Re: IRA patch: move clobbers downwards

[Bernd Schmidt <bernds at codesourcery dot com>]

On 07/14/2010 09:23 PM, Jeff Law wrote:

> When I first looked at this, I kept hearing a little voice saying I'd
> seen this discussion before.  So I did some searching today :-)
> 
> The first thread starts here:
> 
> http://gcc.gnu.org/ml/gcc-patches/2002-03/msg01651.html

Lovely :-)

>> The real problem here is that IRA does a backwards scan; just as in
>> peep2, it would probably be better to use a forwards scan using REG_DEAD
>> notes.  However, I'm told it's expensive to create those.
>>   
> Just so I'm clear, we're doing a backwards life analysis.  We see one or
> more assignments to the components of a double-word pseudo which makes
> the pseudo live.  The pseudo is assumed to still be live until we find
> its clobber thus making the pseudo live between the clobber and the
> component assignments, even though it holds no useful value.  Right?

That's what happens.  SETs of DF_REF_PARTIAL things are ignored for the
purpose of tracking liveness.
With a forward scan we'd ignore the clobbers and mark the reg live on
the first partial set, getting the right answer.

> I guess we could compensate for this with some special clobber handling
> when computing lifetimes, but just moving the clobbers after sched might
> be the best solution.

Since we seem to have decided to use my IRA subword patch, the issue is
a bit moot.  We can still run into trouble for

(clobber (reg:SI x))
(set (zero_extract (reg x) (low half)) (something))
(set (zero_extract (reg x) (high half)) (something else))

which IIRC happens for CHImode and maybe some other cases.  It's
probably no longer that important.


Bernd