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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 - Regression for three way min/max"
href="https://bugs.llvm.org/show_bug.cgi?id=35717">35717</a>
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<th>Summary</th>
<td>Regression for three way min/max
</td>
</tr>
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<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>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
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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>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>david.green@arm.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>This is a regression that came in with rL320749. We have some code with three
way min/max that is now doing a lot worse than it did previously. The most
basic for I could come up with is this:
int test(int xc, int xm, int xy)
{
int xk;
if (xc < xm)
xk = xc < xy ? xc : xy;
else
xk = xm < xy ? xm : xy;
return xk;
}
This used to come out as (roughly "min(min(a,b),c)"):
define i32 @test(i32 %xc, i32 %xm, i32 %xy) {
entry:
%cmp = icmp slt i32 %xc, %xm
%xm.sink15 = select i1 %cmp, i32 %xc, i32 %xm
%cmp2 = icmp slt i32 %xm.sink15, %xy
%cond6 = select i1 %cmp2, i32 %xm.sink15, i32 %xy
ret i32 %cond6
}
But now ends up with extra selects (roughly "min(min(a,b), min(b,c))"):
define i32 @test(i32 %xc, i32 %xm, i32 %xy) {
entry:
%cmp = icmp slt i32 %xc, %xm
%cmp1 = icmp slt i32 %xc, %xy
%cond = select i1 %cmp1, i32 %xc, i32 %xy
%cmp2 = icmp slt i32 %xm, %xy
%cond6 = select i1 %cmp2, i32 %xm, i32 %xy
%xk.0 = select i1 %cmp, i32 %cond, i32 %cond6
ret i32 %xk.0
}
The original case came from a cmy conversion, where the values were i8 (not
i32) so there is a number of zext and trunc's making things more messy. They
are also inverted (subtracted from 255). It runs especially worse on our small
thumb1 cores.</pre>
</div>
</p>
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