Re: [Patch/rtl-expand] Take tree range info into account to improve LSHIFT_EXP expanding

[Jeff Law <law at redhat dot com>]

On 04/29/2015 03:36 PM, Jiong Wang wrote:
> Jeff Law writes:
>
> > On 04/27/2015 02:21 PM, Jiong Wang wrote:
> >
> > > Jeff,
> > >
> > >     Sorry, I can't understand the meaning of "overlap between t_low and low",
> > >     assume "right" in "right value" means the opposite of "left" not
> > >     "correct".
> > >
> > >     So what you mean is t_low and low share the same pseudo regiser?
> > My concern is sharing the same pseudo or memory location.  But thinking
> > more about it, the shifted value has to have range information, so it
> > must have been an SSA_NAME, right?  If so, then it can't overlap with
> > the destination, so this is a non-issue.  Sorry for the confusion.
>
> Thanks for the light. By looking at related code, looks like even it's
> SSA_NAME, it's still possible to share the same pseudo given the
> destination is in the same SSA map parition after ssa name coleascing?
If they're the same size and have non-overlapping lifetimes, then yes, 
they could be the same pseudo.  That ought to be easy to check. 
Thankfully we don't have to worry about MEMs, which is a harder check.

> OK. I will rework the patch, and I found there is a function named
> "expand_doubleword_shift" which looks like a more natural place to do
> this optimization, although it's hard to get range info there. I will do
> further explore on this.
Sounds good.

jeff