Faster implementation of long and int rotation#10187
Faster implementation of long and int rotation#10187niloc132 merged 2 commits intogwtproject:mainfrom
Conversation
Co-authored-by: Thomas May <cogman10@hotmail.com>
zbynek
added
Category-JRE
ready
| i = i << 1 | ((i < 0) ? 1 : 0); | ||
| } | ||
| return i; | ||
| int rotateValue = distance % SIZE; |
There was a problem hiding this comment.
I feel like you should try shifting by any value outside the range this guarantees (which isn't 0 to 31, by the way, it's -31 to 31) in the browser's JS Console. You shouldn't just take my word for it, but the modulus is totally unnecessary here; shifts are implicitly done with a mask of 31 in JS, and in Java for ints.
I would not be at all surprised if performing modulus on JS Number types is slower than the loop before, and it's completely unnecessary. A mask would arguably be more clear, and wouldn't permit negative rotateValues if that's a concern. A mask isn't needed either.
| i = i << 1 | ((i < 0) ? 1 : 0); | ||
| } | ||
| return i; | ||
| int rotateValue = distance % SIZE; |
There was a problem hiding this comment.
Just use distance & 63 here, if it's needed at all. GWT may already abide by the JLS and implicitly mask shifts on longs by 63, but I can't confirm it. This is internally a floating-point modulus because GWT int is JS Number (IEEE double-precision float that JS sometimes treats like a 32-bit integer), and floating-point modulus is the slowest basic arithmetic operation according to https://www.agner.org/optimize/ ; on Intel architectures, x87 FPREM takes reliably twice as long as FDIV, and there isn't a vector floating-point remainder operation that I can see. There doesn't appear to be an integer remainder operation there either, so it's probably produced by DIV or IDIV, which isn't much faster, unless a mask can be substituted in (which it probably can't, because if distance is negative, the result is different for distance % 64 and distance & 63, even if the shifts for negative rotateValue behave the same).
tommyettinger
left a comment
tommyettinger
left a comment
There was a problem hiding this comment.
Looks good to me! All the expensive operations are gone and we have correct bitwise rotations, AKA barrel shifts. They were called barrel shifts long before Java existed. I wonder a little about what JS gets emitted here and what common JS engines compile it down to, but I'm sure the handful of loop-less, branch-less operations here will be much better.
3 participants
Fixes #9563
I picked up the implementation by @cogman from gerrit and added a few testcases.