[Bug tree-optimization/20643] [4.0/4.1/4.2 Regression] Tree loop optimizer does worse job than RTL loop optimizer

["dberlin at dberlin dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #16 from dberlin at gcc dot gnu dot org  2006-07-18 17:03 -------
Subject: Re:  [4.0/4.1/4.2 Regression] Tree loop
 optimizer does worse job than RTL loop optimizer

anemo at mba dot ocn dot ne dot jp wrote:
> ------- Comment #15 from anemo at mba dot ocn dot ne dot jp  2006-07-18 16:53 -------
> (In reply to comment #14)
>> This is because it's an incoming parameter, and as a result, this
>> doesn't look at all like an array access, but just a random pointer access.
>>
>> I have no plans to make the alias analysis algorithm reconstruct array
>> indexes from random pointer arithmetic.
> 
> I do not think reconstructing array indexes are needed,
> but is it hard to tell that *(a+0) never be an alias of *(a+1) ?

We already do say this when we know the offsets.  The point is that
without reconstructing the array indexes, we'd have to follow use-def
chains for *every single pointer access* on *every single operand
update*, in order to attempt to get the offsets and disambiguate them.

This is incredibly slow.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20643