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[priority_deque] simplify & optimize isMinLayer

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LLVM has trouble compiling the old implementation, (presumably) because `leading_zeros` is thought to be a `u7` rather than a `u6`, which means `63 - clz` is not equivalent to `63 ^ clz`, which means it can't deduce that the final condition can simply be flipped. (I am assuming `usize` is a `u64` here for ease of understanding, but it's the same for any power of 2)

https://zig.godbolt.org/z/Pbj4P7ob3

The new version is slightly better too because `isMinLayer(maxInt(usize))` is now well-defined behavior.
This commit is contained in:
Niles Salter 2023-06-20 17:32:07 -06:00 committed by Andrew Kelley
parent 991e00c270
commit ff5850183e
1 changed files with 1 additions and 3 deletions
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lib/std/priority_dequeue.zig
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@ -69,9 +69,7 @@ pub fn PriorityDequeue(comptime T: type, comptime Context: type, comptime compar
// The first element is on a min layer;
// next two are on a max layer;
// next four are on a min layer, and so on.
const leading_zeros = @clz(index + 1);
const highest_set_bit = @bitSizeOf(usize) - 1 - leading_zeros;
return (highest_set_bit & 1) == 0;
return 1 == @clz(index +% 1) & 1;
}
fn nextIsMinLayer(self: Self) bool {