[llvm] r256620 - [ptr-traits] Implement the base pointer traits using the actual

[Chandler Carruth via llvm-commits]

The right solution is to not use incomplete types here as all of my other
commits indicated. Sorry I didn't catch the uses in Polly last night. They
just need to be fixed like every other LLVM project. I'll finish this
morning and re-land.

On Wed, Dec 30, 2015, 06:45 Tobias Grosser via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> On 12/30/2015 05:00 AM, Chandler Carruth via llvm-commits wrote:
> > Author: chandlerc
> > Date: Tue Dec 29 22:00:24 2015
> > New Revision: 256620
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=256620&view=rev
> > Log:
> > [ptr-traits] Implement the base pointer traits using the actual
> > alignment of the pointee type!
> >
> > This is the culmination of the ptr-traits work. Now the compiler will
> > catch me if I try to use a pointer to an empty struct as a key in
> > a dense map or inside a PointerIntPair or PointerUnion! This is much,
> > much better than sometimes corrupting data (and other times working
> > fine) due to insufficient alignment.
> >
> > It also means that we will be much more diligent about rejecting other
> > uses of these constructs that aren't safe.
> >
> > It also means that we can now be more aggressive with the constructs
> > when we actually have guaranteed higher alignment without specializing
> > stuff. I'll be going through and cleaning up all the current overrides
> > of these traits which are no longer necessary.
> >
> > Many thanks to Richard, David, and others who helped me get all of this
> > together.
>
> This broke the use of incomplete types (and as a result Polly). I
> temporarily reverted this commit in 256642.
>
> I spent some more time to look into what the right solution for
> incomplete pointer types might be and came up with three possible
> solutions:
>
> 1) Generally avoid incomplete pointer types
>
> 2) Add PointerLikeTypeTraits for any incomplete type used
>
> 3) Teach PointerLikeTypeTraits to be conservative in case of incomplete
>     types.
>
>
> 1) does not seem to be a solution as in incomplete pointer types are in
> our case used to hide implementation details. I do not think exposing
> the actual type is the right approach.
>
> Solution 2) can be easily implemented (I realized this after reverting).
> I attached a patch that adds two PointerLikeTypeTraits with
> "enum { NumLowBitsAvailable = 0 };" for the two incomplete types in
> Polly. This is sufficient to pass 'make check'. There are likely more
> bits available, but at least the isl interface does not give any
> guarantees. I assume these patches would be sufficient to get Chandlers
> change while ensuring that Polly still builds. If you think this is
> indeed the right approach, I would be happy to add this to Polly and go
> on with life.
>
> Regarding 3), I wonder if we could make the behavior of 2) the default
> for all incomplete types. Maybe there is some C++-magic available that
> would allow us to do this easily?
>
> Best,
> Tobias
>
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