Improve new C23 checked arithmetic feature

libc/intrin/muloti4.c

@@ -0,0 +1,63 @@
+/* clang-format off */
+/*===-- muloti4.c - Implement __muloti4 -----------------------------------===
+ *
+ *                     The LLVM Compiler Infrastructure
+ *
+ * This file is dual licensed under the MIT and the University of Illinois Open
+ * Source Licenses. See LICENSE.TXT for details.
+ *
+ * ===----------------------------------------------------------------------===
+ *
+ * This file implements __muloti4 for the compiler_rt library.
+ *
+ * ===----------------------------------------------------------------------===
+ */
+
+#include "third_party/compiler_rt/int_lib.h"
+
+#ifdef CRT_HAS_128BIT
+
+/* Returns: a * b */
+
+/* Effects: sets *overflow to 1  if a * b overflows */
+
+COMPILER_RT_ABI ti_int
+__muloti4(ti_int a, ti_int b, int* overflow)
+{
+    const int N = (int)(sizeof(ti_int) * CHAR_BIT);
+    const ti_int MIN = (tu_int)1 << (N-1);
+    const ti_int MAX = ~MIN;
+    *overflow = 0;
+    ti_int result = (tu_int)a * (tu_int)b;
+    if (a == MIN)
+    {
+        if (b != 0 && b != 1)
+	    *overflow = 1;
+	return result;
+    }
+    if (b == MIN)
+    {
+        if (a != 0 && a != 1)
+	    *overflow = 1;
+        return result;
+    }
+    ti_int sa = a >> (N - 1);
+    ti_int abs_a = (a ^ sa) - sa;
+    ti_int sb = b >> (N - 1);
+    ti_int abs_b = (b ^ sb) - sb;
+    if (abs_a < 2 || abs_b < 2)
+        return result;
+    if (sa == sb)
+    {
+        if (abs_a > MAX / abs_b)
+            *overflow = 1;
+    }
+    else
+    {
+        if (abs_a > MIN / -abs_b)
+            *overflow = 1;
+    }
+    return result;
+}
+
+#endif /* CRT_HAS_128BIT */