[llvm-dev] RFC: Absolute or "fixed address" symbols as immediate operands

[Chris Lattner via llvm-dev]

On Oct 26, 2016, at 10:10 AM, Chandler Carruth <chandlerc at google.com> wrote:
> 
> To what Reid said, I'm not really worried about impact on the middle end of any of this. We can handle the code changes, etc.
> 
> I agree with Chris about what we're trading off here:
> 
> On Tue, Oct 25, 2016 at 10:48 PM Chris Lattner via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> I’d argue the other side of it.  The quality of the code is higher if we have invariants (like all globals are pointers) because that simplifies assumptions by eliminating cases where “is a pointer” appears to be true, but isn’t actually true in all cases.  I’m not an expert on CFI or how widely it will ultimately impact the compiler hacker consciousness, but I’m pretty sure that the current model for globals and functions will remain more prominent.  If you choose to break this invariant, you’ll be continually swimming upstream against assumptions made throughout the compiler, both in code written today but also in code written in the future.
> 
> I agree that mental assumptions the developers on the middle end hold are the primary challenge here. But I think we are going to run into challenges either way.
> 
> If the type of these entities is an integer, we will have a non-pointer global, yes. But as Peter points out, this is caught effectively by asserts in the cast infrastructure and other programming aids. Essentially, the checking of LLVM's type system helps protect the random middle end developer from getting this wrong.
> 
> On the other hand, if the type of these entities remains consistently pointers, we will still break assumptions that middle end developers routinely make about pointers to globals:
> - They aren't dereferencable
> - They aren't aligned
> - They may be null
> - The difference between them might not be representable in a pointer-sized-integer

Aren’t all these true of “inttoptr’d” integer constants already?  This is already part of the model for things that are PointerType’s, so I don’t see how either resolution would change that.

-Chris

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20161026/958018e8/attachment.html>