[PING][PATCH v2] combine: Don't turn (mult (extend x) 2^n) into extract [PR96998]

[Richard Sandiford]

Segher Boessenkool <segher@kernel.crashing.org> writes:
> On Fri, Oct 09, 2020 at 09:38:09AM +0100, Alex Coplan wrote:
>> Hi Segher,
>> 
>> On 08/10/2020 15:20, Segher Boessenkool wrote:
>> > On Thu, Oct 08, 2020 at 11:21:26AM +0100, Alex Coplan wrote:
>> > > Ping. The kernel is still broken on AArch64.
>> > 
>> > You *cannot* fix a correctness bug with a combine addition.
>> 
>> https://gcc.gnu.org/pipermail/gcc-patches/2020-September/555158.html
>> explains why we do precisely that.
>
> And it still is wrong.
>
>> Also, as your own testing confirmed, the patch does indeed fix the issue.
>
> No, it did not.  It showed that before the patch the bug was hit, and
> after it it was not.  It does not show the bug was solved.

I agree there's a target bug here.  Please see the explanation I posted
in: https://gcc.gnu.org/pipermail/gcc-patches/2020-September/554518.html
(especially the first sentence quoted below :-)).

    The situation as things stand is that aarch64 has a bug: it accepts
    an odd sign_extract representation of addresses, but doesn't accept
    that same odd form of address as an LEA.  We have two options:

    (a) add back instructions that recognise the odd form of LEA, or
    (b) remove the code that accepts the odd addresses

    I think (b) is the way to go here.  But doing that on its own
    would regress code quality.  The reason we recognised the odd
    addresses in the first place was because that was the rtl that
    combine happened to generate for an important case.

So if we go for (b) but fix the aarch64 bug strictly before the
combine patch, we would need to:

(1) Apply the target fix and adjust the testsuite markup to make sure
    that the git commit doesn't regress anyone's test results.

(2) Apply the combine patch and revert the testsuite markup changes
    from (1).

That seems like make-work, and would still show as a blip for
people doing performance tracking.

If you prefer, we could fix the aarch64 bug and patch combine as a
single commit.  See:

https://gcc.gnu.org/pipermail/gcc-patches/2020-September/554257.html

for the full patch, including the aarch64 bugfix.

Thanks,
Richard