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Add SignextLowering pass to wasm-opt #1280

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merged 2 commits into from
Aug 18, 2023
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Add SignextLowering pass to wasm-opt #1280

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6a74070
Add `SignextLowering` pass to `wasm-opt`
ascjones Aug 17, 2023
feab5f4
Remove check for Rust version
ascjones Aug 17, 2023
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10 changes: 0 additions & 10 deletions crates/build/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -308,16 +308,6 @@ fn exec_cargo_for_onchain_target(
env.push(("RUSTC_BOOTSTRAP", Some("1".to_string())))
}

// newer rustc versions might emit unsupported instructions
if rustc_version::version_meta()?.semver >= Version::new(1, 70, 0) {
eprintln!(
"{} {}",
"warning:".yellow().bold(),
"You are using a rustc version that might emit unsupported wasm instructions. Build using a version lower than 1.70.0 to make sure your contract will deploy."
.bold()
);
}

// merge target specific flags with the common flags (defined here)
// We want to disable warnings here as they will be duplicates of the clippy pass.
// However, if we want to do so with either `--cap-lints allow` or `-A
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8 changes: 7 additions & 1 deletion crates/build/src/wasm_opt.rs
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Expand Up @@ -18,6 +18,7 @@ use anyhow::Result;
use wasm_opt::{
Feature,
OptimizationOptions,
Pass,
};

use std::{
Expand Down Expand Up @@ -68,9 +69,14 @@ impl WasmOptHandler {
);

OptimizationOptions::from(self.optimization_level)
// Since rustc 1.70 `SignExt` can't be disabled anymore. Hence we have to allow it.
.mvp_features_only()
// Since rustc 1.70 `SignExt` can't be disabled anymore. Hence we have to allow it,
// in order that the Wasm binary containing these instructions can be loaded.
.enable_feature(Feature::SignExt)
// This pass will then remove any `signext` instructions in order that the resulting
// Wasm binary is compatible with older versions of `pallet-contracts` which do not
// support the `signext` instruction.
.add_pass(Pass::SignextLowering)
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Since paritytech/substrate#14565 we do allow sign_extension in smart contracts on the pallet side. Shouldn't we make this pass optional for the cargo-contract so that the users of the newer versions of pallet-contracts could still opt in to have signext instructions in their contracts?

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I am aware, and I did consider making this optional. I decided against it because removing them by default means that both old and new versions of pallet_contracts will be supported with this, so will make users and our lives easier if the resulting wasm is compatible with as wide a range of target nodes as possible.

I assume the majority of node implementations currently are using an older version of pallet_contracts which does not support signext.

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I see. Could adding smth like --allow-signext to the cargo-contract contract build so that wasm_opt runs without the pass, be an option then?

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@ascjones ascjones Aug 17, 2023 •
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Yes, it would be preferable to do that if the user really does want signext instructions.

It would be interesting to do some analysis to the benefits or otherwise of contracts with or without the signext lowering pass. Could check for differences in contract sizes and gas efficiency.

// the memory in our module is imported, `wasm-opt` needs to be told that
// the memory is initialized to zeroes, otherwise it won't run the
// memory-packing pre-pass.
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