Re: Question about RTL code hoisting

[Jeff Law <law at redhat dot com>]

On 03/25/10 10:19, Jie Zhang wrote:
> On 03/25/2010 11:22 PM, Jeff Law wrote:
> > I never bothered to implement hoisting which touched hard regs -- I
> > never thought the cost/benefit analysis made much sense. It's quite a
> > bit more work to implement and code motion of hard regs is much more
> > restricted than code motion involving just pseudos.
> Thanks Bernd and Jeff.
>
> This case is not common. I'm wondering how likely this kind of 
> optimization pass will be accepted into GCC.
If the code is clean and doesn't add significant compile-time overhead, 
it'd probably be accepted.

> Another way to fix it is teaching register allocator to allocate r0 to 
> REG. But I guess it's more difficult.
Well, the register allocator will already attempt to allocate REG to r0  
-- when it sees a copy between a pseudo and a hard reg it will adjust 
its cost model appropriately to encourage the pseudo to be allocated to 
the hard reg (thus eliminating the copy).

It's probably at least worth looking into why the pseudos in question 
aren't being assigned to the appropriate hard register to eliminate the 
copy.

Another alternative would be to look at something like copy motion (see 
Morgan's text).  I've always wondered if it was generally useful and 
what it's compile-time impact might be.

Jeff