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assume the runtime range of align_offset

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Found when I saw code with `align_to` having extraneous checks.
This commit is contained in:
Scott McMurray
2023-05-02 17:23:54 -07:00
parent 74c4821045
commit a1e5c65aa4
3 changed files with 89 additions and 4 deletions
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14
library/core/src/ptr/mod.rs
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@@ -1632,8 +1632,8 @@ pub(crate) const unsafe fn align_offset<T: Sized>(p: *const T, a: usize) -> usiz
// FIXME(#75598): Direct use of these intrinsics improves codegen significantly at opt-level <=
// 1, where the method versions of these operations are not inlined.
use intrinsics::{
cttz_nonzero, exact_div, mul_with_overflow, unchecked_rem, unchecked_shl, unchecked_shr,
unchecked_sub, wrapping_add, wrapping_mul, wrapping_sub,
assume, cttz_nonzero, exact_div, mul_with_overflow, unchecked_rem, unchecked_shl,
unchecked_shr, unchecked_sub, wrapping_add, wrapping_mul, wrapping_sub,
};
/// Calculate multiplicative modular inverse of `x` modulo `m`.
@@ -1724,12 +1724,18 @@ pub(crate) const unsafe fn align_offset<T: Sized>(p: *const T, a: usize) -> usiz
// in a branch-free way and then bitwise-OR it with whatever result the `-p mod a`
// computation produces.
let aligned_address = wrapping_add(addr, a_minus_one) & wrapping_sub(0, a);
let byte_offset = wrapping_sub(aligned_address, addr);
// FIXME: Remove the assume after <https://github.com/llvm/llvm-project/issues/62502>
// SAFETY: Masking by `-a` can only affect the low bits, and thus cannot have reduced
// the value by more than `a-1`, so even though the intermediate values might have
// wrapped, the byte_offset is always in `[0, a)`.
unsafe { assume(byte_offset < a) };
// SAFETY: `stride == 0` case has been handled by the special case above.
let addr_mod_stride = unsafe { unchecked_rem(addr, stride) };
return if addr_mod_stride == 0 {
let aligned_address = wrapping_add(addr, a_minus_one) & wrapping_sub(0, a);
let byte_offset = wrapping_sub(aligned_address, addr);
// SAFETY: `stride` is non-zero. This is guaranteed to divide exactly as well, because
// addr has been verified to be aligned to the original type’s alignment requirements.
unsafe { exact_div(byte_offset, stride) }