[PATCH] D18298: ThinLTO: do not promote GlobalVariable that have a specific section.

[Teresa Johnson via llvm-commits]

tejohnson added a comment.

In http://reviews.llvm.org/D18298#408779, @joker.eph wrote:

> I'm hitting a wall: even with adding the "hasSection" flag to the summary: no renaming a function breaks the model.
>
>   Module A {
>     define internal foo() { ... };
>     define bar() { foobar() };
>   }
>  
>   Module B {
>     define foo() { ... };
>     define foobar() { foo() };
>   }
>
>
> If we leave `internal foo()` internal in module A and we don't rename it, we have a problem when we want to import `foobar()` because of the collision on the call to `foo()` in module B.
>  There is nothing in the index that can help to address this situation. We reach a point were the only conservative strategy I can imagine is to detect the collision "late" (i.e. during the FunctionImporter, in the "backend" job) and do not perform the import.
>  This is annoying because it prevents future optimizations (like "moving" a global for instance).


How frequent/important are the modules where specific sections are used? Another conservative possibility is to simply prevent importing into any module that contains a GV with local linkage and the hasSection flag set (which is info available in the index).

Also, I assume the plan is to use the proposed hasSection flag to prevent importing any function containing a reference to a local with that flag set (to avoid introducing a cross-module reference requiring promotion). If renaming is ok, but not the promotion, then a local function with hasSection can itself be imported, if we keep the imported copy local (in which case functions containing references to it can also be imported).


http://reviews.llvm.org/D18298