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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - SCEV incorrectly flags add recurrences as nsw"
href="http://llvm.org/bugs/show_bug.cgi?id=22179">22179</a>
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<th>Summary</th>
<td>SCEV incorrectly flags add recurrences as nsw
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Loop Optimizer
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>sanjoy@playingwithpointers.com
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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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<p>
<div>
<pre>When running opt -analyze -scalar-evolution on the following program
%struct.anon = type { i8 }
%struct.S = type { i32 }
@a = common global %struct.anon zeroinitializer, align 1
@b = common global %struct.S zeroinitializer, align 4
; Function Attrs: nounwind ssp uwtable
define i32 @main() {
store i8 0, i8* getelementptr inbounds (%struct.anon* @a, i64 0, i32 0),
align 1
br label %loop
loop:
%storemerge1 = phi i8 [ 0, %0 ], [ %inc, %loop ]
%m = load volatile i32* getelementptr inbounds (%struct.S* @b, i64 0, i32 0),
align 4
%inc = add nuw i8 %storemerge1, 1
%exitcond = icmp eq i8 %inc, -128
br i1 %exitcond, label %exit, label %loop
exit:
store i8 -128, i8* getelementptr inbounds (%struct.anon* @a, i64 0, i32 0),
align 1
ret i32 0
}
we get the SCEV {1,+,1}<nuw><nsw><%loop> for %inc = add nuw i8 %storemerge1, 1.
However, the llvm expression can legitimately signed-overflow.
The real issue is that in several places in ScalarEvolution, we have
// If FlagNSW is true and all the operands are non-negative, infer FlagNUW.
// And vice-versa.
The vice-versa part is incorrect I think -- it is possible for {1,+,1} to not
unsigned-overflow, but it is okay for it to signed-overflow, for instance.</pre>
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