compiler: header for compiler-specific wrappers.

[ccan] / ccan / compiler / compiler.h

diff --git a/ccan/compiler/compiler.h b/ccan/compiler/compiler.h
new file mode 100644 (file)
index 0000000..3601749
--- /dev/null
+++ b/ccan/compiler/compiler.h
@@ -0,0 +1,142 @@
+#ifndef CCAN_COMPILER_H
+#define CCAN_COMPILER_H
+#include "config.h"
+
+#if HAVE_ATTRIBUTE_COLD
+/**
+ * UNLIKELY_FUNCTION_ATTRIBUTE - a function is unlikely to be called.
+ *
+ * Used to mark an unlikely code path and optimize appropriately.
+ * It is usually used on logging or error routines.
+ *
+ * Example:
+ *     void UNLIKELY_FUNCTION_ATTRIBUTE moan(const char *reason)
+ *     {
+ *             fprintf(stderr, "Error: %s (%s)\n", reason, strerror(errno));
+ *     }
+ */
+#define UNLIKELY_FUNCTION_ATTRIBUTE __attribute__((cold))
+#else
+#define UNLIKELY_FUNCTION_ATTRIBUTE
+#endif
+
+#if HAVE_ATTRIBUTE_PRINTF
+/**
+ * PRINTF_ATTRIBUTE - a function takes printf-style arguments
+ * nfmt: the 1-based number of the function's format argument.
+ * narg: the 1-based number of the function's first variable argument.
+ *
+ * This allows the compiler to check your parameters as it does for printf().
+ *
+ * Example:
+ *     void PRINTF_ATTRIBUTE(2,3) my_printf(char *prefix, char *format, ...);
+ */
+#define PRINTF_ATTRIBUTE(nfmt, narg) \
+       __attribute__((format(__printf__, nfmt, narg)))
+#else
+#define PRINTF_ATTRIBUTE(nfmt, narg)
+#endif
+
+#if HAVE_ATTRIBUTE_CONST
+/**
+ * IDEMPOTENT_ATTRIBUTE - a function's return depends only on its argument
+ *
+ * This allows the compiler to assume that the function will return the exact
+ * same value for the exact same arguments.  This implies that the function
+ * must not use global variables, or dereference pointer arguments.
+ */
+#define IDEMPOTENT_ATTRIBUTE __attribute__((const))
+#else
+#define IDEMPOTENT_ATTRIBUTE
+#endif
+
+#if HAVE_ATTRIBUTE_UNUSED
+/**
+ * UNNEEDED_ATTRIBUTE - a parameter/variable/function may not be needed
+ *
+ * This suppresses warnings about unused variables or parameters, but tells
+ * the compiler that if it is unused it need not emit it into the source code.
+ *
+ * Example:
+ *     // With some config options, this is unnecessary.
+ *     static UNNEEDED_ATTRIBUTE int counter;
+ *     ...
+ *             #ifdef DEBUG
+ *             counter++;
+ *             #endif
+ *     ...
+ *     // With some config options, this is unnecessary.
+ *     static UNNEEDED_ATTRIBUTE int add_to_counter(int add)
+ *     {
+ *             counter += add;
+ *     }
+ */
+#define UNNEEDED_ATTRIBUTE __attribute__((unused))
+
+#if HAVE_ATTRIBUTE_USED
+/**
+ * NEEDED_ATTRIBUTE - a parameter/variable/function is needed
+ *
+ * This suppresses warnings about unused variables or parameters, but tells
+ * the compiler that it must exist even if it (seems) unused.
+ *
+ * Example:
+ *     // Even if this is unused, these are vital for debugging.
+ *     static UNNEEDED_ATTRIBUTE int counter;
+ *     static UNNEEDED_ATTRIBUTE void dump_counter(void)
+ *     {
+ *             printf("Counter is %i\n", counter);
+ *     }
+ */
+#define NEEDED_ATTRIBUTE __attribute__((used))
+#else
+/* Before used, unused functions and vars were always emitted. */
+#define NEEDED_ATTRIBUTE __attribute__((unused))
+#endif
+#else
+#define UNNEEDED_ATTRIBUTE
+#define NEEDED_ATTRIBUTE
+#endif
+
+#if HAVE_BUILTIN_CONSTANT_P
+/**
+ * IS_COMPILE_CONSTANT - does the compiler know the value of this expression?
+ * @expr: the expression to evaluate
+ *
+ * When an expression manipulation is complicated, it is usually better to
+ * implement it in a function.  However, if the expression being manipulated is
+ * known at compile time, it is better to have the compiler see the entire
+ * expression so it can simply substitute the result.
+ *
+ * This can be done using the IS_COMPILE_CONSTANT() macro.
+ *
+ * Example:
+ *     enum greek { ALPHA, BETA, GAMMA, DELTA, EPSILON };
+ *
+ *     // Out-of-line version.
+ *     const char *greek_name(enum greek greek);
+ *
+ *     // Inline version.
+ *     static inline _greek_name(enum greek greek)
+ *     {
+ *             switch (greek) {
+ *             case ALPHA: return "alpha";
+ *             case BETA: return "beta";
+ *             case GAMMA: return "gamma";
+ *             case DELTA: return "delta";
+ *             case EPSILON: return "epsilon";
+ *             default: return "**INVALID**";
+ *             }
+ *     }
+ *
+ *     // Use inline if compiler knows answer.  Otherwise call function
+ *     // to avoid copies of the same code everywhere.
+ *     #define greek_name(g)                                           \
+ *              (IS_COMPILE_CONSTANT(greek) ? _greek_name(g) : greek_name(g))
+ */
+#define IS_COMPILE_CONSTANT(expr) __builtin_constant_p(expr)
+#else
+/* If we don't know, assume it's not. */
+#define IS_COMPILE_CONSTANT(expr) 0
+#endif
+#endif /* CCAN_COMPILER_H */