On 17/12/2014 10:06 AM, Martin Buchholz wrote:
On Thu, Dec 11, 2014 at 1:08 AM, Andrew Haley <a...@redhat.com> wrote:
On 11/12/14 00:53, David Holmes wrote:
There are many good uses of storestore in the various lock-free
algorithms inside hotspot.
There may be many uses, but I am extremely suspicious of how good
they are. I wonder if we went through all the uses of storestore in
hotspot how many bugs we'd find. As far as I can see (in the absence
of other barriers) the only things you can write before a storestore
are constants.
Hans has provided us with the canonical writeup opposing store-store
and load-load barriers, here:
http://www.hboehm.info/c++mm/no_write_fences.html
Few programmers will be able to deal confidently with causality
defying time paradoxes, especially loads from "the future".
Well I take that with a grain of salt - Hans dismisses ordering based on
dependencies which puts us into the realm of those "causality defying
time paradoxes" in my opinion. Given:
x.a = 0;
x.a++
storestore
x_init = true
Hans allows for the nonsensical, in my view, possibility that the load
of x.a can happen after the x_init=true store and yet somehow be subject
to the ++ and the ensuing store that has to come before the x_init = true.
David
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