Re: [PATCH, PR62167] Fix tail-merge pass for dead type-unsafe code

[Richard Biener <rguenther at suse dot de>]

On Tue, 18 Nov 2014, Tom de Vries wrote:

> Richard,
> 
> this (trunk) patch fixes PR62167.
> 
> The patch fixes a problem that triggers with the test-case on the 4.8 branch,
> when tail-merge makes a dead type-unsafe load alive.
> 
> I'm not able to reproduce this bug on 4.9 and trunk with the same test-case.
> On those branches, the tail-merge already does not happen.
> 
> The reason for the difference is as follows: With 4.8 the two phi arguments of
> the phi in the tail block are value-numbered identically:
> ...
> SCC consists of: p_14
> Value numbering p_14 stmt = p_14 = MEM[(struct head *)_13].first;
> Setting value number of p_14 to p_14 (changed)
> 
> SCC consists of: p_15
> Value numbering p_15 stmt = p_15 = _13->next;
> Setting value number of p_15 to p_14 (changed)
> ...
> 
> With 4.9 (and trunk), that's not the case:
> ...
> SCC consists of: p_14
> Value numbering p_14 stmt = p_14 = MEM[(struct head *)&heads][k.1_9].first;
> Setting value number of p_14 to p_14 (changed)
> 
> SCC consists of: p_15
> Value numbering p_15 stmt = p_15 = _13->next;
> Setting value number of p_15 to p_15 (changed)
> ...
> 
> I'm not sure the bug triggers on trunk and 4.9, but I see no reason why it
> could not trigger, so I'd prefer to apply the patch to 4.9 and trunk as well.
> 
> The patch introduces an xfail for pr51879-12.c. I can follow up with a patch
> to improve upon that, but I think that's better limited to trunk only.

Yeah.

> Bootstrapped and reg-tested on x86_64/trunk.
> 
> OK for trunk/stage3, 4.8, 4.9?

Ok.  Note that this goes well with disabling the (re-)use of 
value-numbering in tail-merging (which causes me quite some headaches...).
On trunk it shouldn't be necessary as much as it was earlier as we now
fully propagate constants and copies during FRE/PRE elimination.

Thanks,
Richard.