Re: [patch/committed] PR middle-end/65233 make walk-ssa_copies handle empty PHIs

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Mar 4, 2015 at 6:27 AM, Jeff Law <law@redhat.com> wrote:
> On 03/02/15 01:38, Richard Biener wrote:
>>
>> On Mon, Mar 2, 2015 at 6:34 AM, Aldy Hernandez <aldyh@redhat.com> wrote:
>>>
>>> As I mention in the PR...
>>>
>>> What's happening here is that the ipa_polymorphic_call_context
>>> constructor
>>> is calling walk_ssa_copies on a PHI node that has no arguments.  This
>>> happens because finalize_jump_threads eventually removes some PHI
>>> arguments
>>> as it's redirecting some edges, leaving a PHI with no arguments:
>>>
>>> SR.33_23 = PHI <>
>>>
>>> This should get cleaned up later, but the IPA polymorphic code gets
>>> called
>>> during the actual CFG clean-up, and walk_ssa_copies cannot handle an
>>> empty
>>> PHI.
>>>
>>> Approved by Honza.
>>>
>>> Fully tested on x86-64 Linux and verified that the patch fixes the ICE on
>>> an
>>> x86-64 Linux cross aarch64-linux-gnu cc1plus.
>>>
>>> Committed to mainline.
>>
>>
>> I think the real issue is that the walking code is executed via fold_stmt
>> when
>> called with an API that tells you not to walk SSA use-def chains.
>
> ?  We have something that tells us not to walk the chains?  I don't see it
> in an API for fold_stmt.  How is the ipa-polymorphic code supposed to know
> when it can't follow the chains?

It gets passed the valueize callback now which returns NULL_TREE for
SSA names we can't follow.

> The restrictions on what we can do while we're in the inconsistent state
> prior to updating the ssa graph aren't defined anywhere and I doubt anyone
> really knows what they are.  That's obviously concerning.
>
> We might consider trying to narrow the window in which these inconsistencies
> are allowed.  To do that I think we need to split cfgcleanup into two
> distinct parts.  First is unreachable block removal (which is needed so that
> we can compute the dominators).  Second is everything else.
>
> The order of operations would be something like
>
> remove unreachable blocks
> ssa graph update
> rest of cfg_cleanup
>
> That just feels too intrusive to try at this stage though.

Well, not folding statements from cfg-cleanup would be better.

I'll have a look at the testcase in the PR and will come back with a
suggestion on what to do for GCC 5.

Richard.

>
> jeff