Add opt-for-size core lib feature flag #125011
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rustbot
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You may also need to add it to |
Oh right, the abort feature is there too. I've got no idea really what sysroot is, but I've added it there too |
scottmcm
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This sounds like a team question, not a reviewer question. |
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No objection to the concept, but I'd be very, very hesitant to add a large number of alternative algorithm implementations. (And if we ever want to support this option in any stable way, I think we'd need to run the testsuite on a version compiled that way, to make sure that the alternatives work.) |
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We discussed this in the library team meeting today and we're happy to add this flag. However one concern was raised: we would like the alternative implementations of algorithms behind this flag to actually be tested in CI. The easiest way is probably to re-run the standard library tests with this feature enabled in the CI scripts. Or if this is too expensive then it could be done only for a single target. |
Amanieu
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Should the test infra be added in this PR, a followup PR or together with the first PR that actually uses this flag? |
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I can help with setting up the CI, let me know if you want to add it to this PR. |
@Kobzol yeah let's do it! I've been looking for something to do the next hackday (this friday) at work. (And if it takes longer than a day, that's fine too) Looking at the CI scripts I feel kinda lost. So if you could write down and link to where I should be looking, that'd be much appreciated. |
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I digged into stdlib tests a bit and realized that it's not so simple. I'm not actually sure if we already run tests for panic="abort" for stdlib. I found a way to do this, but maybe it's a bit hacky. I'll lead the discussion here. |
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Additionally it would be useful to track the size of a standard library built this way. This could happen in rustc-perf or as some script that prints out the size which can be used for before/after comparisons when someone lands additional implementations under this cfg. But neither has to happen in this PR. |
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Tracking the size would be nice indeed, but also probably hard to do. Afaik you can't really measure the size of a library very well |
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Added the flag to the CI. I think this only runs on merges. |
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Thanks! @bors r=Amanieu,kobzol We don't currently have an easy way of monitoring the size of stdlib with this flag enabled, but for now it will be probably enough to just post a size diff summary on PRs that do something with this flag. |
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Should we have a tracking issue for the effort? Or some documentation? |
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I'm not sure if it should be a tracking issue, as there is not really any specific end goal here to track. The expected usage is that eventually, people will start sending targeted PRs to reduce specific code bloat in the stdlib when the flag is enabled (e.g. using a sort algorithm optimized for small binary size). That being said, we should definitely document this somehow. Maybe here? This is also relevant for |
Right now the feature isn't useful, so I'd say a minimum goal would be getting it up to some shape where it has tangible benefits so that nightly users would want to opt into it. |
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Once this lands in nightly I want to make some prs that are easy wins based on what I experimented before. |
…iaskrgr Rollup of 7 pull requests Successful merges: - rust-lang#124570 (Miscellaneous cleanups) - rust-lang#124772 (Refactor documentation for Apple targets) - rust-lang#125011 (Add opt-for-size core lib feature flag) - rust-lang#125218 (Migrate `run-make/no-intermediate-extras` to new `rmake.rs`) - rust-lang#125225 (Use functions from `crt_externs.h` on iOS/tvOS/watchOS/visionOS) - rust-lang#125266 (compiler: add simd_ctpop intrinsic) - rust-lang#125348 (Small fixes to `std::path::absolute` docs) Failed merges: - rust-lang#125296 (Fix `unexpected_cfgs` lint on std) r? `@ghost` `@rustbot` modify labels: rollup
Rollup merge of rust-lang#125011 - diondokter:opt-for-size, r=Amanieu,kobzol Add opt-for-size core lib feature flag Adds a feature flag to the core library that enables the possibility to have smaller implementations for certain algorithms. So far, the core lib has traded performance for binary size. This is likely what most people want since they have big simd-capable machines. However, people on small machines, like embedded devices, don't enjoy the potential speedup of the bigger algorithms, but do have to pay for them. These microcontrollers often only have 16-1024kB of flash memory. This PR is the result of some talks with project members like `@Amanieu` at RustNL. There are some open questions of how this is eventually stabilized, but it's a similar question as with the existing `panic_immediate_abort` feature. Speaking as someone from the embedded side, we'd rather have this unstable for a while as opposed to not having it at all. In the meantime we can try to use it and also add additional PRs to the core lib that uses the feature flag in areas where we find benefit. Open questions from my side: - Is this a good feature name? - `panic_immediate_abort` is fairly verbose, so I went with something equally verbose - It's easy to refactor later - I've added the feature to `std` and `alloc` as well as they might benefit too. Do we agree? - I expect these to get less usage out of the flag since most size-constraint projects don't use these libraries often.
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Tracking issue for this flag: #125612 |
Adds a feature flag to the core library that enables the possibility to have smaller implementations for certain algorithms.
So far, the core lib has traded performance for binary size. This is likely what most people want since they have big simd-capable machines. However, people on small machines, like embedded devices, don't enjoy the potential speedup of the bigger algorithms, but do have to pay for them. These microcontrollers often only have 16-1024kB of flash memory.
This PR is the result of some talks with project members like @Amanieu at RustNL.
There are some open questions of how this is eventually stabilized, but it's a similar question as with the existing
panic_immediate_abortfeature.Speaking as someone from the embedded side, we'd rather have this unstable for a while as opposed to not having it at all. In the meantime we can try to use it and also add additional PRs to the core lib that uses the feature flag in areas where we find benefit.
Open questions from my side:
panic_immediate_abortis fairly verbose, so I went with something equally verbosestdandallocas well as they might benefit too. Do we agree?