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Problem in CodeIterator.newOffset()
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created by <a href="http://community.jboss.org/people/alasdair">Alasdair Mackintosh</a> in <i>Javassist Development</i> - <a href="http://community.jboss.org/message/547651#547651">View the full discussion</a>
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<div class="jive-rendered-content"><pre><span style="font-family: arial, helvetica, sans-serif;">I've been having some problems with Javassist which I think I've finally narrowed down to the way that</span></pre><pre><span style="font-family: arial, helvetica, sans-serif;">"goto" offsets are updated when you </span><span style="font-family: arial, helvetica, sans-serif;">insert new bytecode. It looks as though newOffset is not correctly</span></pre><pre><span style="font-family: arial, helvetica, sans-serif;">handling the case when you insert new opcodes at a location which already contains a "goto" opcode.</span></pre><pre><span style="font-family: arial, helvetica, sans-serif;"><br/></span></pre><pre><span style="font-family: arial, helvetica, sans-serif;">The code for newOffset() looks like this:</span></pre><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><div id="_mcePaste"><span style="font-family: 'courier new', courier;">    private static int newOffset(int i, int offset, int where,</span></div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">                                 int gapLength, boolean exclusive) {</span></div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">        int target = i + offset;</span></div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">        if (i < where) {</span></div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">            if (where < target || (exclusive && where == target))</span></div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">                offset += gapLength;</span></div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">        }</span></div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">        else if (i == where) {</span></div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">            if (target < where && exclusive)</span></div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">                offset -= gapLength;</span></div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">            else if (where < target && !exclusive)</span></div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">                offset += gapLength;</span></div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">        }</span></div><div> </div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">        else</span></div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">            if (target < where || (!exclusive && where == target))</span></div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">                offset -= gapLength;</span></div><div id="_mcePaste"> </div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">        return offset;</span></div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">    }</span></div><div id="_mcePaste"> </div><div id="_mcePaste"><span style="font-family: 'courier new', courier;"><br/></span></div><p>And I think the middle clause should look like this:</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><div id="_mcePaste"><span style="font-family: 'courier new', courier;">        else if (i == where) {</span></div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">            if (target < where)</span></div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">                offset -= gapLength;</span></div><div id="_mcePaste"><span style="font-family: 'courier new', courier;">        }</span></div><div> </div><div><span style="font-family: arial, helvetica, sans-serif;">If you insert new bytecode at a location containing a goto (so that the goto opcode is about to be moved forward by n bytes) then you need to modify the offset if it's negative, but keep it the same if it's already positive.</span></div><div> </div><div><span style="font-family: arial, helvetica, sans-serif;">I've made this change and re-run the tests, which all pass, but I wanted to check to see if this fix looks reasonable.</span></div><div> </div><div><span style="font-family: arial, helvetica, sans-serif;">I don't have a simple test case that shows the problem. I found this when I tried to run my testing framework ThreadWeaver (</span><a class="jive-link-external-small" href="http://code.google.com/p/thread-weaver/">http://code.google.com/p/thread-weaver/</a>) with the latest version of Javassist. The problem can be demonstrated by building ThreadWeaver and running the unit tests. </div><div> </div><div>If yuo want a simpler test case let me know. I think THreadWeaver triggered this because it uses Javassist to insert lots of small code snippets into the instrumented bytecode.</div><div> </div><div>Alasdair</div></div>
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