[PATCH v2 2/5] compiler.h: Add generic support for 'autoterminating nested for() loops'

[david.laight.linux@gmail.com]

From: David Laight <david.laight.linux@gmail.com>

Autoterminating nested for() loops can be used inside #defines to
declare variables that are scoped to the statement that follows.
These are used by __scoped_user_access() but may have other uses
and the gory details are best separated from the use.

Using 'with (declaration)' and 'and_with (declaration)' seems to
read reasonably well and doesn't seem to collide with any existing
code.

As an example the scoped user access definition becomes:
	with (auto _tmpptr = __scoped_user_access_begin(mode, uptr, size, elbl)) \
		/* Force modified pointer usage within the scope */		\
		and_with (const auto uptr __cleanup(...) = _tmpptr)

Signed-off-by: David Laight <david.laight.linux@gmail.com>
---
 include/linux/compiler.h | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index af16624b29fd..1098a91b5591 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -369,6 +369,32 @@ static inline void *offset_to_ptr(const int *off)
  */
 #define prevent_tail_call_optimization()	mb()
 
+/*
+ * Sometimes a #define needs to declare a variable that is scoped
+ * to the statement that follows without having mismatched {}.
+ *	with (int x = expression) {
+ *		statements
+ *	}
+ * is the same as:
+ *	{
+ *		int x = expression;
+ *		statements
+ *	}
+ * but lets it all be hidden from the call site, eg:
+ *	frobnicate(x, args) {
+ *		statements
+ *	} 
+ * Only a single variable can be defined, and_with() allows extra ones
+ * without adding an additional outer loop.
+ *
+ * The controlled scope can be terminated using return, break, continue or goto.
+ */
+#define with(declaration) \
+	for (bool _with_done = false; !_with_done; _with_done = true)	\
+		and_with (declaration)
+#define and_with(declaration) \
+	for (declaration; !_with_done; _with_done = true)
+
 #include <asm/rwonce.h>
 
 #endif /* __LINUX_COMPILER_H */
-- 
2.39.5