[Xen-devel] [PATCH 2/2] xen/mm: Introduce {G, M}FN_INVALID_INITIALIZER

[Julien Grall <julien.grall@xxxxxxx>]

The current implementation of {G,M}FN_INVALID cannot be used to
initialize global variable because the initializer element is not a
constant.

Due to a bug in GCC 4.9 and older ([1]), it is not easy to find a common
value to initialize a variable and directly passed as an argument.

Introduce 2 news define {G,M}FN_INVALID_INITIALIZER to be used for
initializing a variable.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64856

Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

---
    Build tested it with:
        * ARM: GCC 4.9.4, 5.1, 4.3, 6.1.1, 7.1.0
        * x86: Clang 3.5.0, 3.6.0, 3.6.2, 3.8.0, 3.9.0, 4.0.0

    With introducing a dummy global variable common/mm.c:

    mfn_t foo = INVALID_MFN_INITIALIZER
---
 xen/include/xen/mm.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/xen/include/xen/mm.h b/xen/include/xen/mm.h
index 0050fba498..251db4ffa1 100644
--- a/xen/include/xen/mm.h
+++ b/xen/include/xen/mm.h
@@ -57,6 +57,11 @@
 TYPE_SAFE(unsigned long, mfn);
 #define PRI_mfn          "05lx"
 #define INVALID_MFN      _mfn(~0UL)
+/*
+ * To be used for global variable initialization. This workaround a bug
+ * in GCC < 5.0.
+ */
+#define INVALID_MFN_INITIALIZER {~0UL}
 
 #ifndef mfn_t
 #define mfn_t /* Grep fodder: mfn_t, _mfn() and mfn_x() are defined above */
@@ -90,6 +95,11 @@ static inline bool_t mfn_eq(mfn_t x, mfn_t y)
 TYPE_SAFE(unsigned long, gfn);
 #define PRI_gfn          "05lx"
 #define INVALID_GFN      _gfn(~0UL)
+/*
+ * To be used for global variable initialization. This workaround a bug
+ * in GCC < 5.0 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64856
+ */
+#define INVALID_GFN_INITIALIZER { ~0UL }
 
 #ifndef gfn_t
 #define gfn_t /* Grep fodder: gfn_t, _gfn() and gfn_x() are defined above */
-- 
2.11.0


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