PRE (sometimes) confuses ivopts/scev?

[Richard Guenther]

Daniel Berlin wrote:
> 
> 
> On Mon, 24 Jan 2005, Daniel Berlin wrote:
> 
>>
>>
>> On Mon, 24 Jan 2005, Richard Guenther wrote:
>>
>>> Hi!
>>>
>>> Are there known bad interaction PRs wrt PRE and loop optimizations?
>>> I found only PR18848 for PRE problems.  Sometimes a lot better code
>>> is created with -fno-tree-pre -- SCEV seems to be confused
>>> and produces (for only extra IVs?) scev_not_known with PRE, but not
>>> without, i.e. some uses are not detected?
>>
>>
>> Yes
>> This is the wraparound variable problem.
>> I have fixes for PRE to stop it from creating induction variables.
>> Sebastian has improvements to SCEV that let it handle the type of 
>> induction variables PRE was creating (wraparound variables).
>>
>> Both patches are queued for 4.1
>>
> 
> Just as a followup, sebastian's patch is at (the actual patch is ta the 
> end of the message. You sahouldn't need the other two parts he's pasted 
> in front of it anymore, i believe)
> http://gcc.gnu.org/ml/gcc-patches/2004-11/msg02444.html

This patch applied, but didn't help, I'll try to get more details and 
hopefully a reduced testcase.

> it should be applied because it helps us detect more simple evolutions 
> properly.
> " during a bootstrap
> there are about 7000 cases that are in fact just simple affine
> evolution functions, but that are mishandled for the moment because
> the scalar evolution analyzer was not enough smart to transform them."
> 
> In addition, the patch to teach PRE to stop creating *more* wraparounds 
> is also in the mail archives, see 
> http://gcc.gnu.org/ml/gcc-patches/2004-11/msg02222.html

This patch doesn't apply any more.

Richard.

> If this problem is causing some performance regression or something with 
> regards to 3.x, please file a bug and i'll commit the PRE fix for 4.0.
> --Dan
>