On 6/9/2021 2:39 PM, Aldy Hernandez wrote:
On 6/9/21 9:47 PM, Jeff Law wrote:
On 6/9/2021 9:34 AM, Aldy Hernandez wrote:
On 6/9/21 2:09 PM, Richard Biener wrote:
On Wed, Jun 9, 2021 at 1:50 PM Aldy Hernandez via Gcc
<gcc@gcc.gnu.org> wrote:
Hi Jeff. Hi folks.
What started as a foray into severing the old (forward) threader's
dependency on evrp, turned into a rewrite of the backwards threader
code. I'd like to discuss the possibility of replacing the current
backwards threader with a new one that gets far more threads and can
potentially subsume all threaders in the future.
I won't include code here, as it will just detract from the high
level
discussion. But if it helps, I could post what I have, which just
needs
some cleanups and porting to the latest trunk changes Andrew has
made.
Currently the backwards threader works by traversing DEF chains
through
PHIs leading to possible paths that start in a constant. When such a
path is found, it is checked to see if it is profitable, and if
so, the
constant path is threaded. The current implementation is rather
limited
since backwards paths must end in a constant. For example, the
backwards threader can't get any of the tests in
gcc.dg/tree-ssa/ssa-thread-14.c:
if (a && b)
foo ();
if (!b && c)
bar ();
etc.
After my refactoring patches to the threading code, it is now
possible
to drop in an alternate implementation that shares the profitability
code (is this path profitable?), the jump registry, and the actual
jump
threading code. I have leveraged this to write a ranger-based
threader
that gets every single thread the current code gets, plus 90-130%
more.
Here are the details from the branch, which should be very similar to
trunk. I'm presenting the branch numbers because they contain
Andrew's
upcoming relational query which significantly juices up the results.
New threader:
ethread:65043 (+3.06%)
dom:32450 (-13.3%)
backwards threader:72482 (+89.6%)
vrp:40532 (-30.7%)
Total threaded: 210507 (+6.70%)
This means that the new code gets 89.6% more jump threading
opportunities than the code I want to replace. In doing so, it
reduces
the amount of DOM threading opportunities by 13.3% and by 30.7%
from the
VRP jump threader. The total improvement across the jump threading
opportunities in the compiler is 6.70%.
However, these are pessimistic numbers...
I have noticed that some of the threading opportunities that DOM
and VRP
now get are not because they're smarter, but because they're
picking up
opportunities that the new code exposes. I experimented with
running an
iterative threader, and then seeing what VRP and DOM could
actually get.
This is too expensive to do in real life, but it at least shows
what
the effect of the new code is on DOM/VRP's abilities:
Iterative threader:
ethread:65043 (+3.06%)
dom:31170 (-16.7%)
thread:86717 (+127%)
vrp:33851 (-42.2%)
Total threaded: 216781 (+9.90%)
This means that the new code not only gets 127% more cases, but it
reduces the DOM and VRP opportunities considerably (16.7% and 42.2%
respectively). The end result is that we have the possibility of
getting almost 10% more jump threading opportunities in the entire
compilation run.
Yeah, DOM once was iterating ...
You probably have noticed that we have very man (way too many)
'thread' passes, often in close succession with each other or
DOM or VRP. So in the above numbers I wonder if you can break
down the numbers individually for the actual passes (in their order)?
Sure, I can do that. Let me whip up the old branch and gather some
info.
I'll save you some time. The jump threaders in VRP are doing the
least amount of lifting and the ones we want to kill first. IIRC the
one from vrp2 is doing nearly nothing at this point.
Sure, that was going to be my next target.
What are your thoughts on replacing the current backwards threader,
though? That's basically ready to go.
Going to take a deep dive into it Saturday.
Jeff
