Document costs of various types.

[mudgen]

Does it cost any gas to cast a bytes32 memory value to uint256? What is the assembly op code for this?

Does it cost more gas to use =, >, <. operators with bytes32 than it does with uint256?

Spreadsheets about gas costs say it cost 3 gas for =, >, < operations, so is that true with all types that support those operators? Or is the gas cost for these operators different depending on the type?

I tested some of these things in remix and got mixed results -- sometimes it cost more gas, sometimes less.

Does it make any sense to cast bytes32 to unint256 for doing an operation like this: require(uint256(myvalue) != 0);

If you know the answers to these, can you please tell me how you know?

[chriseth]

In general, bytes32 and uint256 are mostly equivalent with regards to gas and I'm actually surprised you got different results. Did you change the function signature somewhere?

[mudgen]

Yes, my two test functions have different names.

[mudgen]

From my investigation it seems that there is a gas cost to cast a value if the cast changes the size of the value.

For instance casting bytes32 to uint256 does not cost gas because they are the same size. I wish there were a list of rules about these things somewhere that was easy to read.

Using an operator such as =,>,< will cost more gas if a cast needs to happen for the operation to occur and the cast causes one of the arguments to change size.

[chriseth]

Yes, some guidelines about the costs of various types should be added to the documentation.

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[itsN1X]

@chriseth , I had to have a few things debriefed:
Like,

• : Type of format for /docs
  e.g., Jekyll, Pandoc, rtm, PDF, html, etc.
• : Level of detailing:
  e.g.: I'm planning on covering all opcodes, with child --heads on each /dataType.
  Also, what all are the checkboxes I must tick?

My basic reference is the spresdsheet .1 from the wikipedia footnotes on its eth page.
i.e. .30, .32, and the one tagged in this issue.

Anything , or everything else you'd want me to know?
Any better citations, sources?
Also, my /ETA for the v0.0.1 draft shall be 100,000secs, from now.

cc/ @vs77bb
tysm!
~N1X

[chriseth]

@itsN1X this was more meant to be a high level description of the tradeoffs of different types with respect to gas, and not a detailed table with costs per opcode (although that could also be beneficial), since users usually do not know how they code is translated into opcodes (and I guess this is also where most confusion with regards to gas comes from).

[chriseth]

Oh and please don't use the spreadsheet but for example an implementation of a compliant client or the yellowpaper.

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[itsN1X]

@gitcoinbot Yep, still here! W-I-P.

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