[Bug libstdc++/55815] switch hash function of libstdc++ hash tables to siphash

["miyuki at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55815

Mikhail Maltsev <miyuki at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |miyuki at gcc dot gnu.org

--- Comment #6 from Mikhail Maltsev <miyuki at gcc dot gnu.org> ---
SipHash is not very slow, BTW. I remember doing some benchmarks (there was some
discussion about changing the hash function used for incremental hash in GCC),
https://gcc.gnu.org/ml/gcc-patches/2015-03/msg00179.html

Result for small strings (1-20 bytes) on x86_64, elapsed time (i.e. less is
better):

function  | gcc -O3 | clang -O3
----------|---------|----------
murmur32  |   0.91  |   0.95 
murmur128 |   1.33  |   1.24 
sip24     |   1.88  |   1.45 
sip13     |   1.66  |   1.22 
lookup3   |   1.18  |   0.90 
spooky    |   1.24  |   0.90 

I don't remember all the details about the workload used and which
implementation of SipHash I used (but most likely my own one).

(In reply to felix-gcc from comment #0)
> It may even make sense to replace the hash code gcc itself uses, as there
> are now web pages where you can paste code and see which code gcc generates
> for it, turning this problem into a security issue if someone pastes code
> with colliding symbols to exploit this problem.

There is a much more efficient way to attack them: search GCC bugzilla for open
bugs of type "compile time hog" or "memory hog" and simply use the code
provided there.