[AArch64] Re: [PATCH] Relax check against commuting XOR and ASHIFTRT in combine.c

[Alan Lawrence <alan.lawrence@arm.com>]

Ok thanks Jeff. In that case I think I should draw this to the attention of the 
AArch64 maintainers to check the testsuite updates are OK before I commit...?

Methinks it may be possible to get further, or at least increase our confidence, 
if I "mock" out try_widen_shift_mode, and/or try injecting some dubious RTL from 
a builtin, although this'll only give a momentary snapshot of behaviour. I may 
or may not have time to look into this though ;)...

Cheers, Alan

Jeff Law wrote:
> On 09/18/14 03:35, Alan Lawrence wrote:
>> Moreover, I think we both agree that if result_mode==shift_mode, the
>> transformation is correct. "Just" putting that check in, achieves
>> what I'm trying for here, so I'd be happy to go with the attached
>> patch and call it a day. However, I'm a little concerned about the
>> other cases - i.e. where shift_mode is wider than result_mode.
> Let's go ahead and get the attached patch installed.  I'm pretty sure 
> it's correct and I know you want to see something move forward here.  We 
> can iterate further if we want.
> 
>> If I understand correctly (and I'm not sure about that, let's see how
>> far I get), this means we'll perform the shift in (say) DImode, when
>> we're only really concerned about the lower (say) 32-bits (for an
>> originally-SImode shift).
> That's the class of cases I'm concerned about.
> 
> 
>   try_widen_shift_mode will in this case
>> check that the result of the operation *inside* the shift (in our
>> case an XOR) has 33 sign bit copies (via num_sign_bit_copies), i.e.
>> that its *top* 32-bits are all equal to the original SImode sign bit.
>> <count> of these bits may be fed into the top of the desired SImode
>> result by the DImode shift. Right so far?
> Correct.
> 
>> AFAICT, num_sign_bit_copies for an XOR, conservatively returns the
>> minimum of the num_sign_bit_copies of its two operands. I'm not sure
>> whether this is behaviour we should rely on in its callers, or for
>> the sake of abstraction we should treat num_sign_bit_copies as a
>> black box (which does what it says on the, erm, tin).
> Treat it as a black box.  It returns the number of known sign bit 
> copies.  There may be more, but never less.
> 
> 
>> If the former, doesn't having num_sign_bit_copies >= the difference
>> in size between result_mode and shift_mode, of both operands to the
>> XOR, guarantee safety of the commutation (whether the constant is
>> positive or negative)? We could perform the shift (in the larger
>> mode) on both of the XOR operands safely, then XOR together their
>> lower parts.
> I had convinced myself that when we flip the sign bit via the XOR and 
> commute the XOR out that we invalidate the assumptions made when 
> widening.  But I'm not so sure anymore.  Damn I hate changes made 
> without suitable tests :(
> 
> I almost convinced myself the problem is in the adjustment of C2 in the 
> widened case, but that's not a problem either.  At least not on paper.
> 
>> If, however, we want to play safe and ensure that we deal safely with
>>  any XOR whose top (mode size difference + 1) bits were the same,
>> then I think the restriction that the XOR constant is positive is
>> neither necessary nor sufficient; rather (mirroring
>> try_widen_shift_mode) I think we need that num_sign_bit_copies of the
>> constant in shift_mode, is more than the size difference between
>> result_mode and shift_mode.
> But isn't that the same?  Isn't the only case where it isn't the same 
> when the constant has bits set that are outside the mode of the other 
> operand?
> 
> Hmm, what about (xor:QI A -1)?  In that case -1 will be represented with 
> bits outside the precision of QImode.
> 
>> Hmmm. I might try that patch at some point, I think it is the right
>> check to make. (Meta-comment: this would be *so*much* easier if we
>> could write unit tests more easily!) In the meantime I'd be happy to
>> settle for the attached...
> No argument on the unit testing comment.  It's a major failing in the 
> design of GCC that we can't easily build a unit testing framework.
> 
> Jeff
>