[cfe-commits] [libcxx] r159836 - in /libcxx/trunk: include/__hash_table test/containers/unord/unord.map/reserve.pass.cpp test/containers/unord/unord.multimap/reserve.pass.cpp test/containers/unord/unord.multiset/reserve.pass.cpp test/containers/unord/unord.set/reserve.pass.cpp

Joerg Sonnenberger joerg at britannica.bec.de
Fri Jul 6 18:58:27 PDT 2012

On Fri, Jul 06, 2012 at 05:31:14PM -0000, Howard Hinnant wrote:
> Author: hhinnant
> Date: Fri Jul  6 12:31:14 2012
> New Revision: 159836
> URL: http://llvm.org/viewvc/llvm-project?rev=159836&view=rev
> Log:
> This commit establishes a new bucket_count policy in the unordered
> containers:  The policy now allows a power-of-2 number of buckets to
> be requested (and that request honored) by the client.
> And if the number of buckets is set to a power of 2, then the constraint
> of the hash to the number of buckets uses & instead of %.

Does the standard place any constraints in this area? So with this
change, power-of-two sized hash tables still have a conditional in the
hot path. The only justification for using non-power-of-two hashes seems
to be extremely poor user-provided hash functions, which sounds like a
lame excuse. The complexity of computing primes basically just goes back
to the argument that linear series with increments smaller than the
prime itself are evenly distributed. The same or similar properties can
be obtained using double-width-multiplication-and-mask universal hash
functions (hash(x) = hi(dw(x) * T), where dw casts x to a type twice the
size and hi gives the upper half). They are no more expensive to compute
than the double multiplication version of the modulo and significant
cheaper than using modulo directly on most (all?) platforms. So: why not
kill the complexity and require power-of-two bucket counts and clients
to use a sane hash function?


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