1 files changed, 44 insertions, 0 deletions
diff --git a/lib/std/special/compiler_rt/mulodi4.zig b/lib/std/special/compiler_rt/mulodi4.zig
new file mode 100644
index 0000000000..82e9ef3253
--- /dev/null
+++ b/lib/std/special/compiler_rt/mulodi4.zig
@@ -0,0 +1,44 @@
+const builtin = @import("builtin");
+const compiler_rt = @import("../compiler_rt.zig");
+const maxInt = std.math.maxInt;
+const minInt = std.math.minInt;
+
+pub extern fn __mulodi4(a: i64, b: i64, overflow: *c_int) i64 {
+    @setRuntimeSafety(builtin.is_test);
+
+    const min = @bitCast(i64, u64(1 << (i64.bit_count - 1)));
+    const max = ~min;
+
+    overflow.* = 0;
+    const result = a *% b;
+
+    // Edge cases
+    if (a == min) {
+        if (b != 0 and b != 1) overflow.* = 1;
+        return result;
+    }
+    if (b == min) {
+        if (a != 0 and a != 1) overflow.* = 1;
+        return result;
+    }
+
+    // Take absolute value of a and b via abs(x) = (x^(x >> 63)) - (x >> 63).
+    const abs_a = (a ^ (a >> 63)) -% (a >> 63);
+    const abs_b = (b ^ (b >> 63)) -% (b >> 63);
+
+    // Unitary magnitude, cannot have overflow
+    if (abs_a < 2 or abs_b < 2) return result;
+
+    // Compare the signs of the operands
+    if ((a ^ b) >> 63 != 0) {
+        if (abs_a > @divTrunc(max, abs_b)) overflow.* = 1;
+    } else {
+        if (abs_a > @divTrunc(min, -abs_b)) overflow.* = 1;
+    }
+
+    return result;
+}
+
+test "import mulodi4" {
+    _ = @import("mulodi4_test.zig");
+}