Re: [PR tree-optimization/71691] Fix unswitching in presence of maybe-undef SSA_NAMEs (take 2)

[Jeff Law <law at redhat dot com>]

On 12/16/2016 07:41 AM, Aldy Hernandez wrote:
> BTW, I don't understand why we don't have auto_bitmap's, as we already
> have auto_sbitmap's.  I've implemented the former based on
> auto_sbitmap's code we already have.
Trevor poked at it a bit.  bitmaps are a bit more complex than sbitmaps 
in terms of implementation details.

https://gcc.gnu.org/ml/gcc/2016-04/msg00013.html

But his suggestion was to first create auto_bitmap, then look to convert 
to using that as opposed to his other approaches.


> The attached patch fixes the bug without introducing any regressions.
>
> I also tested the patch by compiling 242 .ii files with -O3.  These were
> gathered from a stage1 build with -save-temps.  There is a slight time
> degradation of 4 seconds within 27 minutes of user time:
>
>     tainted:    26:52
>     orig:        26:48
>
> This was the average aggregate time of two runs compiling all 242 .ii
> files.  IMO, this looks reasonable.  It is after all, -O3.    Is it
> acceptable?
>
> Aldy
>
> curr
>
>
> commit 2310bcd0e2552a40ca1de354faf005ed3e9daf4e
> Author: Aldy Hernandez <aldyh@redhat.com>
> Date:   Fri Dec 16 03:44:52 2016 -0500
>
>             PR tree-optimization/71691
>             * bitmap.h (class auto_bitmap): New.
>             * tree-ssa-loop-unswitch.c (ssa_maybe_undefined_value_p): New.
>             (tree_may_unswitch_on): Call ssa_maybe_undefined_value_p.
So I'm going to defer to Richi since he was reviewing my original 
attempt in this space.

It probably doesn't matter in practice (when I looked at this I couldn't 
get into the code in question with a -O3 bootstrap or with the 
testsuite, just with the testcase in the BZ) but you might consider 
handling an already visited node slightly differently.

If the the node was visited and resolved as undefined, then we would 
have already exited the loop.

If the node was visited and resolved as defined, then we could just keep 
processing other items on the the worklist.

The case where you want to conservatively return false is when you're 
actively processing the name in question.

Jeff