Re: [PATCH] Fix PR63184

[Jeff Law <law@redhat.com>]

On 12/7/18 3:55 AM, Richard Biener wrote:
> On Fri, 7 Dec 2018, Richard Biener wrote:
> 
>>
>> The following fixes PR63184 by using tree-affine to resolve pointer
>> comparisons.  Instead of trying to stick this into a match.pd pattern
>> the following does this in the more constrained forwprop environment.
>>
>> I've only implemented the cases where the comparison resolves to a
>> compile-time value, not the case where for example &a[i] < &a[j]
>> could be simplified to i < j.  I'm not sure I can trust the
>> tree-affine machinery enough here to do that.
>>
>> Both testcases require some CSE to happen thus the first forwprop
>> pass doesn't catch it.
>>
>> Bootstrap & regtest running on x86_64-unknown-linux-gnu.
>>
>> I'll collect some statistics from bootstrap.
> 
> Somewhat depressing.  There's a single instance in
> libiberty/rust-demangle.c that gets resolved (a ordered compare).
> This instance triggers 4 times during a c,c++ bootstrap compared
> to 258098 affine expansion combinations tried.
> 
> It doesn't trigger in tramp3d at all (just to try a C++ code base).
> 
> I suspect the cases in PR63184 are arcane enough and usually we
> have simpler addresses that are resolved with the existing
> patterns.
> 
> I'll attach the patch to the PR and leave it alone.
Seems reasonable to me as well.  It's originally your BZ and somewhere
in it I think you indicated you didn't think it was terribly important.

I'd suggest pushing it out to P4.  But I know you don't like that, so I
won't actually do it :-)

Jeff