1 files changed, 162 insertions, 0 deletions
diff --git a/arch/riscv/user/basics/basics.c b/arch/riscv/user/basics/basics.c
new file mode 100644
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+++ b/arch/riscv/user/basics/basics.c
@@ -0,0 +1,162 @@
+#include <assert.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdint.h>
+#include <string.h>
+
+/*
+ * Returns the factorial for the given unsigned 64-bit integer.
+ *
+ * Implemented in factorial.S
+ */
+int factorial(uint64_t n);
+
+void test_factorial(void)
+{
+	assert(factorial(0) == 0);
+	assert(factorial(1) == 1);
+	assert(factorial(2) == 2);
+	assert(factorial(3) == 6);
+	assert(factorial(4) == 24);
+
+	printf("factorial:\t\tOK\n");
+}
+
+/*
+ * Returns a pointer with the same given string but reversed. Note that the
+ * string cannot be in read-only space since the reversal is done in-place.
+ *
+ * Implemented in string.S
+ */
+char *reverse_string(char *str);
+
+void test_reverse_string(void)
+{
+	assert(reverse_string(NULL) == NULL);
+	assert(strlen(reverse_string("")) == 0);
+
+	char s1[] = "This is a string.";
+	assert(strcmp(reverse_string(s1), ".gnirts a si sihT") == 0);
+
+	char s2[] = ".";
+	assert(strcmp(reverse_string(s2), ".") == 0);
+
+	printf("reverse_string:\t\tOK\n");
+}
+
+/*
+ * Returns true if the given string is a palindrome, false otherwise.
+ *
+ * Implemented in string.S
+ */
+bool is_palindrome(char *str);
+
+void test_is_palindrome(void)
+{
+	assert(is_palindrome(NULL) == 0);
+	assert(is_palindrome("") == 0);
+	assert(is_palindrome("aba") == 1);
+	assert(is_palindrome("aaa") == 1);
+	assert(is_palindrome("This is a a si sihT") == 1);
+
+	printf("is_palindrome:\t\tOK\n");
+}
+
+/*
+ * This function computes the following: ((a + b) * b) + b. Don't try to make
+ * sense of it, that's the computation I ended up while messing with RISC-V
+ * floating point instructions. Anyways, with this in mind, it will return if
+ * the computed value is greater than 10.
+ *
+ * Implemented in float.S
+ */
+bool greater_than_ten(double a, uint64_t b);
+
+void test_greater_than_ten(void)
+{
+	assert(!greater_than_ten(2.3, 1)); // 4.3
+	assert(greater_than_ten(2.3, 2)); // 10.6
+
+	printf("greater_than_ten:\tOK\n");
+}
+
+/*
+ * Atomically add the integer pointed by `a` with the given `value`.
+ * this is a function, I've checked that the assembly produced by GCC on a
+ * decent optimization level actually inlines this.
+ */
+static inline void atomic_add(uint64_t *a, uint64_t value)
+{
+	/*
+	 * The execution is a mere `amoadd` instruction, but it will set on `t0` the
+	 * result. If it fails (e.g. the address was already being used), then `t0 =
+	 * 0` and `beqz` will instruct it to try again.
+	 *
+	 * Note that on the board I'm using the 'Zawrs' extension is not available.
+	 * Otherwise I could have re-arranged to code to something like this:
+	 *
+	 *     amoadd.d t0, %1, %0
+	 *     bnez t0, .Latomic_add_done
+	 *     wrs.nto
+	 *  .Latomic_add_done:
+	 *
+	 * This is an extra instruction but it saves on power if the memory address
+	 * happens to be used at access time.
+	 *
+	 * As for the 'A' RISC-V specific constraint, it means "An address that is
+	 * held in a general-purpose register". GCC will then do the magic and
+	 * convert it to `amoadd.d t0, a1, (a0)` or something like that. Note that
+	 * we have to pass the '+' constraint modifier because the address will be
+	 * both read and written atomically.
+	 *
+	 * See: https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html.
+	 */
+	asm volatile(".Latomic_add_retry:\n\t"
+				 "amoadd.d t0, %1, %0\n\t"
+				 "beqz t0, .Latomic_add_retry"
+				 : "+A"(*a)
+				 : "r"(value)
+				 : "t0", "memory");
+}
+
+void test_atomic_add(void)
+{
+	uint64_t i = 4;
+
+	atomic_add(&i, 0);
+	assert(i == 4);
+
+	atomic_add(&i, 2);
+	assert(i == 6);
+
+	printf("atomic_add:\t\tOK\n");
+}
+
+extern int mstrcmp(const char *s1, const char *s2);
+
+void test_mstrcmp(void)
+{
+	char *strings[] = {
+		"hello", "", "iello", "helloa", "contammusaaquellhomedegranarditquetantissimerra",
+	};
+
+	for (uint64_t i = 0; i < 1000000000; i++) {
+		assert(mstrcmp(strings[0], strings[0]) == 0);
+		assert(mstrcmp(strings[0], strings[1]) > 0);
+		assert(mstrcmp(strings[0], strings[2]) < 0);
+		assert(mstrcmp(strings[0], strings[3]) < 0);
+		assert(mstrcmp(strings[0], strings[4]) > 0);
+	}
+
+	printf("mstrcmp:\t\tOK\n");
+}
+
+int main()
+{
+	test_factorial();
+	test_reverse_string();
+	test_is_palindrome();
+	test_greater_than_ten();
+	test_atomic_add();
+	test_mstrcmp();
+}