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// 1.57 stable, 2022-01-16 nightly // LLVM should be able to deduce that it is UB if len exceeds 4 // But no, we get a memcpy call pub unsafe fn small_copy(src: [u8; 4], dst: &mut[u8], len: usize) { for i in 0..len { *dst.get_unchecked_mut(i) = *src.get_unchecked(i); } } // Hint harder that len is really not greater than 4 // Still get a memcpy call pub unsafe fn small_copy_assume(src: [u8; 4], dst: &mut[u8], len: usize) { if len > 4 { core::hint::unreachable_unchecked(); } for i in 0..len { *dst.get_unchecked_mut(i) = *src.get_unchecked(i); } } // The bounds check in slice indexing gives LLVM the information it needs, // but unreachable_unchecked does not. memcpy becomes an unrolled loop. // Except we have a branch to a panic just in case len is out of bounds pub unsafe fn small_copy_checked(src: [u8; 4], dst: &mut[u8], len: usize) { for i in 0..len { *dst.get_unchecked_mut(i) = src[i]; } } // This is the codegen I want from small_copy. No call to memcpy, no panic. pub unsafe fn small_copy_checked_assume(src: [u8; 4], dst: &mut[u8], len: usize) { if len > 4 { core::hint::unreachable_unchecked(); } for i in 0..len { *dst.get_unchecked_mut(i) = src[i]; } } // I know that I'm kind of asking for a memcpy here, but it just feels // wrong that an open-coded copy loop is optimized better pub unsafe fn small_copy_checked_assume_ptr(src: [u8; 4], dst: &mut[u8], len: usize) { core::ptr::copy_nonoverlapping(src.as_ptr(), dst.as_mut_ptr(), len); }
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