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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Induction variable simplification does not simplify truncates"
href="https://llvm.org/bugs/show_bug.cgi?id=24670">24670</a>
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<th>Summary</th>
<td>Induction variable simplification does not simplify truncates
</td>
</tr>
<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>
</tr>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<tr>
<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>Scalar Optimizations
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>sanjoy@playingwithpointers.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Currently LLVM does not optimize `%c0` to `i1 true` in the following example:
declare void @use()
define void @f(i32 %len) {
entry:
%e = icmp slt i32 0, %len
br i1 %e, label %loop, label %leave
loop:
%idx = phi i64 [ 0, %entry ], [ %idx.inc, %be ]
%idx.inc = add i64 %idx, 1
%idx.tr = trunc i64 %idx to i32
call void @use()
%c0 = icmp slt i32 %idx.tr, %len
br i1 %c0, label %be, label %leave
be:
%idx.inc.tr = trunc i64 %idx.inc to i32
call void @use()
%c1 = icmp slt i32 %idx.inc.tr, %len
br i1 %c1, label %loop, label %leave
leave:
ret void
}
The interesting bit is that SCEV already has all the smarts required to
optimize this check away; and calling `simplifyLoopIVs` optimizes the check as
expected. -indvars does not get this case because it does not look through
truncates of induction variables, and thus does not try to eliminate `%c0`.</pre>
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