zig

commit ff5850183eee854fdfe0d3f7b7242b9ff56c2116 (tree)
parent 991e00c2702b31dffb7b5ada26c7b19a552502a7
Author: Niles Salter <Validark@pm.me>
Date:   Tue, 20 Jun 2023 17:32:07 -0600

[priority_deque] simplify & optimize isMinLayer

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.
Diffstat:
M
lib/std/priority_dequeue.zig
 | 
4
+---

1 file changed, 1 insertion(+), 3 deletions(-)
diff --git a/lib/std/priority_dequeue.zig b/lib/std/priority_dequeue.zig
@@ -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 {