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8a8717e971
According to https://discourse.llvm.org/t/rfc-volatile-access-to-non-dereferenceable-memory-may-be-well-defined/86303/4, LLVM allows volatile operations on null and handles it correctly. This should be allowed in Rust as well, because I/O memory may be hard-coded to address 0 in some cases, like the AVR chip ATtiny1626. A test case that ensured a failure when passing null to volatile was removed, since it's now valid. Due to the addition of `maybe_is_aligned` to `ub_checks`, `maybe_is_aligned_and_not_null` was refactored to use it. docs: revise restrictions on volatile operations A distinction between usage on Rust memory vs. non-Rust memory was introduced. Documentation was reworded to explain what that means, and make explicit that: - No trapping can occur from volatile operations; - On Rust memory, all safety rules must be respected; - On Rust memory, the primary difference from regular access is that volatile always involves a memory dereference; - On Rust memory, the only data affected by an operation is the one pointed to in the argument(s) of the function; - On Rust memory, provenance follows the same rules as non-volatile access; - On non-Rust memory, any address known to not contain Rust memory is valid (including 0 and usize::MAX); - On non-Rust memory, no Rust memory may be affected (it is implicit that any other non-Rust memory may be affected, though, even if not referenced by the pointer). This should be relevant when, for example, reading register A causes a flag to change in register B, or writing to A causes B to change in some way. Everything affected mustn't be inside an allocation. - On non-Rust memory, provenance is irrelevant and a pointer with none can be used in a valid way. fix: don't lint null as UB for volatile Also remove a now-unneeded `allow` line. fix: additional wording nits
16 lines
364 B
Rust
16 lines
364 B
Rust
//@ run-fail
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//@ compile-flags: -Copt-level=3 -Cdebug-assertions=no -Zub-checks=yes
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//@ error-pattern: unsafe precondition(s) violated: ptr::read_volatile requires
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//@ revisions: misaligned
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use std::ptr;
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fn main() {
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let src = [0u16; 2];
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let src = src.as_ptr();
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unsafe {
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#[cfg(misaligned)]
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ptr::read_volatile(src.byte_add(1));
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}
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}
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