[Bug tree-optimization/101014] [12 Regression] Big compile time hog with -O3 since r12-1268-g9858cd1a6827ee7a

[amacleod at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101014

--- Comment #17 from Andrew Macleod <amacleod at redhat dot com> ---
Created attachment 51050
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51050&action=edit
patch to fix the issue

The gift that keeps on giving eh.
OK, this should solve the infinite loop. Give it a try, I'm running it through
testing now.

When I introduced the sparse on-entry cache, it is limited to 15 unique ranges
for any given ssa-name, then  it reverts to varying for any additional values
to be safe.

The cache propagation engine works by combining incoming ranges and if that is
different than that current on-entry range, stores that  and proceeds to push
this new value on outgoing edges.

What was happening here is this new value that was calculated was beyond the 15
allowed. When it was stored, it was stored as VARYING.  This block was in a
cycle feeding back to itself, so when it calculated the on-enty value again and
compared, it though it needed to update again.  Which of course failed again...
and the endless loop of trying to propagate was born.

This patch checks that the value being stored to the cache is the same as the
value when it is immediately reloaded. If that fails, we stop trying to
propagate that value.

Please check that it solves both this problam, and likely the 101148 problem