Some confusion about P4 hardware implemention.

[mmhzheng]

Hi,
I try to implement the hardware version of Elastic Sketch in a P4 programmable switch. I note that the stateful ALU has following restrictions :
1. Each packet can only access a specific register once.
2. RegisterAction can only access 2 PHV container values and 4 constants, with not more than 2 comparsions.
In my implemention, I realize it cannot read the <key, vote+> register first, because it must to perform read-test-write in one RegisterAction. So, I read vote_all first, dividing it by right-shift, and then put the result as the threshhold in a metadata field.
Then , I perform the RegisterAction to read-test-write the <key, vote+> register. But here I meet a challenge:
Challenge: In the RegisterAction, I need to read at least three PHV value (hdr, the computed threashold, and the count). And It cannot fit in one stateful ALU.
I note that if I only want to measure the packet count, there is no problem. But what if I need to count the packet length?
I am comfused of this question a few days. Thanks for any suggestions.

Best wishes,
Hao Zheng.