[Bug target/63783] [4.9/5 Regression] [SH] Miscompilation of boolean negation on SH4 using -O2

["olegendo at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63783

--- Comment #13 from Oleg Endo <olegendo at gcc dot gnu.org> ---
(In reply to Michael Karcher from comment #12)
> Further digging into this showed that there actually is a pass that would
> merge the two "tst r1,r1" instructions - the jump2 pass in cfgclenup.c.
> 
> The optimization is called "crossjumping" in gcc, also known as tail
> merging. For some reason[1], gcc is reluctant to perform crossjumping on
> small common parts. In this case, the common part is just one instruction,
> and the parameter min-crossjump-insns is five by default unless optimizing
> for size. With "-fparam-min-crossjump-insns=1" or "-Os", my first patch
> removing all the special-casing of zero compares produces the same result as
> the patch fixing the logical negations. In the current situation, I see no
> advantage of not cross-jumping in this case, so the minimally invasive
> solution is definitely the second patch to fix logical negation, but still
> it somehow feels ugly to me to have a limited reimplementation of
> crossjumping in the sh-treg-combine pass. Replacing stuff of that pass by
> improving other optimizations is IMHO beyond the scope of this bug, though.

Thanks for digging into that.

One possible way to evaluate the crossjumping optimization in ths case:
- disable/don't register sh_treg_combine pass in sh.c
- set default param -fparam-min-crossjump-insns=1 somehow
- run sh.exp tests
- compare CSiBE code sizes (or similar larger code bases)

If you're interested in doing that, please post your findings in PR 51244.