Proposed doc update for Explicit Reg Vars 3/3

[Segher Boessenkool]

On Tue, Oct 20, 2015 at 10:22:53AM -0600, Jeff Law wrote:
> bz21182 has a testcase that's still helped by local register variables.

I tried it out, and it now is much worse *with* the reg vars than
without (and -O2 vs. -O3 makes no difference at all).  It doesn't look
to have used the pre-determined allocation at all.  The difference at
expand time is huge already; maybe some things weren't optimised at
tree level with the register vars?  It's a bit big to really look into ;-)


Segher