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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Infinite loop in Reassociate"
href="https://llvm.org/bugs/show_bug.cgi?id=30818">30818</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Infinite loop in Reassociate
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>bjorn.a.pettersson@ericsson.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=17507" name="attach_17507" title="Hangs in reassociate">attachment 17507</a> <a href="attachment.cgi?id=17507&action=edit" title="Hangs in reassociate">[details]</a></span>
Hangs in reassociate
When running "opt -reassociate" on the following program (also included as
attachment) we end up in an infinite loop:
define void @xor_deadloop() {
br label %endlabel
deadloop:
%1 = xor i32 %2, 7
%2 = xor i32 %1, 8
br label %deadloop
endlabel:
ret void
}
Problem was detected when doing randomized tests using csmith.
A dead loop similar to the example below was created by -jump-threading.
According to -verify the code is valid. However, the SSA form of the dead loop
is quite interesting.
------------------
One solution (to avoid hanging in Reassociate) is to simply avoid analysis of
dead basic blocks.
For example like this patch:
diff --git a/lib/Transforms/Scalar/Reassociate.cpp
b/lib/Transforms/Scalar/Reassociate.cpp
index 1ec7fce..6fc8dc8 100644
--- a/lib/Transforms/Scalar/Reassociate.cpp
+++ b/lib/Transforms/Scalar/Reassociate.cpp
@@ -2177,8 +2177,19 @@ PreservedAnalyses ReassociatePass::run(Function &F,
FunctionAnalysisManager &) {
// Calculate the rank map for F.
BuildRankMap(F);
+ // Mark all reachable blocks.
+ df_iterator_default_set<BasicBlock*> Reachable;
+ for (BasicBlock *BB : depth_first_ext(&F, Reachable))
+ (void)BB/* Mark all reachable blocks */;
+
MadeChange = false;
for (Function::iterator BI = F.begin(), BE = F.end(); BI != BE; ++BI) {
+ // Skip dead basic blocks
+ if (!Reachable.count(&*BI)) {
+ DEBUG(dbgs() << "Skipping dead BB:"; BI->dump(););
+ continue;
+ }
+
// Optimize every instruction in the basic block.
for (BasicBlock::iterator II = BI->begin(), IE = BI->end(); II != IE;)
if (isInstructionTriviallyDead(&*II)) {
------------------
I noticed that this problem started to appear after this commit
(so it could be an old problem that appeared again after that revert):
commit a537a2bbfe75a0f15deb93efa5dd9f8ad96971e8
Author: Chad Rosier <<a href="mailto:mcrosier@codeaurora.org">mcrosier@codeaurora.org</a>>
Date: Wed Aug 17 15:54:39 2016 +0000
Revert "Reassociate: Reprocess RedoInsts after each inst".
This reverts commit r258830, which introduced a bug described in PR28367.
PR28367
git-svn-id: <a href="https://llvm.org/svn/llvm-project/llvm/trunk@278938">https://llvm.org/svn/llvm-project/llvm/trunk@278938</a>
91177308-0d34-0410-b5e6-96231b3b80d8</pre>
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