r281277 - [Sema] Fix PR30346: relax __builtin_object_size checks.

[Hal Finkel via cfe-commits]

----- Original Message -----
> From: "George Burgess IV" <george.burgess.iv at gmail.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Richard Smith" <richard at metafoo.co.uk>, "Joerg Sonnenberger" <joerg at bec.de>, "cfe-commits"
> <cfe-commits at lists.llvm.org>
> Sent: Monday, September 19, 2016 11:21:33 PM
> Subject: Re: r281277 - [Sema] Fix PR30346: relax __builtin_object_size checks.
> 
> 
> WFM; I'll put together a patch that only allows this under
> -fno-strict-aliasing.
> 
> 
> I'm entirely unfamiliar with struct-path-tbaa, so Hal, do you see a
> reason why struct-path-tbaa wouldn't play nicely with flexible
> arrays at the end of types? Glancing at it, I don't think it should
> cause problems, but a more authoritative answer would really be
> appreciated. :) If it might cause issues now or in the future, I'm
> happy to be conservative here if -fno-strict-path-tbaa is given,
> too.

We currently don't emit struct-path-tbaa for array members. We likely should, and we'll need to keep flexible array members in mind when we implement that extension. I don't think that the current representation has a way to represent an unbounded size (except for using ((size_t) -1), which might be as good as anything else).

 -Hal

> 
> On Tue, Sep 13, 2016 at 2:00 PM, Joerg Sonnenberger via cfe-commits <
> cfe-commits at lists.llvm.org > wrote:
> 
> 
> On Tue, Sep 13, 2016 at 12:51:52PM -0700, Richard Smith wrote:
> > On Tue, Sep 13, 2016 at 10:44 AM, Joerg Sonnenberger via
> > cfe-commits <
> > cfe-commits at lists.llvm.org > wrote:
> > 
> > > IMO this should be restricted to code that explicitly disables
> > > C/C++
> > > aliasing rules.
> > 
> > 
> > Do you mean -fno-strict-aliasing or -fno-struct-path-tbaa or
> > something else
> > here? (I think we're not doing anyone any favours by making
> > _FORTIFY_SOURCE
> > say that a pattern is OK in cases when LLVM will in fact optimize
> > on the
> > assumption that it's UB, but I don't recall how aggressive
> > -fstruct-path-tbaa is for trailing array members.)
> 
> The former immediately, the latter potentially as well. I can't think
> of
> many use cases for this kind of idiom that don't involve type
> prunning
> and socket code is notoriously bad in that regard by necessity.
> 
> 
> 
> Joerg
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> 

-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory