Proposed doc update for Explicit Reg Vars 3/3

[Jeff Law]

On 10/20/2015 01:40 PM, Segher Boessenkool wrote:
> On Tue, Oct 20, 2015 at 12:01:26PM -0600, Jeff Law wrote:
>> On 10/20/2015 11:11 AM, Segher Boessenkool wrote:
>>> 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 ;-)
>> We must be evaluating different things.  I review the state of that bug
>> each release. While the amount of stack slot shuffling has improved over
>> time it still looked better with the register variables during the gcc-5
>> release cycle.
>
> The "#if 0" variant (no reg vars) results in 3597 bytes text, "esp"
> mentioned 29 times.
> The "#if 1" version is 3886 bytes, "esp" mentioned 87 times.
>
> This is a GCC 6 compiler, a week or so old.
Cool.  I'll do a final review during stage3/stage4 and assuming we don't 
go backwards between now and then we'll be able to cross that one off 
the regression list for gcc6!

Jeff