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Optimize RcInnerPtr::inc_strong instruction count

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Inspired by this internals thread: https://internals.rust-lang.org/t/rc-optimization-on-64-bit-targets/16362

[The generated assembly is a bit smaller](https://rust.godbolt.org/z/TeTnf6144) and is a more efficient usage of the CPU's instruction cache. `unlikely` doesn't impact any of the small artificial tests I've done, but I've included it in case it might help more complex scenarios when this is inlined.
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Michael Bradshaw 2022-03-22 20:21:56 -06:00 committed by GitHub
parent a4a5e79814
commit 3f7f5e8a2e
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1 changed files with 24 additions and 10 deletions
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34
library/alloc/src/rc.rs
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@ -2512,14 +2512,21 @@ fn strong(&self) -> usize {
fn inc_strong(&self) {
let strong = self.strong();
// We insert an `assume` here to hint LLVM at an otherwise
// missed optimization.
// SAFETY: The reference count will never be zero when this is
// called.
unsafe { core::intrinsics::assume(strong != 0); }
let strong = strong.wrapping_add(1);
self.strong_ref().set(strong);
// We want to abort on overflow instead of dropping the value.
// The reference count will never be zero when this is called;
// nevertheless, we insert an abort here to hint LLVM at
// an otherwise missed optimization.
if strong == 0 || strong == usize::MAX {
// Checking after the store instead of before allows for
// slightly better code generation.
if core::intrinsics::unlikely(strong == 0) {
abort();
}
self.strong_ref().set(strong + 1);
}
#[inline]
@ -2536,14 +2543,21 @@ fn weak(&self) -> usize {
fn inc_weak(&self) {
let weak = self.weak();
// We insert an `assume` here to hint LLVM at an otherwise
// missed optimization.
// SAFETY: The reference count will never be zero when this is
// called.
unsafe { core::intrinsics::assume(weak != 0); }
let weak = weak.wrapping_add(1);
self.weak_ref().set(weak);
// We want to abort on overflow instead of dropping the value.
// The reference count will never be zero when this is called;
// nevertheless, we insert an abort here to hint LLVM at
// an otherwise missed optimization.
if weak == 0 || weak == usize::MAX {
// Checking after the store instead of before allows for
// slightly better code generation.
if core::intrinsics::unlikely(weak == 0) {
abort();
}
self.weak_ref().set(weak + 1);
}
#[inline]