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Peter,<br>
<br>
I tried ccmp as part of improving itable stub on aarch64, and the
results were not promising. Applying ccmp as suggested increased
geomean from 15.7 ns to 15.9 ns on N1 and from 201 ns to 205 ns on
A72. I don't think micro-architecture specialization in itable stub
would bring universal benefits, it will only make code more
complicated. I would appreciate your review of the AArch64 part of
JDK-8305959 once I post it.<br>
<br>
thanks,<br>
Boris<br>
<br>
<div class="moz-cite-prefix">On 4/29/2023 1:48 PM, Boris Ulasevich
wrote:<br>
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cite="mid:19e76652-f635-5ad9-cf84-37e7b87b8adc@bell-sw.com">
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Hi Peter,<br>
<br>
Please have a look at JDK-8305959. I'm going to rewrite the itable
stub codes to use a single pass over itable! I have an aarch64
implementation which shows improvement on <span
style="font-size:10.5pt;font-family:"Arial",sans-serif;color:#172B4D;background:white">Ampere
Altra.</span><br>
<br>
Boris<br>
<br>
<div class="moz-cite-prefix">On 4/29/2023 6:18 AM, Peter Kessler
OS wrote:<br>
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cite="mid:SJ0PR01MB74149EF5FB147590E6CB9815D4689@SJ0PR01MB7414.prod.exchangelabs.com">
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<p class="MsoNormal">I notice that <span
style="font-size:10.5pt;font-family:"Arial",sans-serif;color:#172B4D;background:white">src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp
MacroAssembler::lookup_interface_method loops over the
itable list with code that uses two branches: one to check
for a null indicating the end of the list, and one to see
if the appropriate entry has been found. aarch64 has a
"ccmp" instruction that can be used to evaluate two
conditions with only one branch. On an out-of-order
implementation with more integer execution units than
branch units, the trading of a branch for a ccmp can be
beneficial. The downside is that one has to check, after
the loop has exited, which of the conditions cause the
loop to exit, but if the loop executes more than once or
twice, that is still a win.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.5pt;font-family:"Arial",sans-serif;color:#172B4D;background:white"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.5pt;font-family:"Arial",sans-serif;color:#172B4D;background:white">There
are other opportunities to use cmp;ccmp;br instead of
cmp;br;cmp;br. I happened to see the one in
MacroAssembler::lookup_interface_method because it was in
what passes for hand-written assembler in HotSpot. For
generic searches for a key in a key-value array the
improvement can be ~10% on a Ampere Altra, depending on
how far down the key-value array one has to look.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.5pt;font-family:"Arial",sans-serif;color:#172B4D;background:white"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.5pt;font-family:"Arial",sans-serif;color:#172B4D;background:white">I
am only proposing to fix the loop in
MacroAssembler::lookup_interface_method, but I would be
interested in talking to people about where else the ccmp
style could be applied.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.5pt;font-family:"Arial",sans-serif;color:#172B4D;background:white"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.5pt;font-family:"Arial",sans-serif;color:#172B4D;background:white">
... peter</span><o:p></o:p></p>
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