[doc] Note variable shadowing at max macro using statement expression

When suggesting to rewrite the unsafe (with respect to multiple evaluation of
arguments) macro definition:
...
  #define max(a,b) ((a) > (b) ? (a) : (b))
...
into the safe macro definition:
...
  #define maxint(a,b) \
    ({int _a = (a), _b = (b); _a > _b ? _a : _b; })
...
mention the variable shadowing problem for:
...
  #define maxint3(a, b, c) \
    ({int _a = (a), _b = (b), _c = (c); maxint (maxint (_a, _b), _c); })
...

2019-04-11  Tom de Vries  <tdevries@suse.de>

	* doc/extend.texi (@node Statement Exprs): Note variable shadowing at
	max macro using statement expression.

From-SVN: r270287

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 7131d58f0adadc264f14c707063165e231dbf499..cf4434a941a78f457f7c344305eafe388b6102e3 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,8 @@
+2019-04-11  Tom de Vries  <tdevries@suse.de>
+
+       * doc/extend.texi (@node Statement Exprs): Note variable shadowing at
+       max macro using statement expression.
+
 2019-04-11  David Edelsohn  <dje.gcc@gmail.com>
 
        * xcoffout.h (xcoff_private_rodata_section_name): Declare.

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 8e0deac26c33b080d9872727fa0a8a34d60051ae..cad7ad49e56ced83f02bce63df0605f8a2466dbe 100644 (file)
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -142,14 +142,36 @@ follows:
 @cindex side effects, macro argument
 But this definition computes either @var{a} or @var{b} twice, with bad
 results if the operand has side effects.  In GNU C, if you know the
-type of the operands (here taken as @code{int}), you can define
-the macro safely as follows:
+type of the operands (here taken as @code{int}), you can avoid this
+problem by defining the macro as follows:
 
 @smallexample
 #define maxint(a,b) \
   (@{int _a = (a), _b = (b); _a > _b ? _a : _b; @})
 @end smallexample
 
+Note that introducing variable declarations (as we do in @code{maxint}) can
+cause variable shadowing, so while this example using the @code{max} macro
+produces correct results:
+@smallexample
+int _a = 1, _b = 2, c;
+c = max (_a, _b);
+@end smallexample
+@noindent
+this example using maxint will not:
+@smallexample
+int _a = 1, _b = 2, c;
+c = maxint (_a, _b);
+@end smallexample
+
+This problem may for instance occur when we use this pattern recursively, like
+so:
+
+@smallexample
+#define maxint3(a, b, c) \
+  (@{int _a = (a), _b = (b), _c = (c); maxint (maxint (_a, _b), _c); @})
+@end smallexample
+
 Embedded statements are not allowed in constant expressions, such as
 the value of an enumeration constant, the width of a bit-field, or
 the initial value of a static variable.