Fix drop handling in hint::select_unpredictable · rust-lang/rust@e5e5fb9 (original) (raw)
`@@ -4,6 +4,7 @@
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`//!
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`//! Hints may be compile time or runtime.
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use crate::mem::MaybeUninit;
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`use crate::{intrinsics, ub_checks};
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`/// Informs the compiler that the site which is calling this function is not
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`@@ -735,9 +736,9 @@ pub const fn cold_path() {
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`crate::intrinsics::cold_path()
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`}
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/// Returns either true_val or false_val depending on the value of b,
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/// with a hint to the compiler that b is unlikely to be correctly
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/// predicted by a CPU’s branch predictor.
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/// Returns either true_val or false_val depending on the value of
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/// condition, with a hint to the compiler that condition is unlikely to be
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/// correctly predicted by a CPU’s branch predictor.
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`///
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`/// This method is functionally equivalent to
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```` /// ```ignore (this is just for illustrative purposes)
`@@ -753,10 +754,10 @@ pub const fn cold_path() {
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`/// search.
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`///
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`/// Note however that this lowering is not guaranteed (on any platform) and
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/// should not be relied upon when trying to write constant-time code. Also
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/// be aware that this lowering might *decrease* performance if `condition`
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/// is well-predictable. It is advisable to perform benchmarks to tell if
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/// this function is useful.
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/// should not be relied upon when trying to write cryptographic constant-time
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/// code. Also be aware that this lowering might *decrease* performance if
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/// `condition` is well-predictable. It is advisable to perform benchmarks to
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/// tell if this function is useful.
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`///
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`/// # Examples
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`///
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`@@ -780,6 +781,17 @@ pub const fn cold_path() {
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```` /// ```
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`#[inline(always)]
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`#[unstable(feature = "select_unpredictable", issue = "133962")]
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`-
pub fn select_unpredictable(b: bool, true_val: T, false_val: T) -> T {
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crate::intrinsics::select_unpredictable(b, true_val, false_val)
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pub fn select_unpredictable(condition: bool, true_val: T, false_val: T) -> T {
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// FIXME(https://github.com/rust-lang/unsafe-code-guidelines/issues/245):
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// Change this to use ManuallyDrop instead.
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let mut true_val = MaybeUninit::new(true_val);
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let mut false_val = MaybeUninit::new(false_val);
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// SAFETY: The value that is not selected is dropped, and the selected one
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// is returned. This is necessary because the intrinsic doesn't drop the
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// value that is not selected.
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unsafe {
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crate::intrinsics::select_unpredictable(!condition, &mut true_val, &mut false_val)
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.assume_init_drop();
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crate::intrinsics::select_unpredictable(condition, true_val, false_val).assume_init()
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}
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`}
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