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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [SCEV] Negative backedge-taken count on signed integer wrap of the induction variable"
href="http://llvm.org/bugs/show_bug.cgi?id=17173">17173</a>
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<th>Summary</th>
<td>[SCEV] Negative backedge-taken count on signed integer wrap of the induction variable
</td>
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<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Global Analyses
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>surfmaths@free.fr
</td>
</tr>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
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</tr></table>
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<div>
<pre>Created <span class=""><a href="attachment.cgi?id=11173" name="attach_11173" title="Simple loop with bit-width minimization of the induction variable">attachment 11173</a> <a href="attachment.cgi?id=11173&action=edit" title="Simple loop with bit-width minimization of the induction variable">[details]</a></span>
Simple loop with bit-width minimization of the induction variable
In case the induction variable undergoes a legal signed wrap, the exit
condition is usually negative. (cf: example code where the induction variable
bit-width has been minimized)
The Scalar Evolution Analysis gives a negative backedge-taken count in such
cases.
To reproduce:
$ opt -scalar-evolution -analyze test.ll
Printing analysis 'Scalar Evolution Analysis' for function 'test':
Classifying expressions for: @test
%i = phi i7 [ 0, %preheader ], [ %i.1, %body.incr ]
--> {0,+,1}<%header> Exits: -28
%tmp = zext i7 %i to i64
--> {0,+,1}<nuw><nsw><%header> Exits: 100
%A.addr = getelementptr [100 x i32]* @A, i64 0, i64 %tmp
--> {@A,+,4}<%header> Exits: (400 + @A)
%A.load = load i32* %A.addr, align 4
--> %A.load Exits: <<Unknown>>
%A.inc = zext i7 %i to i32
--> {0,+,1}<nuw><nsw><%header> Exits: 100
%A.val = add nsw i32 %A.load, %A.inc
--> ({0,+,1}<nuw><nsw><%header> + %A.load) Exits: <<Unknown>>
%i.1 = add i7 %i, 1
--> {1,+,1}<nw><%header> Exits: -27
Determining loop execution counts for: @test
Loop %header: backedge-taken count is -28
Loop %header: max backedge-taken count is -28
git-svn-id: <a href="https://llvm.org/svn/llvm-project/llvm/trunk@190205">https://llvm.org/svn/llvm-project/llvm/trunk@190205</a>
91177308-0d34-0410-b5e6-96231b3b80d8</pre>
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