On Feb 24, 2011, at 12:46 PM, Eric Botcazou wrote:
>> Maybe the misunderstanding occurs when the mode of the subreg is less
>> than word_size? It would certainly make sense that a subreg write of
>> less than word_size leaves the bits undefined.
>>
>> ie, if word_size is SImode and we had a write to
>>
>> (subreg:HI (reg:SI) 0)
>>
>> Then the upper bits are left undefined.
>>
>>
>> Contrast that to the same RTL, but word_size is HImode
>>
>> (subreg:HI (reg:SI) 0)
>>
>> Would leave the upper bits untouched.
>
> That's the conventional wisdom I think. Sub-word and word-sized subregs
> don't
> behave the same, in particular when they are on the LHS of an assignment.
That's certainly what the manual says.
paul
