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.
