combine/dce patch for PR36003, PR42575

[Paolo Bonzini]

On 06/24/2010 03:00 PM, Bernd Schmidt wrote:
> On 06/24/2010 02:53 PM, Eric Botcazou wrote:
>>> Would you be opposed to adding a simpler, basic-block-local pass that
>>> only tries to eliminate unnecessary operations on DImode subregs?
>>
>> The 3 words "adding a pass" always make me cringe. :-)  Couldn't this be
>> integrated into one of the existing RTL passes?
>
> I can tack it onto something else, but I don't see how this would reduce
> the amount of work we need to do?

It may be possible to piggy-back it onto an existing DCE pass, or onto 
lower-subreg, but I'm not sure without understanding exactly what 
transform you need to do (especially if you want to run this before 
register allocation).

But you're talking about replacing a buggy and expensive pass 
(byte-level DCE) with a smaller one, which isn't a bad idea.

Paolo