Made loop bound checking in hardware intrinsics more efficient

[cTxplorer]

Changed style to make loop bound checking more efficient.

Closes #835

[dnfclas]

All CLA requirements met.

[codemzs]

Do you have benchmark numbers with and without this change?

[cTxplorer]

@codemzs No, I don't have benchmark numbers.

[danmoseley]

@cTxplorer thanks for doing this.

It may be that the JIT cannot currently even figure out that pEnd is a constant during the loop, so this does not save any work. It would probably be better to extract out something like var pEndMinus8 = pEnd - 8 and then you can do while (pDstCurrent <= pEndMinus8).

That may be worth doing but benchmarking will say for sure. To benchmark you need to build for .NET Core 3.0 since this codepath is not used otherwise. Hopefully @eerhardt can help me with this.

After that instructions to run the benchmarks are here although again there will be some flag required to make sure it runs on .NET Core 3.0

Thanks for your patience (this is probably more complex than you expected)

cc @tannergooding

[danmoseley]

Instructions for building on 3.0 are here: #1032

[shauheen]

Thanks @cTxplorer for looking at this, do you have any plans to update this PR?

[cTxplorer]

@shauheen Ready to help :)

[tannergooding]

@cTxplorer, the merge conflicts need to be resolved and the same feedback made in #1177 applies here (which is a smaller, but related PR).

• The primary feedback is that pDstEnd - <const> needs to be pulled out into a local to ensure that it isn't done once per loop iteration.

[danmoseley]

@cTxplorer please also share benchmark results, as in #1177.

[danmoseley]

@cTxplorer could you clarify the purpose of the rename (81401e9) ? It seems that was intended for a separate PR, associated with some other issue. (And if renamed, probably the file should be too). We would like to keep each PR for a single purpose to keep things clear. Thanks

[danmoseley]

cc @jwood803

[tannergooding]

@cTxplorer, going to close this for now since we haven't had any update in over 10 days.

If you would like to continue working on this, feel free to open a new PR that resolves the conflicts and we will be happy to help push it through.