[PATCH v14 1/2] eal: add __rte_may_alias to unaligned typedefs

[scott.k.mitch1 at gmail.com]

From: Scott Mitchell <scott.k.mitch1 at gmail.com>

Add __rte_may_alias attribute to unaligned_uint{16,32,64}_t typedefs
to prevent GCC strict-aliasing optimization bugs. GCC has a bug where
it incorrectly elides struct initialization when strict aliasing is
enabled, causing reads from uninitialized memory.

The __rte_may_alias attribute signals to the compiler that these types
can alias other types, preventing the incorrect optimization.

Signed-off-by: Scott Mitchell <scott.k.mitch1 at gmail.com>
---
 lib/eal/include/rte_common.h | 34 +++++++++++++++++++---------------
 1 file changed, 19 insertions(+), 15 deletions(-)

diff --git a/lib/eal/include/rte_common.h b/lib/eal/include/rte_common.h
index 9e7d84f929..ac70270cfb 100644
--- a/lib/eal/include/rte_common.h
+++ b/lib/eal/include/rte_common.h
@@ -121,14 +121,27 @@ extern "C" {
 #define __rte_aligned(a) __attribute__((__aligned__(a)))
 #endif
 
+/**
+ * Macro to mark a type that is not subject to type-based aliasing rules
+ */
+#ifdef RTE_TOOLCHAIN_MSVC
+#define __rte_may_alias
+#else
+#define __rte_may_alias __attribute__((__may_alias__))
+#endif
+
+/**
+ * __rte_may_alias avoids compiler bugs (GCC) that elide initialization
+ * of memory when strict-aliasing is enabled.
+ */
 #ifdef RTE_ARCH_STRICT_ALIGN
-typedef uint64_t unaligned_uint64_t __rte_aligned(1);
-typedef uint32_t unaligned_uint32_t __rte_aligned(1);
-typedef uint16_t unaligned_uint16_t __rte_aligned(1);
+typedef uint64_t unaligned_uint64_t __rte_may_alias __rte_aligned(1);
+typedef uint32_t unaligned_uint32_t __rte_may_alias __rte_aligned(1);
+typedef uint16_t unaligned_uint16_t __rte_may_alias __rte_aligned(1);
 #else
-typedef uint64_t unaligned_uint64_t;
-typedef uint32_t unaligned_uint32_t;
-typedef uint16_t unaligned_uint16_t;
+typedef uint64_t unaligned_uint64_t __rte_may_alias;
+typedef uint32_t unaligned_uint32_t __rte_may_alias;
+typedef uint16_t unaligned_uint16_t __rte_may_alias;
 #endif
 
 /**
@@ -159,15 +172,6 @@ typedef uint16_t unaligned_uint16_t;
 #define __rte_packed_end __attribute__((__packed__))
 #endif
 
-/**
- * Macro to mark a type that is not subject to type-based aliasing rules
- */
-#ifdef RTE_TOOLCHAIN_MSVC
-#define __rte_may_alias
-#else
-#define __rte_may_alias __attribute__((__may_alias__))
-#endif
-
 /******* Macro to mark functions and fields scheduled for removal *****/
 #ifdef RTE_TOOLCHAIN_MSVC
 #define __rte_deprecated
-- 
2.39.5 (Apple Git-154)