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ptr::add/sub: do not claim equivalence with `offset(c as isize)` In https://github.com/rust-lang/rust/pull/110837, the `offset` intrinsic got changed to also allow a `usize` offset parameter. The intention is that this will do an unsigned multiplication with the size, and we have UB if that overflows -- and we also have UB if the result is larger than `usize::MAX`, i.e., if a subsequent cast to `isize` would wrap. ~~The LLVM backend sets some attributes accordingly.~~ This updates the docs for `add`/`sub` to match that intent, in preparation for adjusting codegen to exploit this UB. We use this opportunity to clarify what the exact requirements are: we compute the offset using mathematical multiplication (so it's no problem to have an `isize * usize` multiplication, we just multiply integers), and the result must fit in an `isize`. Cc `@rust-lang/opsem` `@nikic` https://github.com/rust-lang/rust/pull/130239 updates Miri to detect this UB. `sub` still has some cases of UB not reflected in the underlying intrinsic semantics (and Miri does not catch): when we subtract `usize::MAX`, then after casting to `isize` that's just `-1` so we end up adding one unit without noticing any UB, but actually the offset we gave does not fit in an `isize`. Miri will currently still not complain for such cases: ```rust fn main() { let x = &[0i32; 2]; let x = x.as_ptr(); // This should be UB, we are subtracting way too much. unsafe { x.sub(usize::MAX).read() }; } ``` However, the LLVM IR we generate here also is UB-free. This is "just" library UB but not language UB. Cc `@saethlin;` might be worth adding precondition checks against overflow on `offset`/`add`/`sub`? Fixes https://github.com/rust-lang/rust/issues/130211 |
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| unwind | ||
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