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Support opt-level ranges to avoid building twice with different opt-level values #8502

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joshtriplett opened this issue Jul 17, 2020 · 0 comments
Open

Support opt-level ranges to avoid building twice with different opt-level values #8502

joshtriplett opened this issue Jul 17, 2020 · 0 comments
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A-profiles Area: profiles C-feature-request Category: proposal for a feature. Before PR, ping rust-lang/cargo if this is not `Feature accepted`

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@joshtriplett
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#8500 will change Cargo's default opt-level for build dependencies to 0. However, if a crate will also be built as a runtime dependency, that would result in Cargo building it twice, once with opt-level 0 and once with opt-level 3. We should have a way of specifying the default as having a minimum opt-level of 0, but allowing a higher opt-level. Then, Cargo could unify that and just build once, leaving build-time-only dependencies unoptimized, but optimizing dependencies shared between build-time and runtime.
@joshtriplett joshtriplett added the C-feature-request Category: proposal for a feature. Before PR, ping rust-lang/cargo if this is not `Feature accepted` label Jul 17, 2020
@joshtriplett joshtriplett changed the title Support opt-level ranges to avoid building twice with differnet opt-level values Support opt-level ranges to avoid building twice with different opt-level values Jul 17, 2020
@joshtriplett joshtriplett mentioned this issue Jul 17, 2020
Merged
@ehuss ehuss added the A-profiles Area: profiles label Jul 29, 2020
@yangby-cryptape yangby-cryptape mentioned this issue Jun 25, 2021
Merged
bors bot added a commit to nervosnetwork/ckb that referenced this issue Jun 27, 2021
@bors @yangby-cryptape
Merge #2788
6844fa4 
2788: build: customize `opt-level` to avoid building some crates twice r=quake,driftluo a=yangby-cryptape

### Purpose

Since "Cargo PR-8500" merged, as "Cargo Issue-8502" said: "if a crate will also be built as a runtime dependency, that would result in Cargo building it twice, once with opt-level 0 and once with opt-level 3."

So, in this PR I customize the `opt-level` to the default values, to make sure each crates only be compiled once.

### Results

When `make prod`  for `v0.42.0`:
- This PR can reduce the number of compiled dependencies 9.56%  (`596 -> 544`).
- This PR can reduce the size of target directory 14.2% (`1_813_100 bytes -> 1_587_780 bytes`).
- Save total compile time.

**Notice: the size of final binary won't be chaneged.**

### References

- [Cargo PR-8500: Build host dependencies with opt-level 0 by default](rust-lang/cargo#8500)
- [Cargo Issue-8502: Support opt-level ranges to avoid building twice with different opt-level values](rust-lang/cargo#8502)
- [Cargo Reference / Default profiles](https://doc.rust-lang.org/cargo/reference/profiles.html#default-profiles)

Co-authored-by: Boyu Yang <yangby@cryptape.com>
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