Re: [PATCH 3/3] powerpc: bpf: implement in-register swap for 64-bit endian operations

["'Naveen N. Rao'" <naveen.n.rao@linux.vnet.ibm.com>]

On 2017/01/24 04:13PM, David Laight wrote:
> From: 'Naveen N. Rao'
> > Sent: 23 January 2017 19:22
> > On 2017/01/15 09:00AM, Benjamin Herrenschmidt wrote:
> > > On Fri, 2017-01-13 at 23:22 +0530, 'Naveen N. Rao' wrote:
> > > > > That rather depends on whether the processor has a store to load forwarder
> > > > > that will satisfy the read from the store buffer.
> > > > > I don't know about ppc, but at least some x86 will do that.
> > > >
> > > > Interesting - good to know that.
> > > >
> > > > However, I don't think powerpc does that and in-register swap is likely
> > > > faster regardless. Note also that gcc prefers this form at higher
> > > > optimization levels.
> > >
> > > Of course powerpc has a load-store forwarder these days, however, I
> > > wouldn't be surprised if the in-register form was still faster on some
> > > implementations, but this needs to be tested.
> > 
> > Thanks for clarifying! To test this, I wrote a simple (perhaps naive)
> > test that just issues a whole lot of endian swaps and in _that_ test, it
> > does look like the load-store forwarder is doing pretty well.
> ...
> > This is all in a POWER8 vm. On POWER7, the in-register variant is around
> > 4 times faster than the ldbrx variant.
> ...
> 
> I wonder which is faster on the little 1GHz embedded ppc we use here.

Worth a test, for sure.
FWIW, this patch won't matter since eBPF JIT is for ppc64.

Thanks,
Naveen