[Bug target/126411] RISC-V rv64gcv: wrong code at -O0 with simple shuffle

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126411

Raphael M Zinsly <raphael.zinsly at oss dot qualcomm.com> changed:

           What    |Removed                     |Added
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                 CC|                            |raphael.zinsly at oss dot qualcomm
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--- Comment #3 from Raphael M Zinsly <raphael.zinsly at oss dot qualcomm.com> ---
(In reply to Souradipto Das from comment #2)
> Created attachment 65147 [details]
> [PATCH] RISC-V: Reject non-monotonic shuffle masks in slide patterns
> 
> Hi, I added checks so that intermediate elements conform to the expected
> monotonic slide segments. I wasn't able to get a clean make check run
> locally due to DejaGNU board-file configuration issues on my end (unrelated
> to the patch itself) but i was able to verify that the patch holds its
> effectiveness on this bug

This seems to work but I think you could do that without the loop as we already
look for a monotonic sequence later, you could explicitly check if the
beginning and end of the sequence corresponds to OP0 and OP1 respectively (vlen
- slideup_cnt and 2 * vlen - 1 - slideup_cnt). The current code assumes that
but this is not guaranteed as this testcased caught. This could be checked in
the same place as we look at the slideup_cnt positions.
If you go this route you should also check for only one pivot by adding
need_slideup_p in the second pivot check to be safe.