compiler: header for compiler-specific wrappers.

Currently they sit in each module.

ccan/compiler/LICENCE

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+../../licences/LGPL-3

ccan/compiler/_info

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+#include <string.h>
+#include <stdio.h>
+#include "config.h"
+
+/**
+ * compiler - macros for common compiler extensions
+ *
+ * Abstracts away some compiler hints.  Currently these include:
+ * - UNLIKELY_FUNCTION_ATTRIBUTE
+ *	For functions not called in fast paths (aka. cold functions)
+ * - PRINTF_ATTRIBUTE
+ *	For functions which take printf-style parameters.
+ * - IDEMPOTENT_ATTRIBUTE
+ *	For functions which return the same value for same parameters.
+ * - NEEDED_ATTRIBUTE
+ *	For functions and variables which must be emitted even if unused.
+ * - UNNEEDED_ATTRIBUTE
+ *	For functions and variables which need not be emitted if unused.
+ * - IS_COMPILE_CONSTANT
+ *	For using different tradeoffs for compiletime vs runtime evaluation.
+ *
+ * Licence: LGPL (3 or any later version)
+ * Author: Rusty Russell <rusty@rustcorp.com.au>
+ *
+ * Example:
+ *	#include <ccan/compiler/attributs.h>
+ *	#include <stdio.h>
+ *	#include <stdarg.h>
+ *
+ *	// Example of a (slow-path) logging function.
+ *	static int log_threshold = 2;
+ *	static void UNLIKELY_FUNCTION_ATTRIBUTE PRINTF_ATTRIBUTE(2,3)
+ *		logger(int level, const char *fmt, ...)
+ *	{
+ *		va_list ap;
+ *		va_start(ap, fmt);
+ *		if (level >= log_threshold)
+ *			vfprintf(stderr, fmt, ap);
+ *		va_end(ap);
+ *	}
+ *
+ *	int main(int argc, char *argv[])
+ *	{
+ *		if (argc != 1) {
+ *			logger(3, "Don't want %i arguments!\n", argc-1);
+ *			return 1;
+ *		}
+ *		return 0;
+ *	}
+ */
+int main(int argc, char *argv[])
+{
+	/* Expect exactly one argument */
+	if (argc != 2)
+		return 1;
+
+	if (strcmp(argv[1], "depends") == 0) {
+		return 0;
+	}
+
+	return 1;
+}

ccan/compiler/compiler.h

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+#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 */

ccan/compiler/test/compile_fail-printf.c

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+#include <ccan/compiler/compiler.h>
+
+static void PRINTF_ATTRIBUTE(2,3) my_printf(int x, const char *fmt, ...)
+{
+}
+
+int main(int argc, char *argv[])
+{
+	unsigned int i = 0;
+
+	my_printf(1, "Not a pointer "
+#ifdef FAIL
+		  "%p",
+#if !HAVE_ATTRIBUTE_PRINTF
+#error "Unfortunately we don't fail if !HAVE_ATTRIBUTE_PRINTF."
+#endif
+#else
+		  "%i",
+#endif
+		  i);
+	return 0;
+}

ccan/compiler/test/run-is_compile_constant.c

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+#include <ccan/compiler/compiler.h>
+#include <ccan/tap/tap.h>
+
+int main(int argc, char *argv[])
+{
+	plan_tests(2);
+
+	ok1(!IS_COMPILE_CONSTANT(argc));
+#ifdef HAVE_BUILTIN_CONSTANT_P
+	ok1(IS_COMPILE_CONSTANT(7));
+#else
+	pass("If !HAVE_BUILTIN_CONSTANT_P, IS_COMPILE_CONSTANT always false");
+#endif
+	return exit_status();
+}