[Mesa-dev] [PATCH 1/2] mesa: Detect and provide macros for function attributes pure and const.

[Eric Anholt]

These are really useful hints to the compiler in the absence of link-time
optimization, and I'm going to use them in VC4.

I've made the const attribute be ATTRIBUTE_CONST unlike other function
attributes, because we have other things in the tree #defining CONST for
their own unrelated purposes.
---
 configure.ac      |  2 ++
 src/util/macros.h | 20 ++++++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/configure.ac b/configure.ac
index bdfd134..38ad398 100644
--- a/configure.ac
+++ b/configure.ac
@@ -210,6 +210,8 @@ AX_GCC_FUNC_ATTRIBUTE([format])
 AX_GCC_FUNC_ATTRIBUTE([malloc])
 AX_GCC_FUNC_ATTRIBUTE([packed])
 AX_GCC_FUNC_ATTRIBUTE([unused])
+AX_GCC_FUNC_ATTRIBUTE([const])
+AX_GCC_FUNC_ATTRIBUTE([pure])
 AX_GCC_FUNC_ATTRIBUTE([warn_unused_result])
 
 AM_CONDITIONAL([GEN_ASM_OFFSETS], test "x$GEN_ASM_OFFSETS" = xyes)
diff --git a/src/util/macros.h b/src/util/macros.h
index 66698e7..4d16183 100644
--- a/src/util/macros.h
+++ b/src/util/macros.h
@@ -130,6 +130,26 @@ do {                       \
 #define PACKED
 #endif
 
+/* Attribute pure is used for functions that have no effects other than their
+ * return value.  As a result, calls to it can be dead code eliminated.
+ */
+#ifdef HAVE_FUNC_ATTRIBUTE_PURE
+#define PURE __attribute__((__pure__))
+#else
+#define PURE
+#endif
+
+/* Attribute const is used for functions that have no effects other than their
+ * return value, and only rely on the argument values to compute the return
+ * value.  As a result, calls to it can be CSEed.  Note that using memory
+ * pointed to by the arguments is not allowed for const functions.
+ */
+#ifdef HAVE_FUNC_ATTRIBUTE_CONST
+#define ATTRIBUTE_CONST __attribute__((__const__))
+#else
+#define ATTRIBUTE_CONST
+#endif
+
 #ifdef __cplusplus
 /**
  * Macro function that evaluates to true if T is a trivially
-- 
2.1.4