RFA: fix rtl-optimization/56833

[Eric Botcazou]

> But I can see that there could be a problem with an earlier value
> that used to be valid in a multi-hard-register sub register to be
> considered to be still valid.
> Setting the mode of every constituent register but the first one
> (which has the new value recorded) to VOIDmode at the same time
> as updating reg_set_luid should be sufficent to address this issue.

Agreed.

> The pass was originally written with word_mode == Pmode targets like
> the SH in mind, where multi-hard-register values are uninteresting.
> 
> But for targets like the avr, most or all of the interesting values
> will be in multi-hard-register registers.

The patch is OK on principle, but can you factor out the common code?  The 
endings of move2add_use_add2_insn and move2add_use_add3_insn are identical so 
it would be nice to have e.g. a record_reg_value helper function and call it 
from there.  Similarly, the 3 new checks look strictly identical.

-- 
Eric Botcazou