Official support for Unsafe

[Maurizio Cimadamore]

I think the increased instruction count is normal, as C2 had to do more 
work to optimize the bound checks away?

Is there any difference compared to the version that doesn't use the 
universe segment?

Maurizio

On 15/01/2024 13:52, Quân Anh Mai wrote:
> Hi,
>
> I have tried using a universe segment instead of Unsafe, and store the 
> custom hashmap buffer in off-heap instead of using a byte array. The 
> output of perf stat on the program
>
>  Performance counter stats for 'sh calculate_average_merykittyunsafe.sh':
>
>           13573.70 msec task-clock:u              #   10.942 CPUs utilized
>                  0      context-switches:u        #    0.000 /sec
>                  0      cpu-migrations:u          #    0.000 /sec
>             238460      page-faults:u             #   17.568 K/sec
>        61995179870      cycles:u                  #    4.567 GHz
>          261830581      stalled-cycles-frontend:u #    0.42% frontend 
> cycles idle
>           93823680      stalled-cycles-backend:u  #    0.15% backend 
> cycles idle
>       137976098809      instructions:u            #    2.23  insn per 
> cycle
>                                                   #    0.00  stalled 
> cycles per insn
>        18373313803      branches:u                #    1.354 G/sec
>           43579782      branch-misses:u           #    0.24% of all 
> branches
>
>        1.240504612 seconds time elapsed
>
>       12.841563000 seconds user
>        0.652428000 seconds sys
>
> For comparison, this is the unsafe version:
>
>  Performance counter stats for 'sh calculate_average_merykittyunsafe.sh':
>
>           13327.46 msec task-clock:u              #   11.202 CPUs utilized
>                  0      context-switches:u        #    0.000 /sec
>                  0      cpu-migrations:u          #    0.000 /sec
>             269896      page-faults:u             #   20.251 K/sec
>        61258348752      cycles:u                  #    4.596 GHz
>          639839262      stalled-cycles-frontend:u #    1.04% frontend 
> cycles idle
>          108018676      stalled-cycles-backend:u  #    0.18% backend 
> cycles idle
>       113476168983      instructions:u            #    1.85  insn per 
> cycle
>                                                   #    0.01  stalled 
> cycles per insn
>        11442665370      branches:u                #  858.578 M/sec
>           44590172      branch-misses:u           #    0.39% of all 
> branches
>
>        1.189768677 seconds time elapsed
>
>       12.628512000 seconds user
>        0.620083000 seconds sys
>
> This program running on my machine expresses dependency bound so the 
> difference in execution time is not as significant as on the test 
> machine but it can be seen that removing Unsafe results in over 21% 
> increase in instruction count.
>
> Regards,
> Quan Anh
>
> On Sat, 13 Jan 2024 at 01:29, Maurizio Cimadamore 
> <maurizio.cimadamore at oracle.com> wrote:
>
>
>     On 12/01/2024 17:26, Quân Anh Mai wrote:
>     > FYI, in my submission to 1brc, using Unsafe decreases the execution
>     > time from 3.25s to 2.57s on the test machine.
>
>     Just curious - what is the difference compared with the everything
>     segment trick?
>
>     (While I know it can't do on-heap access, perhaps you can tweak
>     the code
>     to be all off-heap?)
>
>     Maurizio
>
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