decrement_and_branch_until_zero pattern

[Paul Koning]

> On Jun 8, 2018, at 2:29 PM, Jim Wilson <jimw@sifive.com> wrote:
> 
> On 06/08/2018 06:21 AM, Paul Koning wrote:
>> Interesting.  The ChangeLog doesn't give any background.  I suppose I should plan to approximate the effect of this pattern with a define-peephole2 ?
> 
> The old RTL loop optimizer was replaced with a new RTL loop optimizer. When the old one was written, m68k was a major target, and the dbra optimization was written for it.  When the new one was written, m68k was not a major target, and this support was written differently.  We now have doloop_begin and doloop_end patterns that do almost the same thing, and can be created by the loop-doloop.c code.
> 
> There is a section in the internals docs that talks about this.
> https://gcc.gnu.org/onlinedocs/gccint/Looping-Patterns.html
> 
> The fact that we still have decrement_and_branch_until_zero references in docs and target md files looks like a bug.  The target md files should use doloop patterns instead, and the doc references should be dropped.

Thanks.  I saw those sections and interpreted them as support for signal processor style fast hardware loops.  If they can be adapted for dbra type looping, great.  I'll give that a try.

Meanwhile, yes, it looks like there is a documentation bug.  I can clean that up.  It's more than a few lines, but does that qualify for an "obvious" change?

	paul