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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - SCEV ignores nuw/nsw when computing MaxBackedgeTakenCount"
href="https://bugs.llvm.org/show_bug.cgi?id=50028">50028</a>
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<th>Summary</th>
<td>SCEV ignores nuw/nsw when computing MaxBackedgeTakenCount
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</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
</td>
</tr>
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<th>Reporter</th>
<td>suc-daniil@yandex.ru
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>define void @foo(i32 %n) {
entry:
br label %loop
loop:
%i = phi i32 [ 0, %entry ], [ %i.next, %loop ]
%i.next = add nuw nsw i32 %i, 1
%innercmp = icmp ult i32 %i, %n
br i1 %innercmp, label %loop, label %exit
exit:
ret void
}
Running opt -passes='print<scalar-evolution>' gives us the following result:
Printing analysis 'Scalar Evolution Analysis' for function 'foo':
Classifying expressions for: @foo
%i = phi i32 [ 0, %entry ], [ %i.next, %loop ]
--> {0,+,1}<nuw><nsw><%loop> U: [0,-2147483648) S: [0,-2147483648)
Exits: %n LoopDispositions: { %loop: Computable }
%i.next = add nuw nsw i32 %i, 1
--> {1,+,1}<nuw><%loop> U: [1,0) S: [1,0) Exits: (1 + %n)
LoopDispositions: { %loop: Computable }
Determining loop execution counts for: @foo
Loop %loop: backedge-taken count is %n
Loop %loop: max backedge-taken count is -1
Loop %loop: Predicated backedge-taken count is %n
Predicates:
Loop %loop: Trip multiple is 1
SCEV understands that %i won't exceed 2^31, but says that the backedge can be
taken up to 2^32-1 times. Since it's an upper-bound estimate, it's correct, yet
suboptimal.</pre>
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