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

[Jeff Law]

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On 02/24/11 10:04, Michael Matz wrote:
> 
> Funny.  As far back as I remember we consistently said that bits of the 
> same word, but outside the subreg are left with undefined values after 
> storing into the subreg, except if wrapped with a strict_low_part.  In 
> fact that's the whole point of strict_low_part.  I'm pretty sure we assume 
> this documented semantics in various parts of the compiler.
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.

And perhaps that means my statement to Georg is wrong -- I hadn't
considered the possibility that the subreg was smaller than a word.  I
don't know the last time I saw a subreg like that :-)


jeff
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