Minor correction: 31-bit tagged _signed_ integers are used on ia32, on x64 you get 32-bit tagged _signed_ integers. Neither though are wide enough to contain all values from unsigned 32-bit integer range.
Thus if you are really using them as 32bit _unsigned_ integers, e.g. you are doing something like: var a = (b ^ c) >>> 0; // force into uint32 and then use in non-int32-truncating manner. then unfortunately even V8's optimizing compiler gets confused. It does not have designated uint32 representation and does not try to infer when int32 can be safely used instead of uint32 (another example of this bug: http://code.google.com/p/v8/issues/detail?id=2097). I suggest you post your code here if possible so that we could take a look. -- Vyacheslav Egorov On Wed, May 30, 2012 at 4:40 PM, Jakob Kummerow <jkumme...@chromium.org> wrote: > As long as you're running unoptimized code on a 32-bit V8, this is expected: > 31-bit integers are stored directly as "small integers", the 32nd bit is > used to tag them as such, whereas 32-bit integers are converted to doubles > and stored as objects on the heap, which makes accessing them more > expensive. > > When your code runs long enough for the optimizer to kick in, it should > recognize this situation, use untagged 32-bit integer values in optimized > code, and the difference between 31-bit and 32-bit values should go away. If > it doesn't, please post a reduced test case that exhibits the behavior so > that we can investigate. (Running the code for a second or so should be > enough to get the full effect of optimization and make the initial > difference negligible.) > > > On Wed, May 30, 2012 at 4:31 PM, Joran Greef <jo...@ronomon.com> wrote: >> >> I am implementing a table hash >> (http://en.wikipedia.org/wiki/Tabulation_hashing) and noticed that a table >> hash using a table of 31-bit unsigned integers is almost an order of >> magnitude faster than a table hash using a table of 32-bit unsigned >> integers. >> >> The former has an average hash time of 0.00007ms per 20 byte key for a >> 31-bit hash, and the latter has an average hash time of 0.00034ms per 20 >> byte key for a 32-bit hash. >> >> I figured that XOR on 8-bit integers would be faster than XOR on 16-bit >> integers would be faster than XOR on 24-bit integers would be faster than >> XOR on 32-bit integers, but did not anticipate such a difference between >> 31-bit and 32-bit integers. >> >> Is there something regarding XOR that I may be missing that could explain >> the difference? >> >> > -- > v8-users mailing list > v8-users@googlegroups.com > http://groups.google.com/group/v8-users -- v8-users mailing list v8-users@googlegroups.com http://groups.google.com/group/v8-users
