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recover null-ptr optimization by adding a special case to the niching logic

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This commit is contained in:
Moulins 2023-06-26 15:40:13 +02:00
parent 76c49aead6
commit 4fb039ed6c
2 changed files with 13 additions and 8 deletions
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20
compiler/rustc_abi/src/lib.rs
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@ -1503,16 +1503,21 @@ impl Niche {
pub fn reserve<C: HasDataLayout>(&self, cx: &C, count: u128) -> Option<(u128, Scalar)> {
assert!(count > 0);
if count > self.available(cx) {
return None;
}
let Self { value, valid_range: v, .. } = *self;
let size = value.size(cx);
assert!(size.bits() <= 128);
let max_value = size.unsigned_int_max();
let max_value = value.size(cx).unsigned_int_max();
let distance_end_zero = max_value - v.end;
let niche = v.end.wrapping_add(1)..v.start;
let available = niche.end.wrapping_sub(niche.start) & max_value;
if count > available {
return None;
// Null-pointer optimization. This is guaranteed by Rust (at least for `Option<_>`),
// and offers better codegen opportunities.
if count == 1 && matches!(value, Pointer(_)) && !v.contains(0) {
// Select which bound to move to minimize the number of lost niches.
let valid_range =
if v.start - 1 > distance_end_zero { v.with_end(0) } else { v.with_start(0) };
return Some((0, Scalar::Initialized { value, valid_range }));
}
// Extend the range of valid values being reserved by moving either `v.start` or `v.end` bound.
@ -1535,7 +1540,6 @@ impl Niche {
let end = v.end.wrapping_add(count) & max_value;
Some((start, Scalar::Initialized { value, valid_range: v.with_end(end) }))
};
let distance_end_zero = max_value - v.end;
if v.start > v.end {
// zero is unavailable because wrapping occurs
move_end(v)

1
compiler/rustc_middle/src/ty/layout.rs
View file

@ -1087,6 +1087,7 @@ where
// this will continue to work as long as we don't start
// using more niches than just null (e.g., the first page of
// the address space, or unaligned pointers).
// FIXME(reference_niches): well, the day has come...
Variants::Multiple {
tag_encoding: TagEncoding::Niche { untagged_variant, .. },
tag_field,