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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - exact shift right of constant not optimized (instcombine / value tracking)"
href="http://llvm.org/bugs/show_bug.cgi?id=21477">21477</a>
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<th>Summary</th>
<td>exact shift right of constant not optimized (instcombine / value tracking)
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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
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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
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<th>Component</th>
<td>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>spatel+llvm@rotateright.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>[Follow-on from <a class="bz_bug_link
bz_status_NEW "
title="NEW --- - wrong code at -O3 on x86_64-linux-gnu (LICM)"
href="show_bug.cgi?id=21412">bug 21412</a>.]
An exact shift right guarantees that no set bits are shifted out, so this:
define i32 @exact_shift(i32 %shiftval) {
%shr = lshr exact i32 1, %shiftval
ret i32 %shr
}
...should be optimized to:
define i32 @exact_shift(i32 %shiftval) {
ret i32 1
}
Ie, %shiftval must be zero, or we have undefined behavior / poison.
In the other bug, the shift is followed by division, so the ideal optimization
for:
define i32 @exact_div(i32 %x, i32 %shiftval) {
%shr = lshr exact i32 1, %shiftval
%div = udiv i32 %x, %shr
ret i32 %div
}
...will be:
define i32 @exact_div(i32 %x, i32 %shiftval) {
ret i32 %x
}</pre>
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