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<div class="moz-cite-prefix">On 4/26/2018 6:51 AM, Alexandros
Lamprineas via llvm-dev wrote:<br>
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<blockquote type="cite"
cite="mid:DB6PR0802MB2424200BAFEBF790516A1620E28E0@DB6PR0802MB2424.eurprd08.prod.outlook.com">
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<p style="margin-top:0;margin-bottom:0">Hello,</p>
<p style="margin-top:0;margin-bottom:0"><br>
</p>
<p style="margin-top:0;margin-bottom:0">There is a particular
code sequence I would like to optimize at the IR level.</p>
<p style="margin-top:0;margin-bottom:0">I'd like to turn an
Arm/AArch64 table lookup intrinsic that takes a constant
vector mask into a shufflevector instruction:</p>
<p style="margin-top:0;margin-bottom:0">vtbl1(V,mask) ~>
shufflevector(V,undef,mask)</p>
<p style="margin-top:0;margin-bottom:0"><br>
</p>
<p style="margin-top:0;margin-bottom:0">The reason is that if
the mask is {7,6,5,4,3,2,1,0}, then the backend will generate
rev64 instructions instead.</p>
<p style="margin-top:0;margin-bottom:0">If the mask comes from a
vld1 of a global constant I could fold it to allow the above
instruction combining.</p>
<p style="margin-top:0;margin-bottom:0">My question is, does the
constant folding of the vld1 seem a good thing to do in the
general case, as a standalone transformation, or only when
used as a mask for a table lookup?<br>
</p>
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</blockquote>
<br>
Yes, constant-folding vld1 seems like a good idea.<br>
<br>
Actually, we should probably just lower the NEON vld1 intrinsics to
an LLVM "load" (which would give us constant-folding for free), but
that would be more work to make sure it doesn't have any unexpected
effects.<br>
<br>
-Eli
<p><br>
</p>
<pre class="moz-signature" cols="72">--
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project</pre>
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