[Bug rtl-optimization/112415] [14 regression] Python 3.11 miscompiled on HPPA with new RTL fold mem offset pass, since r14-4664-g04c9cf5c786b94

["manolis.tsamis at vrull dot eu" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112415

--- Comment #15 from Manolis Tsamis <manolis.tsamis at vrull dot eu> ---
(In reply to Sam James from comment #13)
> Created attachment 56527 [details]
> compile.c.323r.fold_mem_offsets.bad.xz
> 
> Output from
> ```
> hppa2.0-unknown-linux-gnu-gcc -c  -DNDEBUG -g -fwrapv -O3 -Wall -O2  
> -std=c11 -Werror=implicit-function-declaration -fvisibility=hidden 
> -I/home/sam/git/cpython/Include/internal -IObjects -IInclude -IPython -I.
> -I/home/sam/git/cpython/Include    -DPy_BUILD_CORE -o Python/compile.o
> /home/sam/git/cpython/Python/compile.c -fdump-rtl-fold_mem_offsets-all
> ```
> 
> If I instrument certain functions in compile.c with no optimisation
> attribuet or build the file with -fno-fold-mem-offsets, Python works, so I'm
> reasonably sure this is the relevant object.

Thanks for the dump file! There are 66 folded/eliminated instructions in this
object file; I did look at each case and there doesn't seem to be anything
strange. In fact most of the transformations are straightforward:

 - All except a couple of cases don't involve any arithmetic, so it's just
moving a constant around.
 - The majority of the transformations are 'trivial' and consist of a single
add and then a memory operation: a sequence like X = Y + Const, R = MEM[X + 0]
is folded to X = Y, R = MEM[X + Const]. I wonder why so many of these exist and
are not optimized elsewhere.
 - There are some cases with negative offsets, but the calculations look
correct.
 - There are few more complicated cases, but I've done these on paper and also
look correct.

Of course I could be missing some more complicated effect, but what I want to
say is that everything looks sensible in this particular file.

> Thanks! You are very welcome to have access to some HPPA machines for
> this kind of work. Please email me an SSH public key + desired username
> if that sounds helpful.

Yes, since I couldn't find anything interesting in the dump, that would
definitely be helpful. Thanks!

Manolis