On 10/14/2015 09:43 AM, Jan Hubicka wrote:
I think he asked for trivial forward threads though due to repeated
tests.
I hacked FRE to do this (I think), but maybe some trivial cleanup
opportunities
are still left here. Honza?
Well, unthreaded jumps quite confuse profile prediction and create profiles
that we can't fix later. An of course they count in time (and size sometimes)
estimates.
From cases I commonly see it is the usual lazyness of repeated tests comming
from early inlining/macro expansion and also C++ love to introduce
if (ptr != NULL)
ptr2 = &ptr->foo;
else
ptr2 = NULL
for instances of multiple inheritance. usually ptr is known to be non-NULL.
And also cases where if is uses to check individual cases without having proper
esles.
Yea. I still see a variety of trivial jump threads lying around early
in the pipeline.
The nice thing about the backwards walking stuff in this context is we
can control how hard it looks for jump threads much better.
The difficult thing is it's not currently prepared to find the implicit
sets from conditionals. Re-using the ASSERT_EXPR mechanisms from vrp
may be the solution. I haven't tried that yet, but it's in the back of
my mind for solving that class of problems cleanly.
jeff
