gcc-4.5/4.4: Bug in .subreg1 pass?

[Eric Botcazou]

> 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.

-- 
Eric Botcazou