Re: [RFC, doc] Note variable shadowing at max macro using statement expression
On 4/8/19 5:38 AM, Tom de Vries wrote:
Hi,
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); })
...
Any comments?
The content looks reasonable, but I have some copy-editing nits.
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 8e0deac26c3..27ed0fb014f 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -143,13 +143,34 @@ follows:
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:
+the macro safe (from evaluating operands more than once) as follows:
That doesn't read well. I suggest
...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 maxint) can
@code{maxint}
+cause variable shadowing, so while this example using the max macro will
@code{max}
+produce correct results:
s/will produce/produces/
+@smallexample
+int _a = 1, _b = 2, c;
+c = max (_a, _b);
+@end smallexample
+this example using maxint will not:
@noindent
this example using @code{maxint} does 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.
-Sandra
[RFC, doc] Note variable shadowing at max macro using statement expression
Hi,
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); })
...
Any comments?
Thanks,
- Tom
[doc] Note variable shadowing at max macro using statement expression
2019-04-08 Tom de Vries
* doc/extend.texi (@node Statement Exprs): Note variable shadowing at
max macro using statement expression.
---
gcc/doc/extend.texi | 23 ++-
1 file changed, 22 insertions(+), 1 deletion(-)
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 8e0deac26c3..27ed0fb014f 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -143,13 +143,34 @@ follows:
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:
+the macro safe (from evaluating operands more than once) 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 maxint) can
+cause variable shadowing, so while this example using the max macro will
+produce correct results:
+@smallexample
+int _a = 1, _b = 2, c;
+c = max (_a, _b);
+@end smallexample
+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.
