Re: [RFA] New pass for sign/zero extension elimination

[Richard Sandiford <richard.sandiford@arm.com>]

Joern Rennecke <joern.rennecke@embecosm.com> writes:
>  On 11/20/23 11:26, Richard Sandiford wrote:
>>> +      /* ?!? What is the point of this adjustment to DST_MASK?  */
>>> +      if (code == PLUS || code == MINUS
>>> +  || code == MULT || code == ASHIFT)
>>> + dst_mask
>>> +  = dst_mask ? ((2ULL << floor_log2 (dst_mask)) - 1) : 0;
>>
>> Yeah, sympathise with the ?!? here :)
> Jeff Law:
>> Inherited.  Like the other bit of magic I think I'll do a test with them
>> pulled out to see if I can make something undesirable trigger.
>
> This represents the carry effect.  Even if the destination only cares about
> some high order bits, you have to consider all lower order bits of the inputs.
>
> For ASHIFT, you could refine this in the case of a constant shift count.

Ah, right.  Think it would be worth a comment.

But I wonder whether we should centralise all this code-specific
information into a single place.  I.e. rather than having one switch to
say "PLUS is OK" or "AND is OK", and then having code-specific handling
elsewhere, we could enumerate how to handle a code.

Thanks,
Richard