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Change Rc::inc_{weak,strong} to better hint optimization to LLVM #53080

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Merged
merged 6 commits into from
Aug 21, 2018
Merged

Change Rc::inc_{weak,strong} to better hint optimization to LLVM #53080

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1553ea2
Changed `Rc::inc_{weak,strong}` to better hint optimization to LLVM
dshynkev Aug 4, 2018
4e17fbd
Fixed typo
dshynkev Aug 5, 2018
85b92c1
Added test
dshynkev Aug 19, 2018
0b83914
Renamed test to match actual issue number
dshynkev Aug 20, 2018
0dd10af
Revert accidental submodule change
dshynkev Aug 20, 2018
79a905e
Added explicit optimization flag to test
dshynkev Aug 20, 2018
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18 changes: 16 additions & 2 deletions src/liballoc/rc.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1359,7 +1359,14 @@ trait RcBoxPtr<T: ?Sized> {

#[inline]
fn inc_strong(&self) {
self.inner().strong.set(self.strong().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
// 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 missied optimization.
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Typo: “missied” (here and below).

if self.strong() == 0 || self.strong() == usize::max_value() {
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Hm, should we instead have unreachable_unchecked for the == 0 case?

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Confusingly, doing this:

-        if self.strong() == 0 || self.strong() == usize::max_value() {
+        if self.strong() == 0 {
+            unsafe { unreachable_unchecked(); }
+        }
+        if self.strong() == usize::max_value() {
             unsafe { abort(); }

produces the same output as before any changes (before in the linked comparison).

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Oh, hm, that might be because LLVM for whatever reason decides that subtraction can overflow or some such. Seems fine to leave it then.

unsafe { abort(); }
}
self.inner().strong.set(self.strong() + 1);
}

#[inline]
Expand All @@ -1374,7 +1381,14 @@ trait RcBoxPtr<T: ?Sized> {

#[inline]
fn inc_weak(&self) {
self.inner().weak.set(self.weak().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
// 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 missied optimization.
if self.weak() == 0 || self.weak() == usize::max_value() {
unsafe { abort(); }
}
self.inner().weak.set(self.weak() + 1);
}

#[inline]
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