Improve simplifyJumps

Improve simplifyJumps to rewrite

  IFEQ L4
  L5
  GOTO L6

to

  IFNE L6
  L5

This rewrite is only correct if L5 is not the target of any jump
instruction (otherwise, removing the GOTO would change semantics).
Previously we did not do the rewrite if there was any label between
the conditional jump and the goto (like L5). Now we track which labels
are jump targets.

1 files changed, 19 insertions, 4 deletions

diff --git a/test/junit/scala/tools/nsc/backend/jvm/opt/SimplifyJumpsTest.scala b/test/junit/scala/tools/nsc/backend/jvm/opt/SimplifyJumpsTest.scala
index a685ae7dd5..99acb318de 100644
--- a/test/junit/scala/tools/nsc/backend/jvm/opt/SimplifyJumpsTest.scala
+++ b/test/junit/scala/tools/nsc/backend/jvm/opt/SimplifyJumpsTest.scala
@@ -96,10 +96,22 @@ class SimplifyJumpsTest {
       instructionsFromMethod(method),
       List(VarOp(ILOAD, 1), Jump(IFLT, Label(3))) ::: rest.tail )
 
-    // no label allowed between begin and rest. if there's another label, then there could be a
-    // branch that label. eliminating the  GOTO would change the behavior.
-    val nonOptMethod = genMethod()(begin ::: Label(22) :: rest: _*)
-    assertFalse(LocalOptImpls.simplifyJumps(nonOptMethod))
+    // branch over goto is OK even if there's a label in between, if that label is not a jump target
+    val withNonJumpTargetLabel = genMethod()(begin ::: Label(22) :: rest: _*)
+    assertTrue(LocalOptImpls.simplifyJumps(withNonJumpTargetLabel))
+    assertSameCode(
+      instructionsFromMethod(withNonJumpTargetLabel),
+      List(VarOp(ILOAD, 1), Jump(IFLT, Label(3)), Label(22)) ::: rest.tail )
+
+    // if the Label(22) between IFGE and GOTO is the target of some jump, we cannot rewrite the IFGE
+    // and remove the GOTO: removing the GOTO would change semantics. However, the jump that targets
+    // Label(22) will be re-written (jump-chain collapsing), so in a second round, the IFGE is still
+    // rewritten to IFLT
+    val twoRounds = genMethod()(List(VarOp(ILOAD, 1), Jump(IFLE, Label(22))) ::: begin ::: Label(22) :: rest: _*)
+    assertTrue(LocalOptImpls.simplifyJumps(twoRounds))
+    assertSameCode(
+      instructionsFromMethod(twoRounds),
+      List(VarOp(ILOAD, 1), Jump(IFLE, Label(3)), VarOp(ILOAD, 1), Jump(IFLT, Label(3)), Label(22)) ::: rest.tail )
   }
 
   @Test
@@ -167,6 +179,9 @@ class SimplifyJumpsTest {
       VarOp(ILOAD, 1),
       Jump(IFGE, Label(target)),
 
+      VarOp(ILOAD, 1), // some code to prevent rewriting the conditional jump
+      Op(IRETURN),
+
       Label(4),
       Jump(GOTO, Label(3)),