[Bug rtl-optimization/68173] gcc takes a long time and a lot of memory with -O0 on source file with very large expression

[vmakarov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68173

--- Comment #13 from Vladimir Makarov <vmakarov at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #12)
> callgrind points at bitmap_set_bit called via process_bb_lives ->
> mark_regno_dead.
> Maybe some code in that (the DCE code?) can be keyed on if (optimize).
> 
> in mark_regno_dead callgrind points to
> 
>       bitmap_set_bit (bb_killed_pseudos, regno);
> 
> being the expensive one.

I've tried to implement bb_killed_pseudos and bb_gen_pseudos as sparsesets but
it gave nothing in term of compiler speed.  I think the major problem is in
processing too many pseudos in -O0 mode which are about 1000 times more than in
-O2 mode.