Re: [PATCH v3] EXPR: Emit an truncate if 31+ bits polluted for SImode

[Jeff Law <jeffreyalaw@gmail.com>]

On 12/24/23 01:11, YunQiang Su wrote:
>>> Yes. I also guess so.  Any new idea?
>> Well, I see multiple intertwined issues and I think MIPS has largely
>> mucked this up.
>>
>> At a high level DI -> SI truncation is not a nop on MIPS64.  We must
>> explicitly sign extend the value from SI->DI to preserve the invariant
>> that SI mode objects are extended to DImode.  If we fail to do that,
>> then the SImode conditional branch patterns simply aren't going to work.
>>
> 
> MIPS64 never claims DI -> SI is nop, instead it claims SI -> DI is nop.
And that just seems wrong, at least for truncation which implies the 
input precision must be larger than the output precision.

If you adjust the mips implementation of TARGET_TRULY_NOOP_TRUNCATION to 
return false when the input precision is smaller than the output 
precision, does that fix this problem?


> And for MIPS64, it has only one type of branch. it works for both SI and DI.
Agreed, but the SImode variant is really just a DImode comparison that 
relies on the sign extending property of the MIPS architecture.  I'm not 
100% sure that's safe in the presence of bit manipulation instructions 
which do not preserve the sign extending property.  We actually don't 
allow some bit manipulations on RV64 for a similar underlying reason.


> 
> Converting from 32 to 64 does be nop, IF the 32 is properly sign extended.
But that's not a *truncation*, that's an *extension*.

Jeff