On 11/19/21 04:21, Richard Biener wrote:
On Thu, Nov 18, 2021 at 8:30 PM Andrew MacLeod via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
At issue here is the dynamic approach we currently use for outgoing edge
calculations. It isn't normally a problem, but once you get a very
large number of possible outgoing values (ie very long unrolled blocks)
with pairs of values on a statement, and individual queries for each
one, it becomes exponential if they relate to each other.
So. We previously introduced a param for PR 100081 which limited the
depth of logical expressions GORI would pursue because we always make 2
or 4 queries for each logical expression, which accelerated the
exponential behaviour much quicker.
This patch reuses that to apply to any statement which has 2 ssa-names,
as the potential for 2 queries can happen then as well.. leading to
exponential behaviour. Each statement we step back through with
multiple ssa-names decreases the odds of calculating a useful outgoing
range anyway. This will remove ridiculous behaviour like this PR
demonstrates.
Next release I plan to revamp GORI to cache outgoing values, which will
eliminate all this sort of behaviour, and we can remove all such
restrictions.
Bootstrapped on x86_64-pc-linux-gnu with no regressions. OK for trunk?
I think it's reasonable. SCEV tries to limit the overall number of SSA -> def
Pushed.
