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<div style="direction:ltr; font-family:Tahoma; color:#000000; font-size:10pt">Hi<br>
<br>
(this picks up from the thread in llvmdev: "PrescheduleNodesWithMultipleUses() causing failure in PickNodeToScheduleBottomUp() ???")<br>
<br>
PrescheduleNodesWithMultipleUses() is a function that tries to transform a DAG where nodes have multiple uses, with the aim of reducing the live range.<br>
(see comment in lib/CodeGen/SelectionDAG/ScheduleDAGRRList.cpp)<br>
<br>
However, it does not check to see if callResources will be affected by the transformation.<br>
<br>
<br>
The problem<br>
========<br>
Take the following correctly scheduled DAG (arrow to predecessor):<br>
<font face="Courier New"><br>
SetUp2 SetUp1<br>
^ ^<br>
| |<br>
| |<br>
Destroy2---->PredSU <----SU<br>
^ ^ ^<br>
| | |<br>
| | |<br>
----------- | ---------<br>
| | |<br>
Destroy1<br>
^<br>
|<br>
</font><br>
In this example there are two successors of 'PredSU' with type<br>
getCallFrameDestroyOpcode (Destroy) and one is a successor of the other.<br>
Taking the successor of the two Destroys (Destroy1), note that it's<br>
matching getCallFrameSetupOpcode (Setup1) is a predecessor of 'SU'.<br>
In this situation, re-routing the dependency on 'PredSU' through 'SU' will<br>
cause a dead lock Viz:<br>
<br>
<font face="Courier New"> SetUp2 PredSU SetUp1<br>
^ ^ ^<br>
| | |<br>
| | |<br>
Destroy2-----> SU -------<br>
^ ^ ^<br>
| | |<br>
| | |<br>
----------- | |<br>
| | |<br>
Destroy1<br>
^<br>
|<br>
</font><br>
<font color="black" face="Tahoma" size="2"><span style="font-size:10pt;" dir="ltr"><font size="2">The new function callResourceDeadLockDanger()</font></span></font> will check for this situation<br>
and prevent PrescheduleNodesWithMultipleUses() from making the transformation<br>
if it will cause a callResource dead lock.<br>
<br>
<br>
Outstanding issues<br>
============<br>
<br>
1. Is it too aggressive in searching predecessors and successors?<br>
Should the algorithm give up and assume the worst if the depth of search reaches a predefined limit?<br>
Can someone confirm that the extra processing time is not onerous.<br>
<br>
2. Should the initial search for 'SetUp1' and 'Destroy1' only search along chains? viz conditional upon II->isCtrl()<br>
This will reduce the search space, but are getCallFrameSetupOpcode & getCallFrameDestroyOpcode always 'chained'?<br>
(Later searches for Destroy2 need to check all predecessors)<br>
<br>
3. What is the best way to construct the test case?<br>
Using an IR as input does not guarantee the required DAG will be output for testing.<br>
The test IR only produces the correct DAG when built for the XCore target.<br>
(see attached test case)<br>
<br>
4. Where should the test be placed?<br>
Currently I have placed it under test/CodeGen/XCore but it relates to lib/CodeGen/SelectionDAG/ScheduleDAGRRList.cpp<br>
<br>
Thank you<br>
<br>
Robert<br>
<br>
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