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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - missed optz'n in mul+div"
href="http://llvm.org/bugs/show_bug.cgi?id=20681">20681</a>
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<th>Summary</th>
<td>missed optz'n in mul+div
</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>Linux
</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>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>nlewycky@google.com
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</tr>
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<th>CC</th>
<td>david.majnemer@gmail.com, llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
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</tr></table>
<p>
<div>
<pre>I noticed this when looking at some vectorized code. We turned the shift+divide
into vector shift and vector divide. It was doing x<<2/12, which is really just
x/3.
Testcases:
define i32 @test1(i32 %a) {
%A = shl nsw i32 %a, 2
%B = sdiv i32 %A, 12
; CHECK: %B = sdiv i32 %a, 3
ret i32 %B
}
define i32 @test2(i32 %a) {
%A = mul nsw i32 %a, 3
%B = sdiv i32 %A, 12
; CHECK: %B = sdiv i32 %a, 4 ;; we don't simplify that to ashr?
ret i32 %B
}
define i32 @test3(i32 %a) {
%A = shl nuw i32 %a, 2
%B = udiv i32 %A, 12
; CHECK: %B = udiv i32 %a, 3
ret i32 %B
}
define i32 @test4(i32 %a) {
%A = mul nuw i32 %a, 3
%B = udiv i32 %A, 12
; CHECK: %B = lshr i32 %a, 2
ret i32 %B
}</pre>
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</p>
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