Re: [PATCH] improve performance of std::allocator::deallocate

[Jonathan Wakely <jwakely at redhat dot com>]

On 06/03/19 02:43 +0000, Pádraig Brady wrote:
> On 02/26/2019 04:23 PM, Padraig Brady wrote:
> > > Note jemalloc >= 5.1 is required to fix a bug with 0 sizes.
> > >
> > > How serious is the bug? What are the symptoms?
> > I've updated the commit summary to say it's a crash.
> > Arguably that's better than mem corruption.
> >
> > > It looks like 5.1.0 is less than a year old, so older versions are
> > > presumably still in wide use.
> > >
> > > We could potentially workaround it by making
> > > new_allocator::allocate(0) call ::operator new(1) when
> > > __cpp_sized_deallocation is defined, and deallocate(p, 0) call
> > > ::operator delete(p, 1). Obviously I'd prefer not to do that, because
> > > the default operator new already has an equivalent check, and only
> > > programs linking to jemalloc require the workaround.
> > Right the jemalloc fix was released May 2018.
> > It would be great to avoid the extra workarounds.
> > Given this would be released about a year after the jemalloc fix was
> > released,
> > and that this would only manifest upon program rebuild,
> > I would be inclined to not add any workarounds.
> > For reference tcmalloc and glibc malloc were seen to work fine with this.
>
> Actually the jemalloc issue will only be fixed with the release of 5.2
> (a few weeks away).
> I've updated the commit message in the attached accordingly.

Hmm, I'm a bit nervous about making a change that will cause crashes
unless using an unreleased version (I know it will be released by the
time GCC 9.1 is released, but some people might upgrade GCC without
upgrading jemalloc).

On the other hand, zero sized allocations should be rare in practice.