[PATCH 01/10] eal: add workaround for __builtin_constant_p

[Andre Muezerie]

There's no MSVC equivalent for compiler extension __builtin_constant_p.
EAL already had __rte_constant which was used as a first attempt to
workaround __builtin_constant_p when using MSVC. However, there are
pieces of code that would benefit from being able to provide a default
value to be used instead of it being always 0 like how it was done by
__rte_constant.

A new macro is added here allowing such default to be provided by the
caller.

Signed-off-by: Andre Muezerie <andremue at linux.microsoft.com>
---
 lib/eal/include/generic/rte_pause.h |  2 +-
 lib/eal/include/rte_common.h        | 12 ++++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/lib/eal/include/generic/rte_pause.h b/lib/eal/include/generic/rte_pause.h
index 968c0886d3..57e13807ea 100644
--- a/lib/eal/include/generic/rte_pause.h
+++ b/lib/eal/include/generic/rte_pause.h
@@ -130,7 +130,7 @@ rte_wait_until_equal_64(volatile uint64_t *addr, uint64_t expected,
  *  rte_memory_order_acquire and rte_memory_order_relaxed.
  */
 #define RTE_WAIT_UNTIL_MASKED(addr, mask, cond, expected, memorder) do { \
-	RTE_BUILD_BUG_ON(!__builtin_constant_p(memorder));               \
+	RTE_BUILD_BUG_ON(!__rte_constant_with_default(memorder, 1));     \
 	RTE_BUILD_BUG_ON((memorder) != rte_memory_order_acquire &&       \
 		(memorder) != rte_memory_order_relaxed);                 \
 	typeof(*(addr)) expected_value = (expected);                     \
diff --git a/lib/eal/include/rte_common.h b/lib/eal/include/rte_common.h
index 3f77b7624e..6bdce70551 100644
--- a/lib/eal/include/rte_common.h
+++ b/lib/eal/include/rte_common.h
@@ -50,6 +50,18 @@ extern "C" {
 #define __rte_constant(e) __extension__(__builtin_constant_p(e))
 #endif
 
+/**
+ * Determine if an expression's value is constant at compile time.
+ * All compilers except MSVC will return 1 if the first argument is a
+ * compile-time constant, and 0 otherwise. MSVC will just return the second
+ * argument as a default value.
+ */
+#ifdef RTE_TOOLCHAIN_MSVC
+#define __rte_constant_with_default(e, def) def
+#else
+#define __rte_constant_with_default(e, def) __extension__(__builtin_constant_p(e))
+#endif
+
 /*
  * RTE_TOOLCHAIN_GCC is defined if the target is built with GCC,
  * while a host application (like pmdinfogen) may have another compiler.
-- 
2.47.2.vfs.0.1