Re: [PATCH v3 1/4] ree: Default ree pass for O2 and above for rs6000 target.

[Peter Bergner <bergner@linux.ibm.com>]

On 4/19/23 3:00 PM, Segher Boessenkool wrote:
> On Wed, Apr 19, 2023 at 11:23:07PM +0530, Ajit Agarwal wrote:
>> 	* common/config/rs6000/rs6000-common.cc: Add REE pass as a
>> 	default rs6000 target pass for O2 and above.
> 
> Why only for -O2?  Only when optimising at all makes sense, people use
> -O0 only when they want to skip as many optimisations as possible, maybe
> because of compilation time concerns, maybe to avoid an ICE or other
> bug.  Isn't REE *always* a good thing, it never degrades code quality?
> Or are there situations where it results in worse code?

I think this is a case of following what the other architectures are doing.
Namely, x86, aarch64, riscv, sparc, alpha and h8300 all enable -free at
-O2 and above, not -O1.  Not to say that is the best answer, but I think
that is why we did the same.  I agree I don't think -free can produce
worse code which makes using it with -O1 and above an option.  Maybe someone
was worried about compile time???  Doesn't seem like an optimization like
this would be too expensive though.

Ajit, one thing that is missing from this specific patch is a change to
gcc/doc/invoke.texi mentioning Power to the list of architectures that
are enabling -free with the -O* options, which currently only mentions
Alpha, AArch64 and x86.  Being good community participants, it'd be
good to add the missing riscv, sparc and h8300 when adding Power.

Peter