[PATCH 1/2] Single thread optimization for malloc atomics

[Steven Munroe]

On Wed, 2014-04-30 at 12:06 -0400, Rich Felker wrote:
> On Wed, Apr 30, 2014 at 04:18:45PM +0200, Ondřej Bílka wrote:
> > On Wed, Apr 30, 2014 at 10:57:07AM -0300, Adhemerval Zanella wrote:
> > > This patch adds a single-thread optimization for malloc atomic usage to
> > > first check if process is single-thread (ST) and if so use normal
> > > load/store instead of atomic instructions.
> > > 
> > How fast is tls on power? When we add a per-thread cache as I suggested
> > then it would have most of time same performance as singlethread, with
> > overhead one tls variable access per malloc call.
> 
> Extremely fast: the TLS address is simply kept in a general-purpose
> register.
> 
Depends on the TLS access model.

General Dynamic TLS Model requires a dynamic up-call to _tld_get_addr().
So slow.

If you can get to the Local Exec or Initial Exec form (where the dvt
slot or TLS offset can be known at static link time) it can be a simple
inline computation.

As we are talking about a dynamic library (libc.so) here, you have to
set this up carefully.