Do not recompute stack frames when instrumenting bytecode.

It turns out that we do not need to do that. See comment in
`ProfilerVisitor.java`. Also, since recomputing stack frame
map was the only reason we needed to implement
`getCommonSuperClass` we can now remove its implementation
that was causing problems on Java 7 due to a cyclic dependency
involving class loader because we would try to load a class
we are currently transforming and transformer is triggered just
before classloading.

//cc @namin who worked on this code with me.

1 files changed, 13 insertions, 0 deletions

diff --git a/src/partest/scala/tools/partest/javaagent/ProfilerVisitor.java b/src/partest/scala/tools/partest/javaagent/ProfilerVisitor.java
index ac83f66506..8306327b14 100644
--- a/src/partest/scala/tools/partest/javaagent/ProfilerVisitor.java
+++ b/src/partest/scala/tools/partest/javaagent/ProfilerVisitor.java
@@ -33,6 +33,19 @@ public class ProfilerVisitor extends ClassVisitor implements Opcodes {
       // only instrument non-abstract methods
       if((access & ACC_ABSTRACT) == 0) {
         assert(className != null);
+        /* The following instructions do not modify compressed stack frame map so
+         * we don't need to worry about recalculating stack frame map. Specifically,
+         * let's quote "ASM 4.0, A Java bytecode engineering library" guide (p. 40):
+         *
+         *   In order to save space, a compiled method does not contain one frame per
+         *   instruction: in fact it contains only the frames for the instructions
+         *   that correspond to jump targets or exception handlers, or that follow
+         *   unconditional jump instructions. Indeed the other frames can be easily
+         *   and quickly inferred from these ones.
+         *
+         * Instructions below are just loading constants and calling a method so according
+         * to definition above they do not contribute to compressed stack frame map.
+         */
         mv.visitLdcInsn(className);
         mv.visitLdcInsn(name);
         mv.visitLdcInsn(desc);