JDK-8307137: aarch64 MacroAssembler::lookup_interface_method could use conditional compare instead of branch

[Boris Ulasevich]

Hi Peter,

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 Ampere Altra.

Boris

On 4/29/2023 6:18 AM, Peter Kessler OS wrote:
>
> I notice that 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.
>
> 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.
>
> 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.
>
> ... peter
>
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