query about commit 666fdc46bc8489 ("RISC-V: Fix bad insn splits with paradoxical subregs")

[Jeff Law]

On 11/4/22 17:38, Vineet Gupta wrote:
>> commit 666fdc46bc848984ee7d2906f2dfe10e1ee5d535
>> Author: Jim Wilson <jimw@sifive.com>
>> Date:   Sat Jun 30 21:52:01 2018 +0000
>>
>>     RISC-V: Add patterns to convert AND mask to two shifts.
>>
>>             gcc/
>>             * config/riscv/predicates.md (p2m1_shift_operand): New.
>>             (high_mask_shift_operand): New.
>
> Indeed Jim introduced the pattern with 666fdc46bc8, but the clobber 
> was added later in 36ec3f57d305 ("RISC-V: Fix bad insn splits with 
> paradoxical subregs"). He attributed this to Jakub, and with Jim not 
> being super active these days, I tried reaching out to this cc list.
>
> Sorry I pasted wrong sha-id in my orig msg, it needs to be 36ec3f57d305

I'd look at the testcases in that hash.  I bet you're going to find one 
where we have a paradoxical subreg destination.  We can't shift bits 
into the paradoxical part, then expect to safely shift them back in.  
The semantics of a paradoxical subreg are that the bits outside the 
inner mode are don't cares.  Of course you *may* need to go back to a 
compiler a that point in time to see the problematical case.


Jeff