This is the mail archive of the
gcc-patches@gcc.gnu.org
mailing list for the GCC project.
Re: [PATCH] Remove unnecessary jump threading restriction
On Fri, Aug 5, 2016 at 10:43 PM, Jeff Law <law@redhat.com> wrote
> On 08/05/2016 01:45 AM, Richard Biener wrote:
>>
>>
>> There is no need to avoid threading to a loop header, the threading
>> code can cope with this just fine. Noticed when working on PR72772.
>>
>> Bootstrapped and tested on x86_64-unknown-linux-gnu, applied to trunk.
>>
>> Richard.
>>
>> 2016-08-05 Richard Biener <rguenther@suse.de>
>>
>> * tree-ssa-threadupdate.c (thread_block_1): Remove unnecessary
>> restriction on threading to a loop header.
>
> This restriction exists to help prevent threading through a loop header into
> the loop which can easily create an irreducible region.
>
> In fact what you've done is revert the fix for 58343. Now there is of
> course a test for 58343 in the regression suite.
>
> It's entirely possible that 58343 is just latent now or other changes in
> tree-ssa-threadupdate.c prevent the problems that we saw with 58343.
Well, I removed the restriction that the forward threading path ends in a loop
header. I did this to restore threadings we've done when there was a forwarder
before the loop header we threaded to -- a forwarder with a degenerate PHI
that was later removed and merged with the header anyway. So I basically
changed nothing here. I also don't see how forward threading to a loop header
(not across it) can cause an irreducible region to appear - the other part of
the condition which I retained certainly guards the case of crossing a
loop header.
But maybe I am missing something.
Richard.
> Jeff
>