limit codegen-units to get the maximum optimization

[sabify]

Hello,

There might be a performance regression due to enabling lto without limiting the number of code generation units (codegen-units = 1).

References:
rust-lang/rust#47745
https://doc.rust-lang.org/rustc/codegen-options/index.html#codegen-units

[cla-bot-2021]

User @sabify, please sign the CLA here.

[melekes]

the change makes sense to me. thanks for contributing 🙏

[tomaka]

Thank you!

[github-actions]

twiggy diff report

Difference in .wasm size before and after this pull request.

---

 Delta Bytes │ Item
─────────────┼──────────────────────────────────────────────────────────────────────────────────────────────────────
      -47269 ┊ smoldot::executor::host::ReadyToRun::run_once::hce8991fa14cad99e
      +46242 ┊ smoldot::executor::host::ReadyToRun::run_once::h3fcc59051bdddd8e
      +42490 ┊ smoldot::json_rpc::methods::MethodCall::from_defs::h1204f543f3cab38f
      -42088 ┊ smoldot::json_rpc::methods::MethodCall::from_defs::h20a5b657120e0df0
      -25760 ┊ data[0]
      +24474 ┊ wasmi::runner::Interpreter::do_run_function::h5ad46fd45798232c
      -23655 ┊ wasmi::runner::Interpreter::do_run_function::h1d9d13ee2562db62
      +20025 ┊ wasmi::prepare::compile::Compiler::compile_instruction::h2c96786de3df97d2
      -20025 ┊ wasmi::prepare::compile::Compiler::compile_instruction::hf489b10a79fd7577
      +14915 ┊ <rand_chacha::chacha::ChaCha12Core as rand_core::block::BlockRngCore>::generate::h183a138761f91427
      -14915 ┊ <rand_chacha::chacha::ChaCha12Core as rand_core::block::BlockRngCore>::generate::h423816d94fa53ff2
      -14915 ┊ <rand_chacha::chacha::ChaCha20Core as rand_core::block::BlockRngCore>::generate::hdab9118613c13bb1
      +14915 ┊ <rand_chacha::chacha::ChaCha20Core as rand_core::block::BlockRngCore>::generate::hddf30a8319a72f4b
      -13086 ┊ sha2::sha512::soft::sha512_digest_block_u64::hbfd626ae3429a7bd
      -13086 ┊ sha2::sha512::soft::sha512_digest_block_u64::hd3baa310419eb6b6
      +13086 ┊ sha2::sha512::soft::sha512_digest_block_u64::hd3eb89d9a3b80320
      +13086 ┊ sha2::sha512::soft::sha512_digest_block_u64::hee89380fd02988b5
      -13021 ┊ <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll::hf2b8c9e998246e96
      -12275 ┊ wasmi_validation::func::FunctionValidationContext::step::he84b45aea78a58d1
      +12183 ┊ wasmi_validation::func::FunctionValidationContext::step::h86de041166276d61
      -47578 ┊ ... and 31561 more.
     -313468 ┊ Σ [31581 Total Rows]