Implement Math.multiplyHigh for Long (#3480)

* Created Math multiplyHigh method * Move multiplyHigh tests to JDK9 tests dir
scala-native · Oct 4, 2023 · 6c89cde · 6c89cde
1 parent 778cbc4
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diff --git a/javalib/src/main/scala/java/lang/Math.scala b/javalib/src/main/scala/java/lang/Math.scala
@@ -188,6 +188,24 @@ object Math {
     else overflow.value
   }
 
+  @alwaysinline def multiplyHigh(a: scala.Long, b: scala.Long): scala.Long = {
+    /* Algorithm from Hacker's Delight, "8–2. Multiply high signed."
+     * Here, `a` is replaced with `u`, and `b` with `v`, and reassignment of
+     * variables with suffix `p`. Unsigned ints correspond to shifting with
+     * `>>>` and performing the `& 0xffffffffL` operations.
+     */
+    val u0 = a & 0xffffffffL
+    val u1 = a >> 32
+    val v0 = b & 0xffffffffL
+    val v1 = b >> 32
+    val w0 = u0 * v0
+    val t = u1 * v0 + (w0 >>> 32)
+    val w1 = t & 0xffffffffL
+    val w2 = t >> 32
+    val w1p = u0 * v1 + w1
+    u1 * v1 + w2 + (w1p >> 32)
+  }
+
   @alwaysinline def multiplyExact(a: scala.Long, b: scala.Int): scala.Long =
     multiplyExact(a, b.toLong)
 

diff --git a/.../shared/src/test/require-jdk9/org/scalanative/testsuite/javalib/lang/MathTestOnJDK9.scala b/.../shared/src/test/require-jdk9/org/scalanative/testsuite/javalib/lang/MathTestOnJDK9.scala
@@ -10,4 +10,41 @@ class MathTestOnJDK9 {
     assertEquals(10.0, Math.fma(2.0, 3.0, 4.0), 0.0)
   }
 
+  @Test def multiplyHighTests(): Unit = {
+    case class MHTest(a: Long, b: Long, expected: Long)
+    val maxval = java.lang.Long.MAX_VALUE
+    val minval = java.lang.Long.MIN_VALUE
+    val halfmax = maxval >> 32
+    val halfmin = minval >> 32
+
+    val testcases: List[MHTest] =
+      MHTest(maxval, maxval, 4611686018427387903L) ::
+        MHTest(maxval, minval, -4611686018427387904L) ::
+        MHTest(minval, minval, 4611686018427387904L) ::
+        MHTest(maxval, 0L, 0L) ::
+        MHTest(minval, 0L, 0L) ::
+        MHTest(0L, 0L, 0L) ::
+        MHTest(maxval, halfmax, 1073741823L) ::
+        MHTest(maxval, halfmin, -1073741824L) ::
+        MHTest(halfmax, halfmin, -1L) ::
+        MHTest(halfmin, halfmin, 0L) ::
+        MHTest(halfmax, 127L, 0L) ::
+        MHTest(halfmax * 42L, halfmax * 1337L, 14038L) ::
+        MHTest(halfmin * 42L, halfmax * 1337L, -14039L) ::
+        MHTest(13L, 37L, 0L) ::
+        MHTest(123456789123456789L, 987654321L, 6609981L) ::
+        MHTest(123123456456789789L, 998877665544332211L, 6667044887047954L) ::
+        MHTest(-123123456456789789L, 998877665544332211L, -6667044887047955L) ::
+        MHTest(123123456456789789L, -998877665544332211L, -6667044887047955L) ::
+        MHTest(-123123456456789789L, -998877665544332211L, 6667044887047954L) ::
+        Nil
+
+    for (tc <- testcases) {
+      val result = Math.multiplyHigh(tc.a, tc.b)
+      assertTrue(
+        s"Math.multiplyHigh(${tc.a}, ${tc.b}) result: ${result} != expected: ${tc.expected}",
+        Math.multiplyHigh(tc.a, tc.b) == tc.expected
+      )
+    }
+  }
 }