UseSuperWord - turned off, still emitting vector instructions?!

[Martin Stypinski]

Thanks Christian!

That explains it, I was pretty sure it had to be something in that
direction.

Best
Martin


Am Do., 17. Aug. 2023 um 15:31 Uhr schrieb Christian Hagedorn <
christian.hagedorn at oracle.com>:

> Hi Martin
>
> -XX:-UseSuperWord will turn off auto-vectorization. What you are observing
> is
> that for some instructions like adding two floats (AddFNode), we directly
> emit
> some vector instructions if the AVX instructions are available. This is
> done
> independently of auto-vectorization.
>
> AddFNode is matched with the addF_reg_mem rule which emits vaddss:
>
> https://github.com/openjdk/jdk/blob/c634bdf9d917c96c38efe826239eab7900c33e74/src/hotspot/cpu/x86/x86.ad#L2987-L2997
>
> For example, this code:
>
>     static float test() {
>         return floatFlield + 3.4f;
>     }
>
> will be compiled to (if UseAVX > 0):
>
>   vmovss -0x4c(%rip),%xmm1        # 0x00007fca3c5b1aa0
>   vaddss 0x70(%r10),%xmm1,%xmm0   ;*fadd {reexecute=0 rethrow=0
> return_oop=0}
>
>
> So, even if you turn off auto-vectorization, C2 will still emit vector
> instructions for some code.
>
> Best regards,
> Christian
>
>
> On 17.08.23 15:00, Martin Stypinski wrote:
> > I am benchmarking some algorithms, wondering how much the JIT applies
> > auto-vectorization. I set up a JMH project and am trying to turn
> > auto-vectorization off.
> >
> >
> > The code snipped, and the results can be found in the following
> > gist: https://gist.github.com/Styp/16e8a20fedfb771107377407fc1cf210
> > <https://gist.github.com/Styp/16e8a20fedfb771107377407fc1cf210> (Sorry
> too long
> > to post here).
> >
> >
> > The results indicate that one is faster than the other, which is
> expected.
> > Nevertheless, the deviation is so high that the benchmark is not
> representative.
> > What irritates me the most is that the version with turned of SuperWord
> still
> > contains vadd and vmove instructions in the Jit-generated code. Do I miss
> > something? How can I enforce vectorization turned off for good?
> >
> > Any ideas?
> >
>
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