minmax: New module, safe min and max macros

Add a 'minmax' module with typesafe macros to compute minimum, maximum and
clamping.  Inspired by the versions used in the Linux kernel, but using
a different implementation based on __builtin_types_compatible_p() and the
build_assert module.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Makefile-ccan

@@ -20,6 +20,7 @@ MODS_NO_SRC := alignof \
 	container_of \
 	darray \
 	endian \
+	minmax \
 	objset \
 	short_types \
 	tcon \

ccan/minmax/LICENSE

+../../licenses/CC0
\ No newline at end of file

ccan/minmax/_info

+#include <string.h>
+#include "config.h"
+
+/**
+ * minmax - typesafe minimum and maximum functions
+ *
+ * The classic implementation of minimum / maximum macros in C can be
+ * very dangerous.  If the two arguments have different sizes, or
+ * different signedness, type promotion rules can lead to very
+ * surprising results.
+ *
+ * This module implements typesafe versions, which will generate a
+ * compile time error, if the arguments have different types.
+ *
+ * Example:
+ *	#include <ccan/minmax/minmax.h>
+ *	#include <stdio.h>
+ *
+ *	int main(int argc, char *argv[])
+ *	{
+ *		printf("Signed max: %d\n", max(1, -1));
+ *		printf("Unsigned max: %u\n", max(1U, -1U));
+ *		return 0;
+ *	}
+ *
+ * Author: David Gibson <david@gibson.dropbear.id.au>
+ * License:  CC0 (Public domain)
+ *
+ * Ccanlint:
+ *      // We need several gcc extensions
+ *      tests_compile_without_features FAIL
+ */
+int main(int argc, char *argv[])
+{
+	/* Expect exactly one argument */
+	if (argc != 2)
+		return 1;
+
+	if (strcmp(argv[1], "depends") == 0) {
+		printf("ccan/build_assert\n");
+		return 0;
+	}
+
+	return 1;
+}

ccan/minmax/minmax.h

+/* CC0 (Public domain) - see LICENSE file for details */
+#ifndef CCAN_MINMAX_H
+#define CCAN_MINMAX_H
+
+#include "config.h"
+
+#include <ccan/build_assert/build_assert.h>
+
+#if !HAVE_STATEMENT_EXPR || !HAVE_TYPEOF
+/*
+ * Without these, there's no way to avoid unsafe double evaluation of
+ * the arguments
+ */
+#error Sorry, minmax module requires statement expressions and typeof
+#endif
+
+#if HAVE_BUILTIN_TYPES_COMPATIBLE_P
+#define MINMAX_ASSERT_COMPATIBLE(a, b) \
+	BUILD_ASSERT(__builtin_types_compatible_p(a, b))
+#else
+#define MINMAX_ASSERT_COMPATIBLE(a, b) \
+	do { } while (0)
+#endif
+
+#define min(a, b) \
+	({ \
+		typeof(a) _a = (a); \
+		typeof(b) _b = (b); \
+		MINMAX_ASSERT_COMPATIBLE(typeof(_a), typeof(_b)); \
+		_a < _b ? _a : _b; \
+	})
+
+#define max(a, b) \
+	({ \
+		typeof(a) _a = (a); \
+		typeof(b) _b = (b); \
+		MINMAX_ASSERT_COMPATIBLE(typeof(_a), typeof(_b)); \
+		_a > _b ? _a : _b; \
+	})
+
+#define clamp(v, f, c)	(max(min((v), (c)), (f)))
+
+
+#define min_t(t, a, b) \
+	({ \
+		t _ta = (a); \
+		t _tb = (b); \
+		min(_ta, _tb); \
+	})
+#define max_t(t, a, b) \
+	({ \
+		t _ta = (a); \
+		t _tb = (b); \
+		max(_ta, _tb); \
+	})
+
+#define clamp_t(t, v, f, c) \
+	({ \
+		t _tv = (v); \
+		t _tf = (f); \
+		t _tc = (c); \
+		clamp(_tv, _tf, _tc); \
+	})
+
+#endif /* CCAN_MINMAX_H */

ccan/minmax/test/compile_fail-wrongsign.c

+#include <ccan/minmax/minmax.h>
+
+static int function(void)
+{
+#ifdef FAIL
+	return min(1, 1U);
+#if !HAVE_TYPEOF||!HAVE_BUILTIN_CHOOSE_EXPR||!HAVE_BUILTIN_TYPES_COMPATIBLE_P
+#error "Unfortunately we don't fail if the typechecks are noops."
+#endif
+#else
+	return 0;
+#endif
+}
+
+int main(int argc, char *argv[])
+{
+	function();
+	return 0;
+}

ccan/minmax/test/compile_fail-wrongsize.c

+#include <ccan/minmax/minmax.h>
+
+static int function(void)
+{
+#ifdef FAIL
+	return min(1, 1L);
+#if !HAVE_TYPEOF||!HAVE_BUILTIN_CHOOSE_EXPR||!HAVE_BUILTIN_TYPES_COMPATIBLE_P
+#error "Unfortunately we don't fail if the typechecks are noops."
+#endif
+#else
+	return 0;
+#endif
+}
+
+int main(int argc, char *argv[])
+{
+	function();
+	return 0;
+}

ccan/minmax/test/run.c

+#include <ccan/minmax/minmax.h>
+#include <ccan/tap/tap.h>
+
+int main(void)
+{
+	int a, b;
+
+	/* This is how many tests you plan to run */
+	plan_tests(23);
+
+	ok1(min(1, 2) == 1);
+	ok1(max(1, 2) == 2);
+	ok1(min(-1, 1) == -1);
+	ok1(max(-1, 1) == 1);
+
+	ok1(min(-1U, 1U) == 1U);
+	ok1(max(-1U, 1U) == -1U);
+
+	ok1(max_t(signed int, -1, 1U) == 1);
+	ok1(max_t(unsigned int, -1, 1) == -1U);
+
+	ok1(min_t(signed int, -1, 1U) == -1);
+	ok1(min_t(unsigned int, -1, 1) == 1U);
+
+	ok1(clamp(1, 2, 5) == 2);
+	ok1(clamp(2, 2, 5) == 2);
+	ok1(clamp(3, 2, 5) == 3);
+	ok1(clamp(5, 2, 5) == 5);
+	ok1(clamp(6, 2, 5) == 5);
+
+	ok1(clamp(-1, 2, 5) == 2);
+	ok1(clamp(-1U, 2U, 5U) == 5U);
+
+	ok1(clamp_t(signed int, -1, 2, 5) == 2);
+	ok1(clamp_t(unsigned int, -1, 2, 5) == 5);
+
+	/* test for double evaluation */
+	a = b = 0;
+	ok1(min(a++, b++) == 0);
+	ok1((a == 1) && (b == 1));
+	ok1(max(++a, ++b) == 2);
+	ok1((a == 2) && (b == 2));
+
+	/* This exits depending on whether all tests passed */
+	return exit_status();
+}