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#![feature(test)] // for black_box(). use std::mem::MaybeUninit; extern "C" { fn snprintf(s: *mut i8, len: usize, fmt: *const i8, ...); } // Based on the Godbolt example from // https://www.ralfj.de/blog/2019/07/14/uninit.html. // // See if we can convince the optimizer to see an // uninitialized chunk of memory as initialized. pub fn always_true_or_false() -> bool { // Just claim we initialized this even though we did not. let mut x: [u8; 16] = unsafe { MaybeUninit::uninit().assume_init() }; // UB: optimized to 'true' or 'false' x[11] == x[12] } pub fn always_true_or_false_vec() -> bool { // same as the first function, but use a Vec. let mut v = Vec::<u8>::with_capacity(256); unsafe { v.set_len(256); } // UB v[11] == v[12] } pub fn init_mem_using_ffi() -> bool { let mut x: [u8; 16] = unsafe { MaybeUninit::uninit().assume_init() }; // An FFI call to some libc function makes the memory // initialized, as far as the optimizer is concerned. unsafe { snprintf((&mut x[..]).as_mut_ptr() as *mut i8, x.len(), b"x\0".as_ptr() as *const i8) }; x[11] == x[12] } pub fn init_mem_black_box() -> bool { let mut x: [u8; 16] = unsafe { MaybeUninit::uninit().assume_init() }; // black_box appears to work as well. however, the docs at // https://doc.rust-lang.org/nightly/std/hint/fn.black_box.html // seem to indicate that this is unreliable. std::hint::black_box(&mut x); x[11] == x[12] } pub fn not_sure_why_but_it_works() -> bool { // Just claim we initialized this even though we did not. let mut x: [u8; 16] = unsafe { MaybeUninit::uninit().assume_init() }; // It appears calling an unsafe function that does a pointer write of just // the first byte works as well. unsafe { init_mem_helper((&mut x[..]).as_mut_ptr() as *mut i8); } x[11] == x[12] } // Need the inline(never) #[inline(never)] pub unsafe fn init_mem_helper(s: *mut i8) { // Write to the first byte. s.write(0); }
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