim Meyering]

The fact that INT_STRLEN_BOUND would overestimate for common sizes like
2, 4, and 16-byte unsigned quantities has been a minor annoyance (albeit
more aesthetic/pedantic than practical).  Today I saw how to fix it,
but only when compiling with gcc.

Reminder: we can't just use TYPE_SIGNED(t) directly because
that macro casts to type (t), and INT_STRLEN_BOUND's `t'
may be an expression as well as a type.

An unexpected bonus: this change makes the comment,
  `Subtract 1 for the sign bit if T is signed;'
obviously true once again.

Any objection?

P.S. The first time I coded this, I did it with two
INT_STRLEN_BOUND definitions, but that definition has
enough grist that I prefer to define a new private macro.

P.P.S: At least two other compilers do support __typeof__,
but I don't think its worth the autoconf test required to
let them use the tighter bound here.

2005-09-27  Jim Meyering  <address@hidden>

        * intprops.h (signed_type_or_expr__): Define.
        (INT_STRLEN_BOUND) [__GNUC__]: Use a slightly tighter bound
        for unsigned types.

Index: lib/intprops.h
===================================================================
RCS file: /cvsroot/gnulib/gnulib/lib/intprops.h,v
retrieving revision 1.6
diff -u -p -r1.6 intprops.h
--- lib/intprops.h      14 May 2005 06:03:58 -0000      1.6
+++ lib/intprops.h      26 Sep 2005 22:42:57 -0000
@@ -53,12 +53,25 @@
        ? (t) -1 \
        : ~ (~ (t) 0 << (sizeof (t) * CHAR_BIT - 1))))
 
+/* Return zero if T can be determined to be an unsigned type.
+   Otherwise, return 1.
+   When compiling with GCC, INT_STRLEN_BOUND uses this macro to obtain a
+   tighter bound.  Otherwise, it overestimates the true bound by one byte
+   when applied to unsigned types of size 2, 4, 16, ... bytes.
+   The symbol signed_type_or_expr__ is private to this header file.  */
+#if __GNUC__
+# define signed_type_or_expr__(t) TYPE_SIGNED (__typeof__ (t))
+#else
+# define signed_type_or_expr__(t) 1
+#endif
+
 /* Bound on length of the string representing an integer type or expression T.
-   Subtract 1 for the sign bit if t is signed; log10 (2.0) < 146/485;
+   Subtract 1 for the sign bit if T is signed; log10 (2.0) < 146/485;
    add 1 for integer division truncation; add 1 more for a minus sign
    if needed.  */
 #define INT_STRLEN_BOUND(t) \
-  ((sizeof (t) * CHAR_BIT - 1) * 146 / 485 + 2)
+  ((sizeof (t) * CHAR_BIT - signed_type_or_expr__ (t)) * 146 / 485 \
+   + signed_type_or_expr__ (t) + 1)
 
 /* Bound on buffer size needed to represent an integer type or expression T,
    including the terminating null.  */