improve undefined behavior case in bzhi emulation #2
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Doing `~(popcnt >> 6)` gives us -2 when popcnt is 64, otherwise it gives us -1. ANDing this value with `1 << popcnt` is fine when `1 << 64` gives us `1`, which it may on a lot of machines, but C does not guarantee that to be the case. Instead, we could do `(popcnt >> 6) - 1` which gives us 0 when the popcnt is 64, and `(0 & undefined)` is 0. When the popcnt is not 64, we get `-1` from the revised expression, and ANDing with that works properly too.
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Doing
~(popcnt >> 6)gives us -2 when popcnt is 64, otherwise it gives us -1. ANDing this value with1 << popcntis fine when1 << 64gives us1, which it may on a lot of machines, but C does not guarantee that to be the case. Instead, we could do(popcnt >> 6) - 1which gives us 0 when the popcnt is 64, and(0 & undefined)is (hopefully?) 0. When the popcnt is not 64, we get-1from the revised expression, and ANDing with that works properly too.Please test before merging!