Result<T, E> where size_of::<T>() < 128 and size_of::<E> >= 128 #4652
Comments
llogiq
added
E-medium
|
feels like the large_enum_variant check but with a limit of 128 by default instead of 200. |
|
Sort of, but specialized to |
|
Also large_enum_variant doesn't fire on Result<T, E>, since (I think?) it's looking at the definition of the enum. |
bors
added a commit
that referenced
this issue
Aug 30, 2022
Initial implementation `result_large_err` This is a shot at #6560, #4652, and #3884. The lint checks for `Result` being returned from functions/methods where the `Err` variant is larger than a configurable threshold (the default of which is 128 bytes). There has been some discussion around this, which I'll try to quickly summarize: * A large `Err`-variant may force an equally large `Result` if `Err` is actually bigger than `Ok`. * There is a cost involved in large `Result`, as LLVM may choose to `memcpy` them around above a certain size. * We usually expect the `Err` variant to be seldomly used, but pay the cost every time. * `Result` returned from library code has a high chance of bubbling up the call stack, getting stuffed into `MyLibError { IoError(std::io::Error), ParseError(parselib::Error), ...}`, exacerbating the problem. This PR deliberately does not take into account comparing the `Ok` to the `Err` variant (e.g. a ratio, or one being larger than the other). Rather we choose an absolute threshold for `Err`'s size, above which we warn. The reason for this is that `Err`s probably get `map_err`'ed further up the call stack, and we can't draw conclusions from the ratio at the point where the `Result` is returned. A relative threshold would also be less predictable, while not accounting for the cost of LLVM being forced to generate less efficient code if the `Err`-variant is _large_ in absolute terms. We lint private functions as well as public functions, as the perf-cost applies to in-crate code as well. In order to account for type-parameters, I conjured up `fn approx_ty_size`. The function relies on `LateContext::layout_of` to compute the actual size, and in case of failure (e.g. due to generics) tries to come up with an "at least size". In the latter case, the size of obviously wrong, but the inspected size certainly can't be smaller than that. Please give the approach a heavy dose of review, as I'm not actually familiar with the type-system at all (read: I have no idea what I'm doing). The approach does, however flimsy it is, allow us to successfully lint situations like ```rust pub union UnionError<T: Copy> { _maybe: T, _or_perhaps_even: (T, [u8; 512]), } // We know `UnionError<T>` will be at least 512 bytes, no matter what `T` is pub fn param_large_union<T: Copy>() -> Result<(), UnionError<T>> { Ok(()) } ``` I've given some refactoring to `functions/result_unit_err.rs` to re-use some bits. This is also the groundwork for #6409 The default threshold is 128 because of #4652 (comment) `lintcheck` does not trigger this lint for a threshold of 128. It does warn for 64, though. The suggestion currently is the following, which is just a placeholder for discussion to be had. I did have the computed size in a `span_label`. However, that might cause both ui-tests here and lints elsewhere to become flaky wrt to their output (as the size is platform dependent). ``` error: the `Err`-variant returned via this `Result` is very large --> $DIR/result_large_err.rs:36:34 | LL | pub fn param_large_error<R>() -> Result<(), (u128, R, FullyDefinedLargeError)> { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ The `Err` variant is unusually large, at least 128 bytes ``` changelog: Add [`result_large_err`] lint
bors
added a commit
that referenced
this issue
Apr 19, 2023
Add size-parameter to unecessary_box_returns Fixes #10641 This adds a configuration-knob to the `unecessary_box_returns`-lint which allows _not_ linting a `fn() -> Box<T>` if `T` is "large". The default byte size above which we no longer lint is 128 bytes (due to #4652 (comment), also used in #9373). The overall rational is given in #10641. --- changelog: Enhancement: [`unnecessary_box_returns`]: Added new lint configuration `unnecessary-box-size` to set the maximum size of `T` in `Box<T>` to be linted [#10651](#10651) <!-- changelog_checked -->
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Those will cause LLVM to issue
memcpycalls to copy them around. Boxing the error type will push them below LLVM's size threshold, thus possibly improving performance, unless the Err case is more likely than the Ok case.The text was updated successfully, but these errors were encountered: