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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - vector approximate reciprocal square root generates bad code on x86"
href="https://bugs.llvm.org/show_bug.cgi?id=37344">37344</a>
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<th>Summary</th>
<td>vector approximate reciprocal square root generates bad code on x86
</td>
</tr>
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<th>Product</th>
<td>new-bugs
</td>
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<tr>
<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>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>gonzalobg88@gmail.com
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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>The following LLVM IR (see it live: <a href="https://godbolt.org/g/88kuky">https://godbolt.org/g/88kuky</a>) computes the
approximate vector reciprocal square root rsqrt(x) ~= 1/ sqrt(x):
declare <4 x float> @llvm.sqrt.v4f32(<4 x float>)
define <4 x float> @rsqrt(<4 x float>) {
%a = call afn <4 x float> @llvm.sqrt.v4f32(<4 x float> %0)
%c = fdiv <4 x float> <float 1.000000e+00, float 1.000000e+00, float
1.000000e+00, float 1.000000e+00>, %a
ret <4 x float> %c
}
On x86_64 with -O3 and sse4.2 they generate the following assembly:
LCPI0_0:
.long 1065353216 # float 1
.long 1065353216 # float 1
.long 1065353216 # float 1
.long 1065353216 # float 1
rsqrt: # @rsqrt
sqrtps xmm1, xmm0
movaps xmm0, xmmword ptr [rip + .LCPI0_0]
divps xmm0, xmm1
ret
However, it should just generate a call to rsqrtps .
I've tried with fast math flags but haven't been able to generate rsqrtps yet.</pre>
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