[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 #14 from dberlin at gcc dot gnu dot org  2006-07-17 13:34 -------
Subject: Re:  [4.0/4.1/4.2 Regression] Tree loop
 optimizer does worse job than RTL loop optimizer

rakdver at gcc dot gnu dot org wrote:
> ------- Comment #13 from rakdver at gcc dot gnu dot org  2006-07-17 11:54 -------
> (In reply to comment #12)
>> The test case in comment #11 looks like a classic store motion opportunity to
>> me.  GCC 3.3 performs the store motion, GCC 4.2 r115467 does not.
>>
>> Zdenek, I thought tree-ssa-lim should be able to do store motion in loops?
> 
> Yes, however again, the alias analysis does not tell it that a[0] does not
> alias a[1]; once that is fixed, tree-ssa-lim should work just fine.

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.

--Dan


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