cppmagic: Allow multiple and deferred evaluation

Recursion (and therefore iteration) in cpp is difficult, since the
preprocessor explicitly looks for and inhibits recursion.

But, it's possible to trick it, up to a point.  CPPMAGIC_DEFER1() and
CPPMAGIC_DEFER2() can "hide" a macro, preventing it from being expanded
and being noticed as recursion.

Along with that we need to cause extra expansion passes to be executed.
There has to be a finite limit here - true recursion is impossible - but
that number can be made very large pretty easily.  CPPMAGIC_EVAL() multiply
expands its argument(s) - up to 1024 times.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

diff --git a/ccan/cppmagic/cppmagic.h b/ccan/cppmagic/cppmagic.h
index b5c35782f78ca5e58ab16fa45bddb0649c87333c..8acbba71a68d4e97c849831f7b5abd55bcc4f1fe 100644 (file)
--- a/ccan/cppmagic/cppmagic.h
+++ b/ccan/cppmagic/cppmagic.h
@@ -98,4 +98,39 @@
 #define CPPMAGIC_IFELSE(cond_)         \
        _CPPMAGIC_IFELSE(CPPMAGIC_NONZERO(cond_))
 
+/**
+ * CPPMAGIC_EVAL - force multiple expansion passes
+ *
+ * Forces macros in the arguments to be expanded repeatedly (up to
+ * 1024 times) even when CPP would usually stop expanding.
+ */
+#define CPPMAGIC_EVAL1(...)            __VA_ARGS__
+#define CPPMAGIC_EVAL2(...)            \
+       CPPMAGIC_EVAL1(CPPMAGIC_EVAL1(__VA_ARGS__))
+#define CPPMAGIC_EVAL4(...)            \
+       CPPMAGIC_EVAL2(CPPMAGIC_EVAL2(__VA_ARGS__))
+#define CPPMAGIC_EVAL8(...)            \
+       CPPMAGIC_EVAL4(CPPMAGIC_EVAL4(__VA_ARGS__))
+#define CPPMAGIC_EVAL16(...)           \
+       CPPMAGIC_EVAL8(CPPMAGIC_EVAL8(__VA_ARGS__))
+#define CPPMAGIC_EVAL32(...)           \
+       CPPMAGIC_EVAL16(CPPMAGIC_EVAL16(__VA_ARGS__))
+#define CPPMAGIC_EVAL64(...)           \
+       CPPMAGIC_EVAL32(CPPMAGIC_EVAL32(__VA_ARGS__))
+#define CPPMAGIC_EVAL128(...)          \
+       CPPMAGIC_EVAL64(CPPMAGIC_EVAL64(__VA_ARGS__))
+#define CPPMAGIC_EVAL256(...)          \
+       CPPMAGIC_EVAL128(CPPMAGIC_EVAL128(__VA_ARGS__))
+#define CPPMAGIC_EVAL512(...)          \
+       CPPMAGIC_EVAL256(CPPMAGIC_EVAL256(__VA_ARGS__))
+#define CPPMAGIC_EVAL1024(...)         \
+       CPPMAGIC_EVAL512(CPPMAGIC_EVAL512(__VA_ARGS__))
+#define CPPMAGIC_EVAL(...)             CPPMAGIC_EVAL1024(__VA_ARGS__)
+
+/**
+ * CPPMAGIC_DEFER1, CPPMAGIC_DEFER2 - defer expansion
+ */
+#define CPPMAGIC_DEFER1(a_)    a_ CPPMAGIC_NOTHING()
+#define CPPMAGIC_DEFER2(a_)    a_ CPPMAGIC_NOTHING CPPMAGIC_NOTHING()()
+
 #endif /* CCAN_CPPMAGIC_H */

diff --git a/ccan/cppmagic/test/run.c b/ccan/cppmagic/test/run.c
index 0f8917d6a4afa5d8e620504a197fd7ae14cec7b8..2aa95303ed52d21a14f3a536cc580e7d7050bf07 100644 (file)
--- a/ccan/cppmagic/test/run.c
+++ b/ccan/cppmagic/test/run.c
@@ -15,9 +15,12 @@ static inline void check1(const char *orig, const char *expand,
 #define CHECK1(orig, match) \
        check1(#orig, CPPMAGIC_STRINGIFY(orig), match)
 
+#define TESTRECURSE()  R CPPMAGIC_DEFER1(_TESTRECURSE)()()
+#define _TESTRECURSE() TESTRECURSE
+
 int main(void)
 {
-       plan_tests(24);
+       plan_tests(27);
 
        CHECK1(CPPMAGIC_NOTHING(), "");
        CHECK1(CPPMAGIC_GLUE2(a, b), "ab");
@@ -51,6 +54,10 @@ int main(void)
        CHECK1(CPPMAGIC_IFELSE(1)(abc)(def), "abc");
        CHECK1(CPPMAGIC_IFELSE(not zero)(abc)(def), "abc");
 
+       CHECK1(TESTRECURSE(), "R R _TESTRECURSE ()()");
+       CHECK1(CPPMAGIC_EVAL1(TESTRECURSE()), "R R R _TESTRECURSE ()()");
+       CHECK1(CPPMAGIC_EVAL2(TESTRECURSE()), "R R R R R _TESTRECURSE ()()");
+
        /* This exits depending on whether all tests passed */
        return exit_status();
 }