Add panic_unreachable_unchecked feature flag to the standard library
#139196
Conversation
This is similar to `panic_immediate_abort` except that all panics are considered immediate UB and can therefore be optimized away as unreachable by the compiler. This has been tested on [regalloc3](https://github.com/Amanieu/regalloc3) where it resulted in a 10% speedup compared to using a normal standard library, mainly due to the elimination of bounds checks. While it may seem that this feature merely to satisfy those with a reckless thirst for performance at any cost, it is also useful for saner heads as a profiling tool to investigate the impact of unnecessary safety check and find places where unsafe code could be used to avoid them.
rustbot
added
S-waiting-on-review
| #[rustc_const_stable_indirect] // must follow stable const rules since it is exposed to stable | ||
| pub const fn panic_fmt(fmt: fmt::Arguments<'_>) -> ! { | ||
| if cfg!(feature = "panic_unreachable_unchecked") { | ||
| // SAFETY: it's not... |
There was a problem hiding this comment.
This safety comment is inadequate, the condition of soundness relies on the programmer asserting panics will never happen, so a more accurate comment is:
| // SAFETY: it's not... | |
| // SAFETY: The user of this flag asserts that a panic will never happen in their codebase |
if you want to do april fools, go all in :^)
|
|
| # Make panics and failed asserts immediately abort without formatting any message | ||
| panic_immediate_abort = ["core/panic_immediate_abort"] | ||
| # Make the optimizer assume panics are unreachable. | ||
| panic_unreachable_unchecked = [ |
There was a problem hiding this comment.
I propose to rename this feature to unsafe_panic_unreachable_unchecked, so that its name includes word unsafe, because this is actually an unsafe API (like #[no_mangle] attribute), and AFAIU all unsafe APIs tend to include unsafe in their usage (as part of their name, as a surrounding unsafe block, etc).
Alternatively, if this PR is a joke, I'd consider crabs_never_panic as an alternative name.
|
It turns out that @saethlin has a much better implementation of this that doesn't even require |
7 participants
This is similar to
panic_immediate_abortexcept that all panics are considered immediate UB and can therefore be optimized away as unreachable by the compiler.This has been tested on regalloc3 where it resulted in a 10% speedup compared to using a normal standard library, mainly due to the elimination of bounds checks.
While it may seem that this feature merely to satisfy those with a reckless thirst for performance at any cost, it is also useful for saner heads as a profiling tool to investigate the impact of unnecessary safety check and find places where unsafe code could be used to avoid them.