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Auto merge of #95224 - mjbshaw:patch-1, r=yaahc

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Optimize RcInnerPtr::inc_strong()/inc_weak() instruction count

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.
This commit is contained in:
bors 2022-04-15 23:15:51 +00:00
commit bb1a03c4fc
1 changed files with 28 additions and 10 deletions
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38
library/alloc/src/rc.rs
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@ -2511,14 +2511,23 @@ trait RcInnerPtr {
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 for overflow 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]
@ -2535,14 +2544,23 @@ trait RcInnerPtr {
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 for overflow 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]