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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Improve instruction selection for addition reductions"
href="http://llvm.org/bugs/show_bug.cgi?id=21371">21371</a>
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<th>Summary</th>
<td>Improve instruction selection for addition reductions
</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>Backend: AArch64
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>james.molloy@arm.com
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</tr>
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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 doing a simple "add" reduction, the code generated to do the final
"reduce" step is sub-optimal. The cleanup code needs to add all elements of a
vector together and return the result.
The following code:
int arr[5000];
int f() {
int a = 0;
for (int i = 0; i < 5000; ++i)
a += arr[i];
return a;
}
Generates the following reduction cleanup (this is outside the loop):
// BB#2: // %middle.block
add v0.4s, v1.4s, v0.4s // add two reduction vectors together
(UF=2)
ext v1.16b, v0.16b, v0.16b, #8 // v1 = high half of $v0 (high half of
v1 and v0 unused after this point)
add v0.4s, v0.4s, v1.4s // v0 = high half of $v0 + low half of
$v0
dup v1.4s, v0.s[1] // v1[0] = 1'st element of v0
add v0.4s, v0.4s, v1.4s // v0[0] = zero'th element of v0 + 1'st
element of v0
fmov w0, s0 // Swap back to GPRs.
ret
Note that the first ADD is adding two reduction vectors together, as the unroll
factor was 2. I've used the notation "$v0" to refer to the value of "v0" at the
beginning of the sequence.
This could be simplified to:
// BB#2: // %middle.block
add v0.4s, v0.4s, v1.4s // Add two reduction vectors together
(UF=2)
addv s0, v0.4s // Cross-lane reduce.
fmov w0, s0 // Swap back to GPRs
ret
This reduces 4 instructions to 1, and would have a bigger impact when reducing
sub-integer vectors (vectors with more than 4 elements).
Other instructions that could be useful for reductions:
* ADDV: integer add reduce across vector.
* FMAXNMV: Floating point maxnum across vector.
* FMAXV: Floating point maximum across vector.
* FMINNMV: Floating point minnum across vector.
* FMINV: Floating point minimum across vector.</pre>
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</p>
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