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jasmin/jasmin-2.4/examples/VerifyTest1.j

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+; --- Copyright Jonathan Meyer 1996. All rights reserved. -----------------
+; File:      jasmin/examples/VerifyTest1.j
+; Author:    Jonathan Meyer, 10 July 1996
+; Purpose:   Trys to pull one on the verifier
+; -------------------------------------------------------------------------
+
+; This file illustrates the bytecode verifier at work - the
+; code in the example() method below seems reasonable, but
+; Java's bytecode verifier will fail the code because the two points leading
+; to the Loop label (from the top of the method and from the ifne
+; statement) have different stack states.  Instead, a different approach
+; must be adopted  - e.g. by allocating an array, or simply writing:
+;
+;    aconst_null
+;    aconst_null
+;    aconst_null
+;    aconst_null
+
+; Note that many interpreters will run this code OK if you don't use
+; a verifier. The code itself is well behaved (it doesn't trash the
+; interpreter), but the approach it uses is disallowed by the verifier.
+;
+
+; Compile the example, then run it using:
+;
+;     % java -verify VerifyTest1
+;     VERIFIER ERROR VerifyTest1.example()V:
+;     Inconsistent stack height 1 != 0
+;
+
+.class public examples/VerifyTest1
+.super java/lang/Object
+
+.method public <init>()V
+   aload_0
+   invokenonvirtual java/lang/Object/<init>()V
+   return
+.end method
+
+.method public example()V
+    .limit locals 2
+    .limit stack  10
+
+    ; this tries to push four nulls onto the stack
+    ; using a loop - Java's verifier will fail this program
+
+    iconst_4     ; store 4 in local variable 1 (used as a counter)
+    istore_1
+
+  Loop:
+    aconst_null  ; push null onto the stack
+    iinc 1 -1    ; decrement local variable 4 (the counter variable)
+    iload_1
+    ifne Loop    ; jump back to Loop unless the variable has reached 0
+
+    return
+.end method
+
+.method public static main([Ljava/lang/String;)V
+    ; - do nothing : this is only to illustrate the bytecode verifier at work.
+    return
+.end method