Conditional statement "!=" is not more gas efficient than ">" or ">="

[kadenzipfel]

Conditional statement "!=" is not more gas efficient than ">" or ">="

[PaulRBerg]

Update: this conversation on Twitter is related to this issue.

As per transmission11's reply there, it does look like in an older version of Solidity, x != 0 used to be more gas efficient than x > 0.

Tagging @hrkrshnn in case he can illuminate us here?

[hrkrshnn]

I will follow up tomorrow.

[PaulRBerg]

Thanks @hrkrshnn, though no rush from my end.

[hrkrshnn]

@PaulRBerg The main difference is the use of via-ir. There are rules for transforming these expressions into an equivalent cheaper expression. But these rules are more effective in the new compilation pipeline. The legacy codegen and its optimizer works across basic blocks of assembly. And sometimes these expressions get split across basic blocks. See: https://hrkrshnn.com/t/devconnect.pdf#page=4 for another example.

I think these expressions should have the same cost if viaIR=true and the optimizer is enabled. Please don't hesitate to ping me again / open an issue in the original repo if that's not the case.

[PaulRBerg]

Thanks @hrkrshnn, makes sense.

What I don't understand though is that when I compiled the the code snippets above in Remix, I did not enable via-ir.

Is it that in the latest versions of Solidity, the compiler automatically applies via-ir to some operations, like != and >?

[hrkrshnn]

@PaulRBerg via-ir is not enabled by default yet. You can expect it to be default in an upcoming breaking release. The easiest way to test would be in Foundry, by adding viaIR = true in the toml config.

[PaulRBerg]

You can expect it to be default in an upcoming breaking release

Good to know!

via-ir is not enabled by default yet

This doesn't clear up the mystery yet of why the code snippets all yield the same gas cost even if --via-ir is not enabled.

[hrkrshnn]

This doesn't clear up the mystery yet of why the code snippets all yield the same gas cost even if --via-ir is not enabled.

It's likely because it uses different opcodes. See https://godbolt.org/z/q8K4WGqzr

[PaulRBerg]

Thank you ser, will take a look.