Finding out what backend instruction pattern matches instruction

[Andrew Hutchinson]

Thank you greatly for the feedback

I took at look at mips.md - and we already use the conditional "length" 
for some instruction.

However, some effective AVR instruction lengths are vastly complicated 
by operands, length and addressing modes. (After all we emulate 16 and 
32 bit operations with only 8 bit CPU) . So it takes a relatively 
complex set of logic to get true length correct. This logic is already 
present as c functions. (move and shifts being the more complex 
variety). We call these routines only for final adjustment - but have to 
figure out which to call based on the insn RTL .

I fear that duplicating this logic in RTL patterns won't be very elegant 
or less error prone.

As I say above, the logic already exists as c function. If we could call 
these directly to determine the attribute value, it would be much easier!

For now, matching the name seems optimal.

Andy

Ian Lance Taylor wrote:
> Andrew Hutchinson <andrewhutchinson@cox.net> writes:
>
>   
>> The alternative, perhaps,  would be to set each length attribute
>> dynamically in each pattern - if that was possible. But that looks
>> like way more work.
>>     
>
> That is certainly the best way.  Search for "length" in mips.md for
> one example of how it can be done.
>
> Ian
>
>