when: emit switch for String if possible

Effectively, the following when structure:

  when (s) {
    s1, s2 -> e1,
    s3 -> e2,
    s4 -> e3,
    ...
    else -> e
  }

is implemented as:

  when (s.hashCode()) {
    h1 -> {
      if (s == s1)
        e1
      else if (s == s2)
        e1
      else if (s == s3)
        e2
      else
        e
    }
    h2 -> if (s == s3) e2 else e,
    ...
    else -> e
  }

where s1.hashCode() == s2.hashCode() == s3.hashCode() == h1,
      s4.hashCode() == h2.

A tableswitch or lookupswitch is used for the hash code lookup.

Change-Id: I087bf623dbb4a41d3cc64399a1b42342a50757a6

compiler/testData/codegen/box/when/edgeCases.kt

@@ -0,0 +1,20 @@
+fun foo(x: Int): String {
+    return when (x) {
+        2_147_483_647 -> "MAX"
+        -2_147_483_648 -> "MIN"
+        else -> "else"
+    }
+}
+
+fun box(): String {
+    if (foo(0) != "else")
+        return "0: " + foo(0).toString()
+
+    if (foo(Int.MAX_VALUE) != "MAX")
+        return "Int.MAX_VALUE: " + foo(Int.MAX_VALUE).toString()
+
+    if (foo(Int.MIN_VALUE) != "MIN")
+        return "Int.MIN_VALUE: " + foo(Int.MIN_VALUE).toString()
+
+    return "OK"
+}

compiler/testData/codegen/box/when/stringOptimization/duplicatingItemsSameHashCode2.kt

@@ -0,0 +1,31 @@
+// IGNORE_BACKEND: NATIVE
+// IGNORE_BACKEND: JS_IR
+// TODO: muted automatically, investigate should it be ran for JS or not
+// IGNORE_BACKEND: JS
+
+// WITH_RUNTIME
+
+import kotlin.test.assertEquals
+
+fun foo(x : String) : String {
+    assert("abz]".hashCode() == "aby|".hashCode())
+
+    when (x) {
+        "abz]" -> return "abz"
+        "ghi"  -> return "ghi"
+        "aby|" -> return "aby"
+        "abz]" -> return "fail"
+    }
+
+    return "other"
+}
+
+fun box() : String {
+    assertEquals("abz", foo("abz]"))
+    assertEquals("aby", foo("aby|"))
+    assertEquals("ghi", foo("ghi"))
+
+    assertEquals("other", foo("xyz"))
+
+    return "OK"
+}

compiler/testData/codegen/box/when/stringOptimization/duplicatingItemsSameHashCode3.kt

@@ -0,0 +1,33 @@
+// IGNORE_BACKEND: NATIVE
+// IGNORE_BACKEND: JS_IR
+// TODO: muted automatically, investigate should it be ran for JS or not
+// IGNORE_BACKEND: JS
+
+// WITH_RUNTIME
+
+import kotlin.test.assertEquals
+
+fun foo(x : String) : String {
+    assert("abz]".hashCode() == "aby|".hashCode())
+
+    when (x) {
+        "abz]" -> return "abz"
+        "ghi"  -> return "ghi"
+        "aby|" -> return "aby"
+        "abz]" -> return "fail"
+        "uvw" -> return "uvw"
+    }
+
+    return "other"
+}
+
+fun box() : String {
+    assertEquals("abz", foo("abz]"))
+    assertEquals("aby", foo("aby|"))
+    assertEquals("ghi", foo("ghi"))
+    assertEquals("uvw", foo("uvw"))
+
+    assertEquals("other", foo("xyz"))
+
+    return "OK"
+}

compiler/testData/codegen/box/when/switchOptimizationDuplicates.kt

@@ -0,0 +1,29 @@
+fun foo(x: Int): Int {
+    return when (x) {
+        1, 1, 2 -> 1001
+        1, 2 -> 1002
+        1 -> 1003
+        2 -> 1004
+        3 -> 1005
+        else -> 1006
+    }
+}
+
+fun box(): String {
+    if (foo(0) != 1006)
+        return "0: " + foo(0)
+
+    if (foo(1) != 1001)
+        return "1: " + foo(1)
+
+    if (foo(2) != 1001)
+        return "2: " + foo(2)
+
+    if (foo(3) != 1005)
+        return "3: " + foo(3)
+
+    if (foo(4) != 1006)
+        return "4: " + foo(4)
+
+    return "OK"
+}